# CLASS 12  PHYSICS -- Chapter 2: CURRENT ELECTRICITY

This is a discussion on CLASS 12  PHYSICS -- Chapter 2: CURRENT ELECTRICITY within the 12th forums, part of the Classes category; CLASS 12  PHYSICS Chapter 2: CURRENT ELECTRICITY Short answer question: Q1. Verify that the equivalent resistance of a parallel ...

1. ## CLASS 12  PHYSICS -- Chapter 2: CURRENT ELECTRICITY

CLASS 12  PHYSICS

Chapter 2: CURRENT ELECTRICITY

Q1. Verify that the equivalent resistance of a parallel combination of resistors is always smaller than the smallest resistance in the combination.

Ans. Consider three resistors of resistance 3Ω, 4Ω and 5Ω which on connected in parallel. In parallel combination the equivalent resistance R is given by:
1/R = 1/R1 + 1/R2 + 1/R3
1/R = 1/3 + ¼ + 1/5
R = 1.276 Ω
In parallel the equivalent resistance R is smaller than the smallest resistance in parallel, which is 3Ω.

Q2. Why are the cows sitting on bare high-tension wires not electrocuted?

Ans. When the crow is sitting on bare high-tension wire, the potential difference between its two claws is zero as both the claws are on the same wire i.e. at the same potential. Therefore no current is flowing through the body of the crow, so the crow is not electrocuted.

Q3. Why is it dangerous to touch a live while standing on earth bare footed?

Ans. The person standing on earth bare footed is at zero potential because the earth is at zero potential when he touches the live wire which is at high potential, the current start flow through his body, which damages the living cells of his body and causes burns.

Q4. Is the principle of energy conservation applicable to electrical circuit?

Ans. Yes the principle of energy conservation applicable to electrical circuits. In electrical circuit the emf source such as battery transform chemical energy into electrical energy, which converts into light energy by using electric bulb, heat energy by using heater and mechanical energy by using electric motor.

Q5. What do a short circuit and open circuit mean to you?

Ans. An electrical circuit in which the emf source (battery/cell) supply current is called close circuit. The close circuit in which the load resistance is of negligible resistance R =0, then such close circuit, is called short circuit in which such a large current flow, that the wire get fire which causes damages. The close circuit in which the load resistance is of infinite resistance R = α, then in such close circuit the current I = 0 i.e. no current is flowing through such circuit. Such close circuit is called open circuit.

Q6. Describe a circuit, which will give a continuously varying potential.

Ans. The circuit, which gives a continuous varying potential, is a potential divider, which is used in potentiometer.
Consider a resistance R in the form of a wire on which a sliding contact C can slide. The resistance between A and C varies from zero to R due to which the potential drop between A and C also varies from zero to maximum. When C is at A, E = 0 and when C is at potential drop becomes E.

Q7. The current in an electric bulb rises to a maximum almost at the instant the bulb is turned on. The current then decreases significantly. Why does the current decrease?

Ans. When the bulb is turned on, its filament is at the room temperature and possess low resistance due to which a large current flow through it. This passage of large current through the filament of bulb according to joule heat law produces heat, which increase the temperature of the filament thereby increase the resistance of filament. For constant potential difference when the resistance increase the electric current decreases.

Q8. Why a tin region of light bulb filament has more possibility to burn than the ticker one?

Ans. The tin region of the filament of bulb has more resistance than the thicker region. As the current in thin region and thick region is same. So in the same time the heat produces in the thin region will be more than heat produces in the thick region. Therefore the thin region of the bulb filament has more possibility to burn than the thicker one.

Q9. In the normal operation of a resistor, what limits the temperature rise due to Joule heating?

Ans. According to the Joules law heat produces in resistance is directly proportional to the square of current flowing through it. This heat resistance the temperature of resistor and hence increases its resistance. This result a decrease in current, which subsequently limit the temperature rise due to Joule heating.

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2. ## Re: CLASS 12  PHYSICS -- Chapter 2: CURRENT ELECTRICITY

thanks alot  Reply With Quote