Current Electricity – Short Questions Collection With Answer | Physics Class 12

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Current ElectricityVery Important Questions Collection
HSEB | Science Faculty
Current Electricity – Physics Grade XII
Questions with Answer (Solution)

1. What are the factors on which the resistance of a conductor depends?
Ans. The resistance(R) of a conductor,

  1. is directly proportional to its length(l).
  2. is inversely proportional to its cross sectional area(A), R α 1/A .
  3. It depends upon the nature of the material.
  4. It changes with temperature (t).

2. Why constantan wire is used for making standard resistances?
Ans. Some alloys like manganin and eureka are used in making standard resisters because,

  1. They have high resistivity.
  2. Their resistivity varies less with temperature.
  3. They have high melting point.
  4. They have low temperature coefficient of resistance.

3. Why do we use connecting wire made of copper?
Ans. We use connecting wire made of copper because:

  1. They have very high conductivity,
  2. They have high melting point,
  3. They are easily available and
  4. They are comparatively cheaper.

4. If you are given three resisters, how do you combine them to increase and decrease the value of effective resistance ?
Ans. Suppose R1,R2,R3 are three given resistance to be combined. Their effective resistance can be increased in series so that its value is:
Rs = R1 + R2 + R3
Also, their effective resistance can be decreased in parallel so that its value is :
1/Rp = 1/R1 + 1/R2 + 1/R3
= x > Rp = R1 R2 R3 / R1R2 + R2R3 + R3R1

5. What do you mean by safe value of fuse wire current ?
Ans. The safe value of fuse wire current is the maximum value of current that a fuse wire can carry without getting melted. Under such conditions, the rate of production of heat in the fuse wire is equal to the rate of lose of heat from it.

6. What do you mean by tolerance of a resistance ?
Ans. the maximum possible error that can occur in the physical quantity resistance is called tolerance of a resistance. For example, 5% tolerance in 10,000 ohm’s of resistance means the resistance will be within 5% of 10,000 ohm’s. In other words, the value of resistance lies between 9,500 ohm’s and 10,500 ohm’s.

7.What is Ohm’s Law?
Ans. The Ohm’s law states that the current(i) flowing through a conductor is directly proportional to the potential difference (V) across its ends provided the physical conditional don’t change. i.e. V α i => V=RI

8. Is it possible to verify the Ohm’s law by using a filament lamp?
Ans. To verify the Ohm’s law, the physical conditions of the current carrying conductors should remain unchanged. But, the temperature of the filament of lamp changes on increasing the voltage across the lamp.This, in turn, changes the resistance of the filament because the resistance varies directly with temperature. Moreover, for Ohm’s law, it is required that the relation between V and I is linear if R is independent of the magnitude of V.

9. You are given a galvanometer. How would you convert this meter into (a) an ammeter and (b) voltmeter? What information would you wish to have about the Galvanometer before you make these conversations?
Ans. (a) A galvanometer can be converted into an ammeter of given range by connecting a suitable small resistance in parallel, called the shunt, with the coil of galvanometer.
(b) A galvanometer can be converted into an voltmeter by connecting a suitable resistance in series with the galvanometer.
Before making these conversations, we require the following two information about the galvanometer :

  1. Resistance of galvanometer (G) and
  2. Current for full scale deflection of galvanometer (ig).

10. Why ammeter is always connected in series?
Ans. An instrument, which is used to measure the electric current flowing in the circuit, is called an ammeter. Ideally, the resistance of an ammeter is nearly zero. Consequently, there is no effect of ammeter on inserting is series.But, in practice, the resistance of an ammeter is not zero. but very low in comparison to the other current. That’s why, an ammeter is always connected in series.

11. A voltmeter is always connected in parallel, why?
Ans. An instrument, which is used to measures the p.d. is called voltmeter. Its resistance is very high so that it is always connected in parallel across any element of a circuit. In such case, it draws a very small current from the main circuit. Most of the current passes through that element. On the other hand, if the voltmeter is connected in series, the resistance of the circuit decrease considerably and voltmeter will not measure the actual p.d.

12. By mistake a voltmeter is connected in series and an ammeter is connected in parallel with a resistance in an electrical circuit. What will happen to the instruments?
Ans. Suppose RA and RV be the resistance of ammeter and voltmeter respectively. Further let R be the resistance of circuit. Obviously R>>RA and RV>>R.
When the voltmeter is connected in series and ammeter in parallel with resistance R, The net resistance, then current flowing in the circuit becomes,
i = E/Rv
This entire current flows through the voltmeter and as R>>RA, nearly whole of current i also passes through the ammeter . If they were connected correctly, then the resistance of circuit had been R and current flowing via thew circuit becomes i0 = E/R, As R< i<< i0 . Therefore no instrument,ammeter or voltmeter will be damaged. 13. Why do the electrons acquire a steady drift velocity? Ans. When a battery is connected across the ends of a conductors, an electric field is set up. This electric field accelerates the electrons in the conductor. Electrons gain Kinetic energy depending on the amount of the applied electric field.During this movement, electrons collide with atomic lattice and lose their energy. As a result, they can’t be accelerated continuously but they soon acquire a steady average drift velocity. 14. What is the relation between electric electric field and drift velocity? Ans. The relation between the electric field ‘E’ and drift velocity ‘Vd’ of the free electron is: Vector Vd = – Vector a*t => Vector Vd α Vector E, Where ‘t’ is relaxation time, e is change and m is mass of a free electron. Here negative sign shows that the direction of drift velocity is opposite to that of the electric field.

15. Why don’t we consider the drift velocity of position ions?
Ans. The positive ions in a conductor also experience a force in the presence of an electric field. Since the positive ions are heavy and tightly bound in the metal, they are hardly able to move making the drift velocity negligibly small.

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