Tests in 7th grade Test work No. 1 " Mechanical movement. Density." Option 1 1) How long will it take the Moon, moving at a speed of 1000 m/s, to travel 60 km? 2) Find the mass of a cast iron slab with a volume of 2.5 m3 if the density of cast iron is 7000 kg/m3. 3) Express the speed of 108 km/h in m/s. Option 2 1) How far will a pedestrian travel in 2 minutes, moving at a speed of 2 m/s? 2) Find the volume ice block weighing 3.6 tons if the ice density is 900 kg/m3. 3) Express the speed of 180 m/min in m/s. Test No. 2 “Strength. Resultant of forces." Option 1 1) Determine the weight of a body weighing 300 g. Draw the body weight in the figure. 2) Find the volume of an ice block subject to a gravity force of 27 kN (ρ ice = 900 kg/m3). 3) Two forces of 300 N and 500 N act on the body, directed along one straight line in one direction. Determine the resultant of the forces. Option 2 1) Find the force of gravity acting on a body weighing 4 tons. Draw this force in the figure. 2) Determine the density of a metal plate with a volume of 4 m3 if its weight is 280 kN. 3) The body is acted upon by two forces of 400 N and 600 N, directed along one straight line in opposite directions. Determine the resultant of the forces. Test No. 3 “Pressure. Pascal's law." Option I. 1. Bucket of water total mass 8 kg exerts a pressure of 2 kPa on the floor. Determine the area of the bottom of the bucket (g=10 N/kg). 2. There is gas in a glass cylinder under the piston. How can we increase its pressure without changing the density of this gas? Option II. 1. What pressure does a table weighing 200 N produce if the area of each of its four legs is 0.0005 m2? 2. The figure shows the same vessel with a piston. The numbers 1, 2 and 3 indicate round holes covered with identical rubber films. When the piston was moved from position A to position B, the films bent outward. Which of the figures shows the convexity of the films correctly? Test No. 4 “Pressure in liquid and gas” I option. 1. Figure 1 shows a U-shaped tube with liquid. Determine what kind of liquid is in the tube if its pressure at the bottom at point A is 1.5 kPa. 2. For a U-shaped tube shown in Figure 1, compare the fluid pressure: a) on the walls at points B and C; b) to the bottom at points A and D; 3. What is the name of the device shown in Figure 2? What is it used for? Record his testimony. Option II. 1. Figure 1 shows a glass with vegetable oil. Determine the pressure and force of oil pressure on the bottom of the glass (g=10 N/kg, ρoil=930 kg/m3). 2.Two liquid barometers - mercury and water - are located next to each other. In which of them will the liquid column be higher and by how many times? (ρрт=13,600 kg/m3, ρв=1000 kg/m3.) 4. What shape should be given (Fig. 2) so that it can hold more water - a or tube b? Test No. 5 on the topic “Pressure solids, liquids and gases" Option 1. 1. A concrete slab 2 m long, 1 m wide and 10 cm thick is completely immersed in water. Calculate the Archimedean force acting on it. 2. Some liquid presses on the bottom of the vessel with a force of 60 N (Fig. 70). (h=0.2 m, 2 S=0.03 m) What is the density of this liquid? What liquid is in the vessel? 3. Why bubble, blown through a tube, takes the shape of a ball? Option 2. 1. The force of gravity acting on a closed metal container with a load is 10,000 N, the volume of the container is 1.5 m3. Will it float or sink if you put it in water? 2. Will the pressure of the liquid at the bottom of the vessel change if a weight on a string is lowered into it as shown in the figure? Will the pressure on the bottom of the vessel be the same at points A and B? Explain your answers. 3. The area of the small hydraulic piston is large. A weight was placed on the small piston, which must be placed on the large piston to be in equilibrium. (The weight of the pistons of the machine is 50 times less than a weight weighing 20 N. Determine the weight of the piston so that the pistons are neglected.) Test No. 6 “Work and power” Option 1. 1. What force must be applied to the left end of the weightless lever so that Was he in balance? 2. A bucket of sand weighing 120 N is lifted using a stationary block to a height of 10 m, acting on a rope with a force of 125 N. Determine the efficiency of the installation. Option 2. 1. Engine power spaceship“Vostok” was equal to 1.5∙107 kW. How much work did the engines of this ship produce in 1 s? 2. With a uniform movement of a load weighing 150 N along inclined plane a dynamometer tied to a load showed a force equal to 40 N. Determine the efficiency of an inclined plane if its length is 1.8 m and its height is 0.3 m. Tests in 8th grade Test work No. 1 on the topic “Thermal phenomena” Option 1 1. A steel part weighing 500 g when processed at lathe heated up to 200C. What is the change? internal energy details? 2. What mass of gunpowder needs to be burned in order to complete combustion 38,000 kJ of energy released? 3. Tin and brass balls of the same mass, taken at a temperature of 200C, were lowered into hot water. Will the water balls receive the same amount of heat when heated? 4. How much will the temperature of water weighing 20 kg change if all the energy released during the combustion of gasoline weighing 20 g is transferred to it? Option 2. 1. Determine the mass of a silver spoon if 250 J of energy is required to change its temperature from 20 to 400C. 2. What amount of heat will be released during the complete combustion of peat weighing 200 g? 3. Steel and lead weights weighing 1 kg each were heated in boiling water and then placed on ice. Which weight will melt the most ice? 4. What mass of kerosene needs to be burned to obtain the same amount of energy as it is released during combustion coal weighing 500 g? Test No. 2 on the topic “Change states of aggregation substances" Option 1. 