Important Questions and Answers: Heat and Power

1. What is Joule’s law of heating?

Ans: Joules law of heating states that, the amount of heat produced in a conductor is directly proportional to:

  • Square of current (H∝I2)
  • Resistance of the conductor (H∝R)
  • Time upto which current is passed (H∝T)

 2. Can a potential difference across a battery be greater than its e.m.f. ?

Ans: Yes. In charging circuit, current flows opposite to the direction of emf. So, voltage is dropped due to the internal resistance. In this case potential difference becomes greater than emf as per the relation.  V = (E + Ir). Here,

V = Voltage
E = E.M.F.
Ir = Internal resistance

3. When a motor car is started, its light becomes slightly dim. Why?

Ans: When a motor car is started, the starter draws large amount of current from the battery. The potential drops across the battery as per the relation V = Ir where r is the internal resistance of the battery. The potential drop causes considerable reduction in p.d. across the battery. So, the light of a motor car becomes slightly dim when it is just started.

 4. Two bulbs of 60 W and 100 W are connected in series and this combination is connected accross the mains. Which bulb will glow more brightly?

Ans: Let us consider the resistance of two bulbs be R1 and R2 respectively.  Let their power be P1 = 60 W and P2 = 100W. Let V be the voltage of both bulb. (Voltage is equal for both bulbs as the bulbs are in series.)

We know,
$$R_1 =  \frac{V}{P_1}……(i)$$

$$R_2 =  \frac{V}{P_2}……(ii) $$

Dividing equation (i) by (ii), we get:

$$\frac{R_1}{R_2} =  \frac{P_1}{P_2}$$

$$\therefore R \propto P$$

Since the bulb having value 60 W has higher value of P, the bulb of 60 watt glows more brightly.

5. The element of heater is very hot, while the current carrying wire is cold. Why?

Ans: From Joule’s law we have the relation H = I2Rt. So, the heat produced in a wire is directly proportional to the resistance. As the resistance of the coil of the heater is very high due to high number of turns but the resistance of current carrying wire is very low compared to that of heater element. With the increase resistance, the collision of electrons becomes very high and the kinetic energy of the electron is transformed into heat energy. So, the element of heater is very hot while current carrying current is cold.

6. What is electric power?

Ans: The time rate at which electric energy is produced is known as electric power. $$\text{i.e., Electric power (E)} = \frac{\text{Work done (W) }}{\text{Time (T)}}$$

The SI unit of electric power is watt.

7. What is internal resistance ? What are the factors does it depend on?

Ans: The resistance offered by the source for a current to pass through it is called internal resistance of a cell. The internal resistance of a cell depends on many factors such as:

  • Distance between the electrodes
  • Nature of electrolytes
  • Area of the electrodes
  • Temperature of the electrolyte

8. What is e.m.f. and potential difference of a cell ?

Ans: The amount of workdone in moving unit amount of charge from one terminal of a cell to another terminal of the cell throughout the circuit is called em.f. of a cell. The emf of a cell generally depends on the material of the electrodes and the electrolyte used is cell.

Potential difference is the energy difference in two terminals of the cell.

9. Batteries are always leveled with emf. Would it be appropriate to put a level on batteries stating how much energy they provide?

Ans: Since the current provided by the battery depends upon the external resistance of the circuit as well as its internal resistance. So, no amount of current is written in a battery.

10. The energy that can be extracted from the storage battery is always less than the energy that goes into it while charging. Why?

Ans: The energy is always less in this case because some of the energy is transformed into heat and other forms of energy due to the internal as well as external resistance of the wire.


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2 Responses to “Important Questions and Answers: Heat and Power”

  1. Sagar jimee

    Better help through numericals as well?

  2. Mohamed Rashid

    It’s helpful article but prepare for kirchhoff’s law questions