Find some physics problems suitable for eighth grade students. A little more difficult. Competition questions are also ok. Better have an answer. Please send it to xn98514@126.com.

when measuring current and voltage with ammeter and voltmeter, the influence of the resistance of the instrument itself on the circuit to be measured is usually not considered, and it is regarded as an "ideal instrument", that is, the internal resistance of ammeter is zero and that of voltmeter is infinite. In fact, both ammeter and voltmeter have certain resistance values, so we can regard ammeter as a resistor with small resistance that can display the current passing through itself. The voltmeter can be regarded as a resistor with a large resistance that can display the voltage across itself. Therefore, when an ammeter or voltmeter is connected to the circuit under test, it is equivalent to connecting a resistor in series or connecting a resistor in parallel in the circuit under test. In this way, there must be a certain voltage across the ammeter and a certain current through the voltmeter, so it is difficult to accurately measure the voltage across the resistor to be measured and the current through the resistor to be measured at the same time.

1. there is an ammeter with a measuring range of .6A and an internal resistance of .13 ohm. can it be used to measure the voltage across a circuit? If not, please explain the reason. If yes, please explain the maximum voltage that can be measured with it.

2. To accurately measure a resistor Rx with a resistance of about 8 ohms, the laboratory provides the following equipment.

ammeter A1: measuring range 1mA, internal resistance r1=4 Ω

ammeter A2: measuring range 5μA, internal resistance r2=75 Ω

voltmeter v: measuring range 1V, internal resistance R3 = 1kΩ,

rheostat R: resistance about 1 Ω

fixed resistance R1: resistance about 1kΩ

.

what physical quantities need to be measured? According to the measured physical quantity, write the expression of the measured resistance Rx.

Circuit Ohm's Law Competition Exercise

1. As shown in Figure 1, when the electric key is closed, the two bulbs are not lit, and the pointers of the ammeter and voltmeter are almost motionless. In the case of finding out that the terminals in the circuit are in good contact with the wires, the positions of the key S and the ammeter are now reversed, and the positions of the two bulbs L1 and L2 are also reversed. When the key is closed again, it is found that the two bulbs are still not lit, and 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.

2. Xiao Ming is a photography enthusiast. He wants to make a small exposure box for developing black and white photos by using the battery and lighting bulbs in his car. 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 S1 is closed, only the red light L1 emits light. At this time, the position of the negative film and photographic paper can be adjusted under the red light illumination. When S2 is closed after adjustment, L1 and L2 both emit light normally, thus making 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 does not emit light when S2 is turned on before S1 is closed. According to the above requirements, please draw the internal circuit diagram of exposure box, and connect the physical diagram shown in Figure 2 into the corresponding working circuit.

3. On a closed part of a 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 lights will not ring; When connecting A and B, the bell light does not light up; When connecting B and C, the light does not light and the bell does not ring. Please draw the circuit diagram of this component according to the above situation.

4. One of the common sense of safe use of electricity is that birds with their feet standing on high-voltage lines do not get an electric shock accident. This is mainly because

A. Bird's claws are good insulators. B. Bird's feet are very dry.

C. There is an insulating protective material on the wire. D. The voltage between bird's feet is very low.

5. Xiaoming wants to use a 3.8 volt. After the experiment, the physics teacher told Xiao Ming that the resistance of this light bulb was about 13 ohms, and at the same time gave him a resistance wire of 1 ohm per cm, asking him to come up with a solution to the problem himself. Do you know what to do about this problem?

6. There are four small bulbs A, B, C and D, of which A and B have the same specifications, both of which are "6．V and .3A"; The specifications of C and D are the same, both of which are "6.V.18a". To connect them to a battery with a 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 design is more ideal.