1. What amount of heat is needed to melt a copper billet weighing 100 g, taken at a temperature of 10750 C? 2. When water boiled, 690 kJ of energy was expended. Find the mass of water evaporated. 3. Why are potatoes, apples and other vegetables and fruits intended for drying cut into pieces? Option 2. 1. What amount of heat is needed to convert water weighing 200 g, taken at a temperature of 500 C, into steam? 2. Determine the mass of a copper bar if 42 kJ of energy is required to melt it. 3. Why to measure low temperatures do they use alcohol rather than mercury thermometers? Test No. 3 on the topic “Electrification of bodies.” Electric field. Structure of the atom" Option 1. 1. When rubbed against silk, the glass is charged... A. positively. B. negative. 2. If an electrified body is repelled by an ebonite stick rubbed against fur, then it... A. has no charge. B. is positively charged. B. negatively charged. 3. The figure shows light balls suspended on silk threads. Which of the figures corresponds to the case when the balls have the same charges? A. 1. B. 2. 4. A stick rubbed on the fur is brought to the ball (fig). What is the sign of the charge on the ball? A. Positive. B. Negative. 5. How to charge metal body What if you bring a charged body B (fig) to it? A. Positive. B. Negative. B. Neutral. 6. What kind of rod - glass, ebonite or steel - should be used to connect the electroscopes so that they are both charged (fig)? A. Glass. B. Ebonitov. V. Steel. 7. A copper rod that had a positive charge was discharged and it became electrically neutral. Will the mass of the rod change? A. It won't change. B. Will increase. B. Will decrease. 8. Which particle has the least negative electrical charge? A. Electron. B. Neutron. B. Proton. 9. The figure shows a diagram of a lithium atom. Is this atom charged? A. The atom is negatively charged. B. The atom is positively charged. B. The atom is electrically neutral. 10. Which one chemical element shown schematically in Fig? A. Hydrogen. B. Lithium. B. Helium Option 2. 1. When an ebonite stick rubs against fur, it becomes charged... A. positively. B. negative. 2. If an electrified body is attracted to a glass rod rubbed on silk, then it... A. is positively charged. B. is negatively charged. V. has no charge. 3. The picture shows balls suspended on silk threads. Which picture shows balls charged with opposite charges? A. 1. B. 2. 4. A glass rod rubbed on silk (rice) is brought to the elderberry ball. What is the sign of the charge on the ball? A. Negative. B. Positive. 5. Which of the charged bodies is affected by the charged ball (fig) with less force? A. 1. B. 2. C. 3. 6. What kind of rod - copper, ebonite or steel - are the electroscopes connected to (fig)? A. Medny. B. Ebonitov. V. Steel. 7. Iron ball, which had negative charge, discharged, and it became electrically neutral. Will the mass of the ball change? A. It won't change. B. Will increase. Q. 8. What particles make up the nucleus of an atom? A. Electrons and protons. B. Neutrons and protons. B. Electrons and neutrons. 9. The figure shows a diagram of a hydrogen atom. A. The atom is negatively charged. B. The atom is positively charged. B. The atom is electrically neutral. 10. Which chemical element is schematically A. Hydrogen. B. Lithium. B. Helium. Will decrease. Is this atom charged? shown in the picture? Test No. 4 “Electric current. Connection of conductors" Option 1. 1. From the drawing, determine: a) the total resistance of the sections CD and (do not take into account the resistance of the ammeters); b) readings of ammeters A1 and A3, if ammeter A2 shows current I2 = 0.1A. 2. The figure shows a graph of current versus voltage in the conductor. Determine: a) at what voltage the current in the conductor is 3A; b) conductor resistance. BD I, A 4 2 2 U,B Option 2. 1. There are two pieces of copper wire of the same length. Square cross section the first wire is 2 times larger than the second. Compare the resistances (R1 and R2) of the wires. Compare the voltages (U1 and U2) on the wires when they are: a) connected in series (Fig. a); 4 6 8 10 b) parallel connection(Fig.b). 2. The iron is connected to a network with a voltage of 220 V. Determine the strength of the current passing through the heating element of the iron if its resistance is 55 Ohms. Test No. 5 on the topic “Electrical phenomena” Option 1. 1. Two conductive balls suspended on threads to each other (picture). attract a) Can one of the balls be charged and the other not? b) Can both balls be charged? If so, is it the same name or different names? 2. Draw a diagram electrical circuit, consisting of a galvanic element, a switch, a rheostat, an ammeter and a voltmeter connected so that, without changing its connection points, they could alternately measure the voltage on galvanic cell and on the rheostat. At what position of the key will the voltmeter show the voltage on the galvanic element, and at what position will it show the voltage on the rheostat? 3. The current flowing through the voltmeter is 1mA. Determine the resistance of the voltmeter if it shows a voltage of 12V. 4. Two electric stoves with the same resistance R are connected to the network in series. How and by how many times will the amount of heat generated by the tiles change if they are connected to the same network in parallel? (The mains voltage is constant). Explain your answer. Option 2. 