## FSc Notes: Physics XII: Chapter 13 Current Electricity Exercise Short Questions:

**Question 13.1 A potential difference is applied across the ends of a copper wire. What is the effect on the drift velocity of free electrons by**

- Increasing the potential difference.
- Decreasing the length and the temperature of the wire.

**Answer 13.1 Drift velocity is actually the resultant velocity of electrons in the presence of an external electric field.**

- By increasing the potential difference, value of drift velocity of free electrons increases.
- When length and temperature of the wire is decreased resistance also decreases, with the decrease in resistance drift velocity of free electrons increases.

**Question 13.2 Do bends in a wire affect its electrical resistance? Explain.**

Answer 13.2No, as we know that

Answer 13.2

**R = ρ L/A**it shows that there is no relationship b/w resistance and the bends of a wire i.e. R is independent of the bends in a wire. Hence Bends in a wire don’t affect its resistance.

**Question 13.3 What are the resistances of the resistors given in the fig A and B? What is the tolerance of each? Explain what is meant by the tolerance?**

Answer 13.3Fig A: Brown 1 Green 5 Red 00 Gold 5% tolerance 1500Ω with 5% tolerance.

Answer 13.3

Fig B: yellow 4 White 9 Orange 000 Silver 10% tolerances 49000Ω with 10% tolerance.

**Question 13.4 Why does the resistance of a conductor rise with temperature?**

Answer 13.4Actually resistance of a conductor is due to collision b/w electrons and lattice atoms with increase in temperature, vibration motion of atoms increases and due to greater amplitude of vibration, their collision probability with electrons increases hence resistance of conductor increases according to the relation:

Answer 13.4

Rt = R0(1+at)

Rt Resistance at t temp, R0 Resistance at 0 temp, a Co-efficient of resistance, t change in temp.

**Question 13.5 What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s Law?**

Answer 13.5Ohm Law states “

Answer 13.5

**Current is directly proportional to the potential difference applied across the conductor provided that the physical state of the conductor remains constant (V=IR).**” The resistance of a lighted electric bulb does not remains constant but gradually increases with increase in temp. Hence the filament of a bulb does not follow Ohm’s Law.

**Question 13.6 Is the filament resistance lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?**

Answer 13.6As we know that: Power P=V2/R or R= V2/P

Answer 13.6

**Case i:**when V=220V & P=500W so R1= (220)2/500 =

**96.8Ω**

Case ii:when V=220V & P=100W so R2= (220)2/100 =

Case ii:

**484Ω**

Conclusion From R2 R1. It is clear that filament resistance in 500W and 220V bulb is lower than in 100W, 220V bulb.

**Question 13.7 Describe a circuit which will give a continuously varying potential.**

Answer 13.7Potentiometer is a circuit that can give a continuously varying potential as shown in figure: A and B are fixed terminals and C is sliding terminal Resistance b/w A and B is ‘R’ and that b/w A and C is ‘r’.

Answer 13.7

Now I=V/R and potential drop across ‘r’ is

**V=I\r**

**V=Vr/R.**

As C moves from A to B r varies from 0 to R. Hence a circuit is described which gives a continuously varying potential 0 to V.

**Question 13.8 Explain why the terminal potential difference of a battery decreases when the current drawn from it is increased?**

Answer 13.8As we know that

Answer 13.8

**vt = E-Ir ----------I**

Here vt = terminal velocity, potential difference E=emf, r=internal resistance of battery.

According to eq when current (I) drawn from the circuit it is increased by factor

**‘Ir’**.

That is why, the value of vt decrease, when current is increased.

**Question 13.9 What is Wheatstone bridge? How can it be used to deter mine an unknown resistance?**

Answer 13.9That device is used to find the unknown resistance is known as

Answer 13.9

**Wheatstone bridge**.

An unknown resistance can be found by the relation:

**R1 / R2 = R3 / R4**.

thanks

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