7. as shown in fig. 5, fig. (a) is a physical diagram of a single-pole double-throw switch, and its symbol is shown in fig. (b), and fig. (c) is an example of the control function of the single-pole double-throw switch on the circuit. When S contacts with "L", the bulb L1 emits light, and when S contacts with "2", the bulb L2 emits light.

there are the following equipment: a power supply (the voltage between the two poles remains unchanged, but the electric value is unknown), a resistance box (denoted by R), a resistance to be measured (denoted by RX), a voltmeter (the maximum range is greater than the voltage between the two 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 can't 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. Xiao Ming's family bought a new 1-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 value of the electric blanket, and the maximum resistance value of the sliding rheostat is 2Ohm. 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 the circuit was found to be connected correctly everywhere. Would you please analyze the reasons for this phenomenon? If the experimental equipment is not replaced, how should the circuit be connected in order 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 of a single wire per unit length is r(r is in 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, give you a DC regulated power supply with voltage of U and an ammeter with appropriate range. How to find out the distance and damage from the damaged part to A terminal?

1. There are two resistors in the electrical black box as shown in Figure 7 (shown by the bold box in the figure), one is the constant resistance R of 8 ohms, and the other is the unknown resistance Rx. Three wires A, B and C are led out from the box, and the value of Rx is measured by the circuit as shown in the figure. When the switches S and C are connected, the current indicator is 1 A and the voltage indicator is 8 V; When S and D are connected, the ammeter still shows 1 ampere, and the voltage representation becomes 1 volts.

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

(2) find the resistance of Rx.

11. As shown in Figure 8, the sealed box is detached from an 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 2 ohms, the resistance RCD between CD is 5 ohms, and the resistance Rad between AD is 3 ohms. If BD is connected with wires, the resistance between AD is measured as RAD=2 ohms. Please draw the connection diagram of resistors in the square box, and mark the resistance value of each resistor in the diagram.

12. There is a small colored light string for home use, which consists of 24 small light bulbs connected in series to a 22v power supply L. In use, all the small bulbs are extinguished because a small bulb is burnt out, and it is impossible to see which small bulb's filament is burnt out because the colored lights are painted. Now 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?

in the circuit shown in 139, when the sliding head p of the sliding rheostat moves to the right, the indicator of the voltmeter _ _ _ _ _ _ _ the indicator of the ammeter _ _ _ _.

14. as shown in figure 1, the power supply voltage is constant, U=9 volts, the total resistance of sliding rheostat R1 is 1 ohms, and the fixed resistance R2 is 2 ohms. When the slider P of the sliding rheostat moves, the voltmeter can obtain the range of _ _ _ _ volts to _ _ _ _ volts. The ratio of the maximum current to the minimum current on the fixed value resistor is _ _ _ _ _ _.

15, R1=4 ohms, R2=6 ohms, and connected to a power supply with a voltage of u, the ratio of currents passing through R1 and R2 is _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

16. in the circuit shown in fig. 11, the power supply voltage is constant, and when the resistance R=1 ohm, i1: I2 = _ _ _ _ _ _; When the resistance R=3 ohms, I1: I2 = _ _ _ _ _.

17. in the circuit shown in fig. 12, R1=2 ohms, R2=18 ohms, and the power supply voltage U=5 volts remains unchanged. In order to make the voltage across R1 only change arbitrarily between and 2 volts, the maximum resistance value of R3 should be _ _ _ _ _ _.

18. As shown in Figure 13, the circuit Uab= 4.4 volts and remains unchanged, R1= R2= R3= R4=.5 ohm, RL1= RL2=2 ohm, then the voltage at both ends of the light bulb L1 = _ _ _ _ _ _ _, and the voltage at both ends of the light bulb L2 = _ _ _ _ _ _

19. Turn back the B end at point A, so that point B and point C are welded together, as shown in Figure 14. Connect both ends of AD to the two poles of power supply (constant voltage). AB is _ _ _ _ _ times the original length L, so that the current passing through point D can be 1.5I.