1. Two conductive sleeves suspended on threads repel each other (Fig. 1). a) Can one of the cartridges be loaded and the other not? b) Can both cartridges be loaded? If so, is it the same name or different names? 2. The figure shows a graph of the current in the circuit versus voltage. Determine the current strength in a section of the circuit at voltages of 10 V and 15 V. What is the resistance of this section of the circuit? 3. Find the ratio of the resistances of two copper conductors if both the length and cross-sectional area of the first conductor are 2 times greater than the second. 4. An electric stove with a resistance R is connected to the lighting network. How and how many times will the amount of heat generated by the electric stove change if a second stove with the same resistance R is connected in series with the first? (The mains voltage is constant). Explain your answer. Test (15-20 min) on the topic “Electromagnetic phenomena” Option 1 1. Figure 65 shows a straight conductor carrying current and a magnetic needle underneath it, established in its magnetic field. Transfer the drawing to your notebook and indicate the direction of the magnetic line of this field. 2. Figure 66 shows two coils suspended on conductors. What needs to be done to make them attract or repel? 3. Figure 67 shows a bar magnet. At which point (1, 2 or 3) is the magnet's weakest action? Option 2 1. The iron rod was brought close to one end north pole magnet. Northern or south pole will be the opposite end of the rod? 2. Figure 68 shows a strip magnet and several lines of it magnetic field. Make a similar drawing and indicate the direction of the magnetic lines. 3. Figure 69 shows a magnetic needle on a stand. When the south pole of the strip magnet was brought closer to it, the needle remained motionless. Make a drawing and show on it which pole of the magnetic needle is located closer to the magnet. Test No. 7 on the topic “Light phenomena” Option 1. Medium 1 1. Based on the picture, determine which medium 1 or 2 – is medium 2 optically denser? 2. The bug crawled closer to flat mirror by 5 cm. How much has the distance between him and his image decreased? 3. The figure shows a mirror and rays 1 – 3 incident on it. Plot the path of reflected rays and indicate the angles of incidence and reflection. 4. Construct and characterize the image of an object in a converging lens if the distance between the lens and the object is greater than double the focal length. 5. The focal length of the lens is 20 cm. At what distance from the lens will the rays incident on the lens parallel to the main optical axis intersect after refraction? Option 2. 1. The figure shows a ray falling from the air onto a smooth surface of water. Draw in your notebook the path of the reflected ray and the approximate path of the refracted ray. 2. The figure shows two parallel rays of light falling from glass into the air. Which of the figures a - c correctly depicts the approximate course of these rays in the air? a b c 3. Where you need to place an object to see it direct image using a converging lens? 4. The object is at double the focal length from the converging lens. Construct its image and characterize it. 5. The student experimentally established that focal length lens is 50 cm. What is its optical power? Tests in 9th grade Test work No. 1 on the topic “Fundamentals of kinematics” Option 1. 1. Is it possible to count balloon material point when determining Archimedean force FA acting on a ball in the air? (FA = g ρair Vball) 2. The ball, falling from a height of 2 m and bouncing off the ground, was caught at a height of 1 m. In both directions, the ball moved along a vertical straight line. Determine the path l and displacement s of the ball during its entire movement. 3. Two cars are moving along a straight section of the highway. The figure shows graphs of the projections of the speeds of these cars on the X-axis parallel to the highway. a) How do cars move: uniformly or uniformly accelerated? b) How are their velocities directed relative to each other? c) At what absolute speed is the first car moving? second? 4. The speed of a skier sliding down a mountain in 3 s increased from 0.2 m/s to 2 m/s. Determine the projection of the skier's acceleration vector onto the X axis, co-directed with the speed of his movement. 5. The train is moving at a speed of 20 m/s. What will be the speed of the train after braking, occurring with an acceleration of 0.25 m/s2, for 20 s? 6. The figure shows how the projection of the body’s velocity vector changes over time. Using the graph, determine the projection ax and the magnitude a of the acceleration vector with which the body is moving. 7. A train moves straight at a speed of 15 m/s. How far will the train travel in 10 s of braking, which occurs with an acceleration of 0.5 m/s2? Test No. 1 on the topic “Fundamentals of kinematics” Option 2. 1. Is it possible to count globe material point when determining the time of sunrise in the eastern and western borders Russia? 2. Midpoint minute hand clock is located at a distance of 2 cm from the center of the dial. Determine the path l and the displacement s of this point in 30 minutes, if in an hour it covers a path equal to 12.56 cm. 3. Two cars are moving along a straight section of the highway. The figure shows graphs of the projections of the speeds of these cars on the X-axis parallel to the highway. a) How do cars move: uniformly or uniformly accelerated? b) How are their velocities directed relative to each other? c) At what absolute speed is the first car moving? second? 