The contest of electric power and ohm's law in the second year of physics (plus 50 points)

Circuit ohm's law competition exercise

1, the circuit shown in figure 1, when the key is closed, both bulbs are not lit, and the pointers of ammeter and voltmeter hardly move. When it is found that the terminals in the circuit are in good contact with the wires, the positions of the key S and the ammeter are reversed, and the positions of the two bulbs L 1 and L2 are also reversed. Close the key again, and find that the two bulbs are still off, the ammeter pointer is still almost motionless, but the voltmeter pointer has obvious deflection. According to the above phenomenon, try to analyze which circuit devices may fail.

Xiaoming is a photographer. He wants to make a small exposure box with batteries and light bulbs on the car to develop black and white photos. According to the requirements, there should be two light bulbs in exposure box, one of which is painted red, and there are two switches outside exposure box. When S 1 is turned off, only the red light L 1 glows. At this time, under the irradiation of red light, the position of negative film and photographic paper can be adjusted. When S2 is turned off after adjustment, L 1 and L2 both emit light normally, which makes the photographic paper sensitive. In order to prevent L2 from emitting light due to operation error before putting the photographic paper away, the circuit design requires that L2 will not emit light before S 1 is turned off and S2 is turned on. According to the above requirements, please draw the internal circuit diagram of exposure box, and connect the physical diagram shown in Figure 2 to the corresponding working circuit.

3. In the closed part of the control circuit, there are three terminals A, B and C, a light bulb and an electric bell, as shown in Figure 3. When connecting A and C with wires, the light does not ring; When connecting a and b, the bell light does not light up; When connecting B and C, the light doesn't work and the bell doesn't ring. Please draw the circuit diagram of this component according to the above situation.

4. One of the common sense of safe electricity use is not to get close to high-voltage electricity. The bird standing on the high-voltage line has no electric shock accident. This is mainly because

Bird's paw is a good insulator. B bird's feet are very dry.

C. There is insulation protection material on wire D, and the voltage between bird feet is very low.

Xiaoming wants to make a toy desk lamp with a 3.8V bulb. The power supply is the power supply of a small tape recorder, but the output voltage of this power supply is 6V, so he finds a physics teacher. After the experiment, the physics teacher told Xiao Ming that the resistance of this light bulb was about 13 ohm, and gave him a resistance wire per cm 1 ohm, so that he could figure out a solution to the problem himself. Do you know how to solve this problem?

6. There are four small light bulbs A, B, C and D, among which A and B have the same specifications, both of which are "6.0V and 0.3A"; The specifications of C and D are the same, both of which are "6.0V0. 18A". In order to connect them to the storage battery with the voltage of 12 volts, Xiao Gang designed several circuits as shown in Figure 4 according to the working characteristics of series-parallel circuits, and tried to analyze which one is the ideal circuit.

7. As shown in Figure 5, Figure (a) is the physical schematic diagram of the single-pole double-throw switch, and its symbol is shown in Figure (b). Figure (c) is an example of the control function of the single-pole double-throw switch pair circuit. When S touches "L", the light bulb L 1 emits light, and when S touches "2", the light bulb L2 emits light.

At present, there are the following equipment: a power supply (the voltage at both poles is constant, but the electrical value is unknown), a resistance box (denoted by R0), a resistance to be measured (denoted by RX), a voltmeter (the maximum range is greater than the voltage at both poles of the power supply), a single-pole double-throw switch and several wires.

(1) According to the equipment given above, design a standard circuit diagram that can measure and calculate the resistance Rx to be measured (it cannot be disassembled during the experiment after connection);

(2) Write out the operation steps of measurement and the measured physical quantities;

(3) Write an expression for calculating Rx with measured physical quantities.

8. Xiaoming's family bought a new 100 watt electric blanket. He wanted to measure the resistance of the electric blanket by voltammetry, so he connected the experimental circuit as shown in Figure 6. The power supply voltage is 6V, Rx is the resistance of the electric blanket, and the maximum resistance of the sliding rheostat is 20 ohms. However, during the experiment, Xiao Ming found that no matter how to adjust the position of the contact P of the sliding rheostat, the indication of the voltmeter hardly changed, and he found that the circuit was connected correctly everywhere. Could you please analyze the reasons for this phenomenon? If the experimental equipment is not changed, how should the circuit be connected to effectively change the voltage value at both ends of the electric blanket during the sliding process of the contact P?

9. The distance between A and B is L, during which a telephone line consisting of two wires is laid. The resistance per unit length of a single wire is R (the unit of R is ohm/meter), and the terminal of B is connected with a load resistance. If two wires are connected due to insulation damage in the middle of the line, in order to find out the connection position, I will give you a DC stabilized power supply with voltage of U and an ammeter with appropriate range. How to find the distance from the damaged end to A and the contact resistance of the damaged end?

