CBSE Guess > Papers > Important Questions > Class XII > 2007 > Physics > Physics (Current & electricity) By: Mr. Abhishek Gupta

Current & Electricity

Q. 1. Define the following terms

1. Electromotive force
2. Drift velocity
3. Relaxation time
4. Internal resistance of a cell
5. Resistance

Q. 2. The resistance of a copper wire at 27^oC was found to be 100Ω and at 127^oC was found to be 105Ω. Find the temperate coeff of resistance

Q. 3. An aluminium wire has a resistance of 10Ω at 27^oC. The wire is heated to a temp of 527^oC. Find its new resistance if α = 10^-3

Q. 4. What type of materials are used for making standard resistance coils. Give examples.

Q. 5. State Kirchoff’s laws

Q. 6. Why are household appliances connected in parallel.

Q. 7. Explain the usefulness of the fuse wire

Q. 8. Explain how a potentiometer can be used to compare the emf of 2 primary cells

(b) A potentiometer of length 4m is connected across 100V Power supply. It is then used to compare the emf of 2 primary cells. The balance point obtained with cell E₁ is 1.5m and with cell E₂ is 2m. Find the ratio E₁/E₂. How can a potentiometer be made more sensitive?

Q. 9. State 3 essential differences b/w a potentiometer and a voltmeter.

Q. 10. A potentiometer wire is of length 4m and has a resistance of 10ohms. It is connected in series with a resistance of 90 ohm to 200V power supply. Balance length for an unknown emf E is found to be 3m. Find the unknown value E.

(b) What is the use of the series resistance of 90 ohm?

Q. 11. A student while performing the potentiometer experiment could not obtain a null point. Mention two possible mistakes that he could have made.

Q. 12. A 100W bulb and a 40W fan are used for 10 hours a day. Find the amount of energy consumed in 30 days. Also find the electricity bill if the cost of electricity is Rs 5 per KWh.

Q. 13. A galvanometer of resistance 10ohm has a current rating of 10mA. Explain how this can be converted into a

1. Ammeter of range 5A
2. Voltmeter of range 25 volts

Q. 14. The maximum power rating of a 20-ohm resistor is 2.0KW ( That is, this is the maximum power the resistor can dissipate without melting ) Would you connect this resistor directly across a 300V DC supply. Explain. Can we connect two such resistors in series across the 300V supply?

Q. 15. An electric bulb is marked as 100W at 230V. Find the power dissipated if instead of 230V supply the bulb is joined to

1. 460 volt supply
2. 1A constant current source

Q. 16. A rod of length L and area A has a resistance R.

1. It is cut into 2 halves. Find the resistance of each part
2. The rod is compressed to half its length. Find the new resistance

Q. 17. Two wires A and B are made of the same material, have the same area but lengths are in the Ratio 2:1. What will be ratio of current flowing through them when the same potential is applied across each of them?

Q. 18. A 100W bulb is joined in series with a 50W bulb across 200V supply. Find the current and power dissipated in each resistor. Which bulb will glow brighter?

Q. 19. What is Seebeck effect? How is it different from Peltier effect?

Q. 20. Explain the effect of temperature on thermo EMF. Draw the corresponding graph.

Q. 21. What is a voltmeter? What should be the resistance of an ideal voltmeter?

(b) What is an ammeter? What should be the resistance of an ideal ammeter?

Explain whether they are connected in series or parallel.

Q. 22. The sequence of bands marked on a carbon resistor are Green, Red, Orange and silver. Find the resistance and the tolerance

Q. 23. A dry cell of EMF 1.5 volts and internal resistance 0.1ohm if connected to a resistor R. If the current drawn from the cell if 2A, find the value R and the power dissipated in it.

Q. 24. Resistors of 2,3 and 6 ohm are connected in series across a 220 Volts power supply. Find the current, voltage and power loss in each resistance.

Q. 25. Resistors of 2,3 and 6 ohm are connected in parallel across a 20 Volts power supply. Find the current, voltage and power loss in each resistance.

Q. 26. A heater wire is rated as 1000W at 220volt. The heater wire is cut into 2 halves and theses two parts are connected in parallel across 220V power supply. Find the power dissipated.

Q. 27. In a thermocouple the Emf varies with temp as E = AT - BT^2. Where T is the temp of the hot junction and the cold junction is kept at 0oC. Here Emf is in μV and T is in ^oC. The value of

A is 4 μV^oC^-1 and B is 0.04 μV^oC^-2 . Find

1. Neutral temp
2. inversion temp
3. Emf at 120^oC
4. Thermoelectric power at 120^oC
5. Thermoelectric power at neutral temp

Q. 28. State Faraday’s laws of electrolysis. Define electrochemical equivalent

(b) Current of 5A is passed through a silver voltameter and a copper voltamter connected in series. Find the amount of substance deposited in each after 10 minutes.

Q. 29. Define neutral and inversion temperature. On what factors does the neutral temp depend.

(b) For A particular thermocouple the EMF varies with temperature according to the relation

E = 100T – 2T² mV . Find the Emf and thermoelectric power at 10^oC. If the cold junction is at 0^oC , find the neutral and inversion temperature.

Q. 30. In a meter bridge experiment to find an unknown resistance, a resistor of 100ohm is used. The balance length is found to be 0.4m. Find the unknown resistance. On what principle is the meter bridge based? When is the meter-bridge most sensitive?