4. A skier who rolled down the mountain moved along the plain for 6 s. At the same time, its speed decreased from 3 m/s to 0. Determine the projection of the acceleration vector onto the X axis, co-directed with the speed of the skier. 5. What speed will a car acquire when accelerating with an acceleration of 0.4 m/s2 for 10 s, if initial speed the car's movement was 10 m/s? 6. The figure shows how the projection of the body’s velocity vector changes over time. Using the graph, determine the projection ax and the magnitude a of the acceleration vector with which this body moves. 7. What displacement will the plane make in 10 s of a straight run at an initial speed of 10 m/s and an acceleration of 1.5 m/s2? Test No. 2 “Laws of Dynamics” Option 1 1) Figure 20 shows a block moving on the surface of a table under the action of two forces: a traction force F equal to 1.95 N, and a resistance force Fc equal to 1.5 N. What is the acceleration of the block if its mass is 0.45 kg? 2) The mass of an apple hanging on a branch is approximately 1025 times less than the mass of the Earth. An apple is attracted to the Earth with a force equal to 3 N. Is the Earth attracted to this apple? If yes, then with what force? 3) A dog weighing 3 kg jumps onto a cart weighing 2 kg, rolling around the circus arena at a speed of 0.5 m/s. The dog's speed is 1 m/s and is directed horizontally along the cart. Determine the speed of the cart with the dog. 4) Figure 21 shows how the cyclist's speed changed over time. The cyclist's movement was linear and was considered in inertial system countdown. At what time intervals was the resultant of all forces applied to the cyclist equal to zero? Option 2 1) A skier weighing 60 kg slides down a mountain. Moreover, in any 3 s its speed increases by 1.5 m/s. Determine the resultant of all forces applied to the skier. 2) flare launched vertically upward at a speed of 30 m/s. After what period of time will its speed decrease to zero? How high will the rocket rise during this time? (g = 10 m/s2.) 3) Force increases or decreases gravitational attraction between Mercury and Venus as the distance between them increases? How many times will the force of gravity change if the distance between these planets doubles? 4) Figure 22 shows two weights hanging at the ends of threads thrown over blocks. The other ends of the threads are tied to a dynamometer D. What force does the dynamometer show if the weight of each weight is 7 N? Test No. 3 " Mechanical vibrations and waves" Option 1. 1. Spring pendulum made 16 oscillations in 4 s. Determine the period and frequency of its oscillations. 2. In the oceans, the wavelength reaches 270 m, and the oscillation period is 13.5 s. Determine the speed of propagation of such a wave. 3. Can forced oscillations happen in oscillatory system? in a system that is not oscillatory? If they can, please provide examples. 4. A graph of the dependence of the coordinates of an oscillating body on time is given. Determine the oscillation period from the graph. Option 2. 1. The boat swings on waves traveling at a speed of 1.5 m/s. The distance between the two nearest wave crests is 6 m. Determine the period of oscillation of the boat. 2. A string pendulum oscillates at a frequency of 2 Hz. Determine the period of oscillation and the number of oscillations per minute. 3. Can free vibrations occur in an oscillatory system? in a system that is not oscillatory? If they can, please provide examples. 4. The coordinate of the middle point of the sewing machine needle changes over time as shown in the figure. With what amplitude does this point oscillate? Test No. 4 “Electromagnetic field” (F-9) Option 1. 1. Magnetic and electric fields can be detected simultaneously: A. Near a stationary charged particle or a stationary magnet. B. Only near a moving charged particle. B. Only near the flow of charged particles. D. Near a moving charged particle and a flow of charged particles. 2. What energy transformations occur in an electric stove? 3. Magnetic poles coils with current will not change if: A. Insert an iron rod into the coil. B. Remove the iron rod from it. B. Change the direction of the current in it. D. Answers A and B are correct. 4. The figure shows a conductor with current in a uniform magnetic field. Determine the direction of the induction lines of the magnetic field acting on the conductor with force F. field. 5. There is a current-carrying conductor in a uniform magnetic field with an induction of 0.1 Tesla. The length of the conductor is 1.5 m. It is located perpendicular to the lines of magnetic induction. Determine the current strength in the conductor if a force of 1.5 N acts on it. 6. The figure shows a graph of voltage across current-carrying coils versus time. Determine the amplitude and frequency of voltage fluctuations. at the ends of the period 7. The distance from the Earth to the Sun is 15 ∙ 1010 m. How long will it take light 8 to overcome it? The speed of light is considered equal to 3 ∙ 10 m/s. 8. At what frequency must a radio transmitter operate so that the length of the electromagnetic waves it emits is 49 m? Option 2. 1. A wire coil is attached to a galvanometer and rotates around a magnet located inside it. show galvanometer? (see figure). She What will A. The galvanometer will show some constant current. B. Its arrow will deviate either to the right or to the left. B. The galvanometer will read zero. D. The arrow will always be deflected in the same direction. force value 2. What energy transformations occur when an electric lamp glows? 