# Physics Class 12 NCERT Solutions: Chapter 3 Current Electricity Part 6 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 13. The number density of free electrons in a copper conductor estimated in Example is . How long does an electron take to drift from one end of a wire long to its other end? The area of cross section of the wire is and it is carrying a current of .

Number density of free electrons in a copper conductor,

Length of the copper wire,

Area of cross-section of the wire,

Current carried by the wire, , Which is given by the relation,

Where,

Electric charge

Drift velocity

Therefore, the time taken by an electron to drift form one end of wire to the other is .

Q: 14. The earth՚s surface has a negative surface charge density of . The potential difference of between the top of the atmosphere and the surface results (due to the low conductivity of the lower atmosphere) in a current of only over the entire globe. If there were no mechanism of sustaining atmospheric electric field, how much time (roughly) would be required to neutralise the earth՚s surface? (This never happens in practice because there is a mechanism to replenish electric charges, namely the continual thunderstorms and lightning in different part of the globe [Radius of earth ] .

Surface charge density of the earth,

Current over the entire globe,

Surface area of the earth,

Charge on the earth surface,

Time taken to neutralize the earth՚s surface ,

Therefore, the time taken to neutralize the earth՚s surface is .

Q: 15. (A) Six lead-acid type of secondary cells each of emf and internal resistance are joined in series to provide a supply to a resistance of . What are the current drawn from the supply and its terminal voltage?

(B) A secondary cell after long use has an emf of and a large internal resistance of . What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?

(a) Number of secondary cells,

Emf of each secondary cell,

Internal resistance of each cell,

Series resistor is connected to the combination of cells.

Resistance of the resistor,

Current drawn from the supply , which is given by the relation,

Terminal voltage,

Therefore, the current drawn from the supply is and terminal voltage is .

(B) After a long use, emf of the secondary cell,

Internal resistance of the cell,

Hence, maximum current

Therefore, the maximum current drawn from the cell is . Since a large current is required to start the motor of a car, the cell cannot be used to start a motor.

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