10, as shown in fig. 7, there are two resistors in the electrical black box (as shown in the thick box in the figure), one is the constant resistance R0 of 8 ohms, and the other is the unknown resistance Rx. Lead out three lines A, B and C from the box, and measure the value of Rx with the circuit as shown in the figure. When switches S and C are turned on, the current indicator is 1 A and the voltage indicator is 8V; When S and D are connected, the ammeter reading is still 1 A, and the voltage reading becomes 10V.

(1) Draw the connection diagram of two resistors and three wires in the black box.

(2) Find the resistance of Rx.

1 1, as shown in Figure 8, is a sealed box detached from the electronic instrument. It is known that there are three resistors in the box, and A, B, C and D are four leads. At present, many meters are used to measure, and it is known that the resistance RAC between "AC" is 20 ohms, the resistance RCD between CD is 50 ohms, and the resistance Rad between ad is 30 ohms. If BD is connected by wires, the resistance between AD is measured as RAD=20 ohms. Please draw the connection diagram of resistors in the box and mark the resistance value of each resistor in the diagram.

12. There is a small household colored light string, which consists of 24 small light bulbs connected in series to a 220v power supply L. In use, because one small light bulb is burnt out, all the small light bulbs are extinguished. Because the colored light is painted, it is impossible to tell which small light bulb has a burnt filament. Now I give you a voltmeter, how do you find the fault? If you only have a small light bulb of the same specification, how can you find the fault?

13 Circuit 9. When the sliding head P of the sliding rheostat moves to the right, the indication of the voltmeter _ _ _ _ _ _ the indication of the ammeter _ _ _.

14. For the circuit shown in figure 10, the power supply voltage is constant, U=9 volts, the total resistance of the sliding rheostat R 1 is 10 ohms, and the fixed resistance R2 is 20 ohms. When the slider P of the sliding rheostat moves, the voltmeter can obtain the range from _ _ _ volts to _ _ _ volts. The ratio of the maximum current to the minimum current on the fixed-value resistor is _ _ _ _ _ _.

15, R 1=40 ohms, R2=60 ohms, connected to a power supply with a voltage of u, then the current ratio through R 1 and R2 is _ _ _ _ R 1, a R2 is connected in parallel and a resistor R3 is connected in series, and this circuit is still connected with a voltage of u.

16. In the circuit shown in figure 1 1, the power supply voltage is constant. When the resistance R= 1 ohm, I1:I2 = _ _ _ _ _ _ _; When the resistance R=3 ohms, I1:I2 = _ _ _ _.

17. In the circuit shown in figure 12, R 1=20 ohms, R2= 18 ohms, and the power supply voltage U=5 volts is constant. In order to make the voltage at both ends of R 1 only change arbitrarily between 0 and 2 volts, the maximum resistance value of R3 should be _ _ _ _ _ _.

18. As shown in figure 13, if the circuit Uab= 4.4 volts is constant, R 1= R2= R3= R4=0.5 ohm, and RL 1= RL2=2 ohm, then the voltage across the bulb L 1 is U.

19. The length of the uniform straight resistance wire BD is L, and the two ends of BD are connected to the two poles of the power supply respectively, and the current passing through point D is i. Turn back the B end at point A to weld point B and point C together, as shown in figure 14. Connect the two ends of the AD to the two ends of the power supply (constant voltage). AB is _ _ _ _ times the original length l, so the current passing through point D can be1.5i.

20. Figure 15 shows a part of the circuit. Resistors R 1 and R2 indicate resistance values of 30 ohms and 60 ohms respectively, and the mark on R3 is ambiguous. In order to identify the resistance of R3, an ammeter and a voltmeter are connected between A and C and between A and D respectively, and the readings on the meter are 50 mA and 2.7 volts respectively. Therefore, R3 = _ _ _ Ou.

2 1. In the circuit shown in Figure 16, it is known that the power supply voltage connected at both ends of AB is 7.5 volts, R 1=2 ohms, R3=5 ohms, the indication of voltmeter U 1 is 2.5 volts, and the indication of U 1 is 4.0 volts. Then R2 should be equal to _ _ _ _ _ _ _, and R4 should be equal to _ _ _ _ _ _.

22. In the circuit shown in Figure 17, the voltage between AB is constant, R 1=R2=R3, and when the switch S is off, the voltmeter reads U; When s is closed, the voltmeter reads _ _ _ _ _ _ u.

23. As shown in figure 18, the power supply voltage is constant, with r1=10Ω, R2 = 20ω and R3 = 30Ω. When the switch S 1 S2 is opened and closed at the same time, the ratio of indicator numbers of the ammeter is 1: 2, and the resistance of R is _ _ _ _ _ _ _ _ _.