3. The magnetic field of a current-carrying coil can be weakened by: A. Inserting an iron core into the coil. B. Remove the core. B. Enlarge electric current in a reel. D. And increase the current and insert an iron core. 4. The figure shows a conductor carrying current in a uniform field. Determine the direction of the force acting on the conductor. 5. A uniform magnetic field with an induction of 0.25 T acts on a conductor located in it with a force of 2N. Determine the length if the current in it is 5 A. 6. The current in the lighting wires changes over time according to the graph presented on Determine the amplitude, period and frequency of oscillations. magnetic conductor, flow pattern. 7. The radar pulse reflected from the target returned 0.8 ∙ 10-6 s after emission by the radar. What is the distance from the locator to the target? 8. The radio station “Europe – Plus” broadcasts on a frequency of 106.2 MHz. Find the length of the emitted electromagnetic wave. Test No. 5 “Structure of the atom and atomic nucleus” All tests are designed to test the skills and abilities that need to be developed in accordance with the “Basic Standard general education in physics." All works contain 2 options and are aimed at the average student. If there are students in the class who study physics intensively, they can be given a task increased difficulty and evaluate it separately.
948. The ball is positively charged. The student touched it with his finger. How did the ball's charge change?
The charge will go into the ground through the student’s body.
949. A metal sphere has a charge of -1.6 nC. How many excess electrons are there on the sphere?
950. After the glass rod was rubbed, its charge became 3.2 µC. How many electrons were removed from the stick by friction?
951. There are 4.8 1010 excess electrons on a metal ball. What is its charge?
952. The electroscope was charged to -3.2 10-10 C. How many excess electrons are there in the electroscope?
953. Is it possible to electrify a piece of metal? What conditions are necessary for this?
It is possible by influencing it with an electric field.
954. With mutual friction, both bodies are electrified, but with charges opposite sign. What experience can demonstrate this?
If you rub dry cloth on an ebonite stick, the stick is attracted to the cloth.
955. Two identical cork balls are suspended on thin silk threads, one charged, the other uncharged. How to determine which ball is charged?
Bring an electrified ebonite stick to the balls. The charged ball will be attracted or repelled from it.
956. Two charges of different sizes are located at a certain distance from each other. A third charge of the same sign is placed between them, which remains in equilibrium. Which of the two charges is the third closest to?
The third charge is closer to the smaller charge, i.e. from more he is repelled more strongly.
957. How can we explain that a light cork ball is first attracted to an electrified stick and then repelled from it?
A ball in an electrostatic field is polarized. An opposite charge is concentrated on the surface and the ball is attracted to the stick. After contact, part of the charge transfers to the ball, which receives a charge of the same sign and is repelled from the stick.
958. Between two horizontal oppositely charged plates an uncharged drop of water hangs in the air (Fig. 88). Why doesn't the drop fall down?
An electrostatic force acts on the drop, opposite to the direction of gravity.
959. Electronic theory states that only electrons - negative charges - can move freely in metal conductors. Then how can we explain that a metal object can be positively charged?
The positive charge can be explained by a lack of electrons.
960. Figure 89 shows two oppositely charged bodies A and B. A light, positively charged ball a was placed near body A. What will happen to ball a? Draw a curve along which ball a will move.
961. Why is an uncharged elderberry bead attracted to both a positively and negatively charged bead?
A charge opposite in sign to the charged ball is concentrated on the uncharged ball and the ball is attracted to it.
962. Why, when holding it in your hand, can you electrify a plastic comb by friction, but not a metal comb?
Because plastic is a dielectric and metal is a conductor.
963. Why is it impossible to electrify a metal rod by friction, even if you touch a charged body with this rod?
Because the charge will immediately go through the body into the ground.
964. If a body with an opposite charge is brought to the charged ball of an electroscope without touching the ball, the leaves of the electroscope will move closer together. Why?
Part of the charge from the leaves will transfer to the electroscope ball under the influence of electrostatic forces.
965. If you touch the ball of a charged electroscope with your hand, the electroscope discharges. Why?
Charge through the body person will pass into the ground.
966. When a hand is brought to the ball of a charged electroscope without touching the ball, the leaves of the electroscope come closer. Why?
A charge is concentrated on the hand, opposite in sign to the charge of the electroscope, and part of the charge from the leaves passes into the ball.
967. A positively charged rod is brought to the ball of an uncharged electroscope (without touching the ball). What charge is obtained on the leaves of the electroscope?
A negative charge will appear on the surface of the ball, and a positive charge on the leaves.