2. Figure 15 shows a part of a certain circuit. The resistances indicated by resistors R1 and R2 are 3 ohms and 6 ohms respectively, and the mark on R3 is blurred. In order to identify the resistance of R3, ammeter and voltmeter are respectively connected between A and C and between A and D. The readings on the meter are 5 mA and 2.7 volts respectively. Therefore, R3 = _ _ _ Ou.

21. in the circuit shown in fig. 16, it is known that the power supply voltage connected to both ends of AB is 7.5 volts, R1=2 ohms, R3=5 ohms, and the indicator of voltmeter U1 is 2.5 volts, and the indicator of U1 is 4. 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 remains unchanged, R1=R2=R3. When the switch S is turned off, the voltmeter reads U; When s is closed, the voltmeter reads _ _ _ _ _ _ _ u.

23. as shown in fig. 18, the power supply voltage is constant, R1=1 ohms, R2=2 ohms and R3 = 3oohms. When the switches S1 S2 are both open and closed, the ratio of the indicators of the ammeter is 1: 2, and the resistance of R is _ _ _ _ _ _ _ _ _ _.

24. as shown in fig. 19, there is a circuit composed of a power supply with a voltage of 5v and a resistor with the same resistance in the known box. There are four terminals outside the box. Measure the voltage with a voltmeter, U12 = 5v, U34 = 3v, U13 = 2v and U42=. Draw the circuit inside.

25. In electrical experiments, when an open circuit is encountered, a voltmeter is often used to detect it. A classmate connects the circuit as shown in Figure 2, and the power supply is intact. After the switch is closed, the light is off, and the ammeter has no indication. At this time, the voltage measured by voltmeter between two points B and C and between two points A and D is zero, which indicates that it may be ().

A power supply terminal has poor contact; B the contact piece or terminal of the light switch is in poor contact;

the terminal of c ammeter has poor contact; D bulb filament is broken or has poor contact

26. As shown in Figure 21, 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 A1 decreases, A2 increases, V1 remains unchanged, and V2 decreases; B A1 increases, A2 decreases, V1 decreases, and V2 remains unchanged;

C A1 remains unchanged, A2 remains unchanged, V1 remains unchanged, and V2 increases; D A1 increases, A2 increases, V1 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's ammeter reading will become larger < P > 28. As shown in Figure 23, the power supply voltage remains unchanged, and after the switch S is closed, the ratio of ammeters A1 and A2 is 8: 5. If a resistor R3 is well connected between points A and B, the ratio of indicators of ammeters A1 and A2 is 3: 2 after switch S is closed. Compare the sizes of R1 and R3:

AR1 > R3; B R1＜R3； C R1=R3； D conditions are insufficient to judge.

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

A the current I through R1 and R2 is equal, and I = u/(R1+R2)

the voltage u1 on br1 = ir1; The voltage U2 on R2 = I R2, and the current in the conductor is zero;

the voltage U1=U on cr1 = u, and the voltage on R2 is zero;

the current I1 in dr1 = u/R1, the current in conductor l = i1, and the current in R2 is zero;

3. In order to adjust the current in the circuit, two sliding varistors are used in the adjustment part of a precision electronic instrument, as shown in Figure 25. It is known that these two sliding varistors are made by winding different resistance wires around the same insulating porcelain tube, in which RL; The total resistance of is 2Ω, and the total resistance of R2 is 5Ω. At the beginning, both varistors are at the maximum resistance. The following methods can make the ammeter pointer point to the required position quickly and accurately ()

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

B. Adjust R2 first to make the ammeter pointer point to the vicinity of the required position. Then adjust Rl

C. Adjust Rl and R2 at the same time to make the ammeter pointer point to the required position

D. Adjust RL and R2 alternately and repeatedly to make the ammeter pointer point to the required position

31. Equivalent substitution method is often used in physical experiments. For example, a 7Ω resistor is used to replace the series resistance of 2Ω and 5Ω in a branch, and in other branches,