24. As shown in figure 19, it is known that there is a circuit consisting of a power supply with a voltage of 5 volts and a resistor with the same resistance value in the box. There are four terminals outside the box. Measure the voltage with voltmeter, u12 = 5v, U34 = 3v, u13 = 2v, U42 = 0v. Draw the circuit in.

25. In electrical experiments, when an open circuit is encountered, a voltmeter is usually used to detect it. A classmate connects the circuit according to Figure 20, and the power supply is intact. After the switch is closed, the light goes out and the ammeter has no indication. At this point, the voltage measured by voltmeter between B and C and between A and D is zero, indicating that it may be ().

Poor contact of power supply terminal; B. The contact piece or terminal of the light switch is in poor contact;

C. poor contact of ammeter terminal; D bulb filament is broken or has poor contact.

26. As shown in Figure 2 1, the power supply voltage remains unchanged. When the slider P of the sliding rheostat slides to the B end, the following statement is correct ().

A A 1 decreases, A2 increases, V 1 remains unchanged, and V2 decreases; B A 1 increases, A2 decreases, V 1 decreases, and V2 remains unchanged;

C A 1 unchanged, A2 unchanged, V 1 unchanged, V2 increased; D A 1 increases, A2 increases, V 1 remains unchanged, and V2 increases.

27? In the four circuits shown in Figure 22, if the voltmeter reading remains unchanged. When the switch S is closed, which circuit has a larger ammeter reading?

28. As shown in the circuit of Figure 23, the supply voltage remains unchanged. When the switch S is closed, the ratio of ammeter A 1 to A2 is 8: 5. If the resistor R3 is well connected between the point A and the point B, after the switch S is closed, the ratio of the ammeter A 1 to the indicator of A2 is 3: 2. Compare the size of R 1 and R3:

A r1> R3; b r 1 < R3； c r 1 = R3； D condition is not enough to judge.

29. in the circuit shown in fig. 24, l is a wire connected in parallel with R2, and the correct one in the following statement is ().

A the current I through R 1 and R2 is equal, and I=U/(R 1+ R2).

The voltage on b r1u1= I r1; +0; The voltage U2 on U2 = I R2, and the current in the conductor is zero;

The voltage on c 1 is U 1=U, and the voltage on R2 is zero;

The current in d is I 1 r 1 = U/r 1, the current in conductor l is equal to I 1, and the current in R2 is zero;

30. In order to regulate the current in the circuit of precision electronic instrument, two sliding rheostats are used in the regulation part, as shown in Figure 25. It is known that these two sliding varistors are made of different resistance wires wound on the same insulating porcelain tube, where RL; The total resistance of is 200Ω, and the total resistance of R2 is 5000Ω. At the beginning, both varistors are at maximum resistance. The following methods can make the ammeter pointer point to the required position quickly and accurately ().

A. first adjust Rl to make the ammeter pointer point to the required position, and then adjust R2.

B first, adjust R2 to make the ammeter pointer point near the required position, and then adjust Rl.

C adjust Rl and R2 at the same time, so that the ammeter pointer points to the required position.

D, alternately and repeatedly adjust Rl and R2, so that the ammeter pointer points to the required position.

3 1. Equivalent substitution method is a common method in physical experiments. For example, a 7 Ω resistor is used to replace the series resistors of 2 Ω and 5 Ω in a branch, and the current in the branch remains unchanged under other conditions, indicating that a 7 Ω resistor is equivalent to the series resistors of 2 Ω and 5 Ω. Therefore, 7 Ω resistor can be used instead of 2 Ω and 5 Ω series resistor. In the experiment of measuring unknown resistance with the circuit shown in Figure 26, the equivalent substitution method is adopted, where RX is the resistance to be measured (the resistance is about several hundred ohms), R is the sliding rheostat, and R0 is the resistance box (the maximum resistance of the resistance box is greater than Rx).

(1) Please fill in the blanks in the following main experimental steps according to the experimental circuit diagram.

① Connect the circuit according to the circuit diagram and adjust the resistance of the resistance box R0 to the maximum.

② Before the switch S 1 is closed, the slider P is located at the _ _ _ _ _ _ _ _ end. (Select "A" or "B")

③ Close the switch S 1.

(4) Close the switch _ _ _ _ _ _ _, adjust the slider P, so that the ammeter pointer is in the proper position, and write down the ammeter pointer I at this time.

⑤ Turn off the switch _ _ _ _ _ _ _ first, then turn off the switch _ _ _ _ _ _, keep the resistance of _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

(2) In this experiment, if the ammeter is changed to voltmeter, and other equipment remains unchanged, draw the experimental circuit diagram of measuring RX resistance by equivalent substitution method (it is required that the designed circuit can only change the resistance connection through the switch after being turned on).

The picture is in the resources.