968. Balls A and B are oppositely charged. Placed between them is a positively charged small ball moves towards body B. Which of the balls is positively charged?
Ball B is negatively charged, ball A is positively charged.
969. Why is the rod of an electroscope made of metal?
So that the charge from the ball is transferred to the leaves.
970. In order for the electroscope to more accurately show the amount of charge, it is grounded - it is connected outer surface with the ground (Fig. 90). Why is this being done?
So that there is no charge on the body of the electroscope.
972. Why does an electrified glass rod attract light objects: pieces of paper, corks, elderberry balls, etc.?
Opposite charges are concentrated on the surface of these bodies and will attract each other.
973. How to determine the sign of a body’s charge using an electroscope?
Touch the electroscope ball with a charged body, then bring the charged body whose charge sign is known. If the leaves go down, the charges are different.
974. Why is it difficult, and sometimes almost impossible, to charge an electroscope at high air humidity?
The charge leaves the electroscope through moisture particles.
975. It is known that if a charged metal ball is touched uncharged, then after separation both balls turn out to be charged. However, when a charged ball is connected to the ground, it is almost completely discharged. Why?
Most of the charge goes to bigger body. The size of the Earth is incommensurably larger than any body located on it.
976. Why are excess charges in conductors located only on the surface?
The electrons repel each other and are distributed so that the field strength inside is minimal.
977. A negatively charged body is brought to the ball of an uncharged electroscope (without touching it). Determine the signs of the charges on the ball and on the leaves of the electroscope.
A positive charge appears on the ball, and a negative charge on the leaves.
978. Two uncharged electroscopes are connected to each other by a metal wire (Fig. 91). A positively charged stick was brought to the ball of one (without touching it). What charges will be on the balls and leaves of each electroscope?
On the right electroscope: on the ball “-” on the leaves “+”; on the left electroscope: on the ball “+” on the leaves “-”.
979. The charged stick was removed from the electroscopes previous task. What happened to the leaves of both electroscopes?
The leaves will fall.
980. What needs to be done to ensure that electroscopes (see Fig. 91) remain charged after removing the rod?
Cut the metal wire.
981. If the electroscopes (see Fig. 91) remained charged after the rod was removed, then what sign of the charges will appear on the balls and leaves of each electroscope?
Left is negative, right is positive.
982. Answer the questions in problems 969-972 for the case when an ebonite rod rubbed on fur is brought to the electroscope.
The stick will have a negative charge. All charges will change to positive.
983. To electrify an electroscope positively, a negatively electrified rod is brought closer to the ball. Then, without removing the sticks, touch the ball with your hand for a moment. After this, the stick is removed and the electroscope is charged.
Do this experiment and explain it.
The negative charge of the stick will change the positive charge from the hand to the ball.
984. Charge the electroscope negatively in the same way. What charge and what rod should be electrified for this and brought to the electroscope? Explain this process based on electron theory.
You need to bring a positively charged stick to it, then touch the ball with your hand for a moment. The positive charge of the stick will transfer the negative charge from the hand to the ball.
985. An insulated metal cylinder is connected to an electroscope. The presence of what charges will be shown by the electroscope in the following cases:
a) a positively charged ball is introduced into the cylinder without being in contact with it;
b) the charged ball is touched to the inner surface of the cylinder;
c) the ball is inserted inside the cylinder (without touching it), then they touch the cylinder with their hand, remove their hand and remove the ball from the cylinder?
A) positive
B) positive if the ball is positively charged.
B) negative if the ball was positively charged.
986. In what cases can a lightning rod pose a danger to a building?
If the lightning rod is not grounded.
Option 1.
1. When rubbed against silk, the glass charges...
2. If an electrified body is repelled by an ebonite stick rubbed on fur, then it...
A. has no charge.
B. is positively charged.
47428153111500B. negatively charged.
3. The picture shows light balls suspended on silk threads. Which of the figures corresponds to the case when the balls have the same charges?
446722514795500A. 1.B. 2.
4. A stick rubbed on the fur (rice) is brought to the ball. What is the sign of the charge on the ball?
A. Positive.B. Negative.
427672564135005. How will metal body A be charged if a charged body B is brought to it (fig)?
A. Positive.
B. Negative.
B. Neutral.
42481508890006. Which rod - glass, ebonite or steel - should be used to connect the electroscopes so that they are both charged (fig)?
A. Glass. B. Ebonitov.V. Steel.
7. A copper rod that had a positive charge was discharged and it became electrically neutral. Will the mass of the rod change?
A. Will not change. B. Will increase.V. Will decrease.
8. Which particle has the least negative electrical charge?
A. Electron.B. Neutron.V. Proton.
46101006985009. The figure shows a diagram of a lithium atom. Is this atom charged?
4581525152400010. Which chemical element is shown schematically in the figure?
Test work in 8th grade physics. Topic: Electrification of bodies. The structure of the atom.
Option 2.
1. When an ebonite stick is rubbed against fur, it charges...
A. positive.B. negative.
2. If an electrified body is attracted to a glass rod rubbed on silk, then it...
A. positively charged.
B. is negatively charged.
V. has no charge.
45319958890003. The picture shows balls suspended on silk threads. Which picture shows balls charged with opposite charges?
A. 1.B. 2.
450850012700004. A glass rod rubbed on silk is brought to the elderberry ball (fig). What is the sign of the charge on the ball?
A. Negative.B. Positive.
right5080005. Which of the charged bodies is affected by the charged ball (fig) with less force?
A. 1.B. 2.V. 3.
6. What kind of rod - copper, ebonite or steel - are the electroscopes connected to (fig)?
4958715139700046132751841500A. Copper.B. Ebonitov.V. Steel.
7. An iron ball that had a negative charge was discharged, and it became electrically neutral. Will the mass of the ball change?
A. Will not change. B. Will increase.V. Will decrease.
8. What particles make up the nucleus of an atom?
A. Electrons and protons.
B. Neutrons and protons.
B. Electrons and neutrons.
460819512319000
9. The figure shows a diagram of a hydrogen atom. Is this atom charged?
A. The atom is negatively charged.
B. The atom is positively charged.
B. The atom is electrically neutral.
437007088900010. Which chemical element is shown schematically in the figure?
A. Hydrogen. B. Lithium.V. Helium.
Attached files
The word electricity comes from Greek name amber - ελεκτρον
.
Amber is a fossilized resin coniferous trees. The ancients noticed that if you rub amber with a piece of cloth, it will attract light objects or dust. This phenomenon, which we today call static electricity, can be observed by rubbing an ebonite or glass rod or simply a plastic ruler with a cloth.
A plastic ruler, which has been thoroughly rubbed with a paper napkin, attracts small pieces of paper (Fig. 22.1). You may have seen discharges of static electricity while combing your hair or taking off your nylon blouse or shirt. You may have experienced an electrical shock when you touched a metal door handle after standing up from a car seat or walking on synthetic carpet. In all these cases, the object acquires an electrical charge through friction; they say that electrification occurs by friction.
Are all electric charges the same or are there different types? It turns out that there are two types of electric charges, which can be proven as follows: simple experience. Hang a plastic ruler by the middle on a thread and rub it thoroughly with a piece of cloth. If we now bring another electrified ruler to it, we will find that the rulers repel each other (Fig. 22.2, a).
In the same way, bringing another electrified glass rod to one, we will observe their repulsion (Fig. 22.2,6). If a charged glass rod is brought to an electrified plastic ruler, they will be attracted (Fig. 22.2, c). The ruler appears to have a different kind of charge than the glass rod.
It has been experimentally established that all charged objects are divided into two categories: either they are attracted by plastic and repelled by glass, or, conversely, repelled by plastic and attracted by glass. There appear to be two types of charges, charges of the same type repel each other, and charges different types are attracted. We say that like charges repel, and unlike charges attract.
American statesman, philosopher and scientist Benjamin Franklin (1706-1790) called these two types of charges positive and negative. It made absolutely no difference what charge to call;
Franklin proposed that the charge of an electrified glass rod be considered positive. In this case, the charge appearing on the plastic ruler (or amber) will be negative. This agreement is still followed today.
Franklin's theory of electricity was in effect a "one fluid" concept: a positive charge was seen as an excess of the "electrical fluid" over its normal content in a given object, and a negative charge as its deficiency. Franklin argued that when, as a result of some process, a certain charge arises in one body, the same amount of charge of the opposite kind simultaneously arises in another body. The names “positive” and “negative” should therefore be understood in an algebraic sense, so that the total charge acquired by bodies in any process is always equal to zero.
For example, when a plastic ruler is rubbed with a paper napkin, the ruler acquires a negative charge, and the napkin acquires an equal positive charge. There is a separation of charges, but their sum is zero.
This example illustrates the firmly established conservation law electric charge
, which reads:
The total electric charge resulting from any process is zero.
Deviations from this law have never been observed, therefore we can consider that it is as firmly established as the laws of conservation of energy and momentum.
Electric charges in atoms
Only in the last century did it become clear that the reason for the existence of electric charge lies in the atoms themselves. Later we will discuss the structure of the atom and the development of ideas about it in more detail. Here we will briefly discuss the main ideas that will help us better understand the nature of electricity.
By modern ideas an atom (somewhat simplified) consists of a heavy, positively charged nucleus surrounded by one or more negatively charged electrons.
IN in good condition the positive and negative charges in an atom are equal in magnitude, and the atom as a whole is electrically neutral. However, an atom can lose or gain one or more electrons. Then its charge will be positive or negative, and such an atom is called an ion.
In a solid, nuclei can vibrate, remaining near fixed positions, while some electrons move completely freely. Electrification by friction can be explained by the fact that in various substances Nuclei hold electrons with different strengths.
When a plastic ruler that is rubbed with a paper napkin acquires a negative charge, this means that the electrons in the paper napkin are held less tightly than in the plastic, and some of them transfer from the napkin to the ruler. The positive charge of the napkin is equal in magnitude to the negative charge acquired by the ruler.
Typically, objects electrified by friction only hold a charge for a while and eventually return to an electrically neutral state. Where does the charge go? It “drains” onto the water molecules contained in the air.
The fact is that water molecules are polar: although in general they are electrically neutral, the charge in them is not uniformly distributed (Fig. 22.3). Therefore, excess electrons from the electrified ruler will “drain” into the air, being attracted to the positively charged region of the water molecule.
On the other hand, the positive charge of the object will be neutralized by electrons, which are weakly held by water molecules in the air. In dry weather, the influence of static electricity is much more noticeable: there are fewer water molecules in the air and the charge does not flow off as quickly. In damp, rainy weather, the item is unable to hold its charge for long.
Insulators and conductors
Let there be two metal balls, one of which is highly charged and the other is electrically neutral. If we connect them with, say, an iron nail, the uncharged ball will quickly acquire an electric charge. If we simultaneously touch both balls with a wooden stick or a piece of rubber, then the ball, which had no charge, will remain uncharged. Substances such as iron are called conductors of electricity; wood and rubber are called non-conductors, or insulators.
Metals are generally good conductors; Most other substances are insulators (however, insulators conduct electricity a little). It's interesting that almost all natural materials fall into one of these two sharply different categories.
There are, however, substances (among which silicon, germanium and carbon should be mentioned) that belong to an intermediate (but also sharply separated) category. They are called semiconductors.
From the point of view atomic theory electrons in insulators are bound to the nuclei very tightly, while in conductors many electrons are bound very loosely and can move freely within the substance.
When a positively charged object is brought close to or touches a conductor, free electrons quickly move towards a positive charge. If an object is negatively charged, then electrons, on the contrary, tend to move away from it. In semiconductors there are very few free electrons, and in insulators they are practically absent.
Induced charge. Electroscope
Let's bring a positively charged metal object to another (neutral) metal object.
Upon contact, free electrons of a neutral object will be attracted to a positively charged one and some of them will transfer to it. Since the second object now lacks a certain number of negatively charged electrons, it acquires a positive charge. This process is called electrification due to electrical conductivity.
Let us now bring the positively charged object closer to the neutral metal rod, but so that they do not touch. Although the electrons will not leave the metal rod, they will nevertheless move towards the charged object; a positive charge will arise at the opposite end of the rod (Fig. 22.4). In this case, it is said that a charge is induced (or induced) at the ends of the metal rod. Of course, no new charges arise: the charges simply separated, but on the whole the rod remained electrically neutral. However, if we were now to cut the rod crosswise in the middle, we would get two charged objects - one with a negative charge, the other with a positive charge.
You can also impart a charge to a metal object by connecting it with a wire to the ground (or, for example, to a water pipe going into the ground), as shown in Fig. 22.5, a. The subject is said to be grounded. Due to its enormous size, the earth accepts and gives up electrons; it acts as a charge reservoir. If you bring a charged, say, negatively, object close to the metal, then the free electrons of the metal will be repelled and many will go along the wire into the ground (Fig. 22.5,6). The metal will be positively charged. If you now disconnect the wire, a positive induced charge will remain on the metal. But if you do this after the negatively charged object is removed from the metal, then all the electrons will have time to return back and the metal will remain electrically neutral.
An electroscope (or simple electrometer) is used to detect electrical charge.
As can be seen from Fig. 22.6, it consists of a body, inside of which there are two movable leaves, often made of gold. (Sometimes only one leaf is made movable.) The leaves are mounted on a metal rod, which is insulated from the body and ends on the outside with a metal ball. If you bring a charged object close to the ball, a separation of charges occurs in the rod (Fig. 22.7, a), the leaves turn out to be similarly charged and repel each other, as shown in the figure.
You can completely charge the rod due to electrical conductivity (Fig. 22.7, b). In any case, than more charge, the more the leaves diverge.
Note, however, that the sign of the charge cannot be determined in this way: a negative charge will separate the leaves exactly the same distance as an equal positive charge. And yet, an electroscope can be used to determine the sign of the charge; for this, the rod must first be given, say, a negative charge (Fig. 22.8, a). If you now bring a negatively charged object to the electroscope ball (Fig. 22.8,6), then additional electrons will move to the leaves and they will move apart further. On the contrary, if a positive charge is brought to the ball, then the electrons will move away from the leaves and they will come closer (Fig. 22.8, c), since their negative charge will decrease.
The electroscope was widely used at the dawn of electrical engineering. Very sensitive modern electrometers operate on the same principle when using electronic circuits.
This publication is based on materials from the book by D. Giancoli. "Physics in two volumes" 1984 Volume 2.
To be continued. Briefly about the following publication:
Strength F, with which one charged body acts on another charged body, is proportional to the product of their charges Q 1 and Q 2 and inversely proportional to the square of the distance r between them.
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