## Very Important Questions Collection

HSEB | Science Faculty

Heat and Power – Physics Grade XII

Questions with Answer (Solution)

**1. Why is heat generated across a resistor when electric field is applied?**

Ans. When electric field is applied across a resister, free electrons drift through it. During their movement, they collide frequently with atoms and their kinetic energy is transferred in the form of heat. As a result, heat is generated cross a resister field is applied. According to joules law of heating, the amount of heat generated across a resister is given by:

`H = i= I square RT, where I is the current flowing for time t.`

**2. Distinguish between e.m.f and potential difference.**

Ans. The differences between e.m.f. and p.d. are :

Electromotive force (E.M.F) | Potential Difference (P.D) |

a. The amount of energy supplied by a cell to move unit coulomb charge in the closed circuit is called e.m.f. of cell. | a. The amount of work done by a cell during the movement of unit coulomb charge in the closed circuit is called p.d. |

b. It id denoted by E. | b. It is denoted by V. |

c. It is measured in open circuit. | c. It’s measure in closed circuit. |

d. During discharge of a cell e.m.f. is greater than p.d. | d. During charging a cell, p.d. is greater than e.m.f. |

e. It is independent of internal resistance the cell. | e. It depends on the internal resistance of the cell. |

f. E.m.f. is cause of p.d. | f. P.d. is and effect of e.m.f. |

**3. The wires supplying current to an electric heater are not heated where as the filament of the heater becomes very hot. Why?**

Ans. According to joule’s law of heating, the amount of heat developed per second by a wire of resistance R due to the flow of electric current I is

`H = I square R => H α R for constant I`

So, heat developed across the wire directly depends on its resistance. As the resistance of the filament of heater is very high than that of connecting wire, the heat developed in the filament is very high and becomes very hot, where as the heat developed in the connecting wire is very low and it is not heated.

**4. What do you mean by internal resistance of a cell?**

Ans. The resistance offered by a cell to that current flow is called internal resistance of the cell.It is denoted by r. In general, the resistance of a cell is low. The internal resistance of a cell depends upon the following factors :

- It increase with the increase in distance between the two plates of cell,
- It depends on the nature of the electrodes,
- It depends on the nature of electrolyte,
- It decreases with the increase in plate area, and
- It increase with the concentration of the electrolyte.

**5. Can the potential difference across a battery be greater than its e.m.f. ?**

Ans. In general, the p.d. across a battery is smaller than its e.m.f. But, when an electric current flows through a battery in such a way that the direction of current is opposite to the direction of e.m.f. of the battery, then p.d. across the battery is greater than its e.m.f. In such case, the current flows from positive to negative terminal inside the battery . The relationship between p.d of a battery and its e.m.f. is given by:

E = V + (-I)r

**Where r is the internal resistance of the battery**

E = V – Ir For ‘r’ is not equal to zero, then E If external resistance is negligible with internal resistance of a cell, then parallel combination of cell will be preferred to draw large current .Suppose r be the internal resistance and E be the e.m.f. of n number of cells connected in parallel. Then current flowing through the circuit is given by :

`I = Total emf / Total resistance = E / (R+r/n)`

For r /n>>R, {R +(r/n)} α r/n

=> I = E/(r+n) = n x (E/r) = n x current due to one cell.

**7. What do you mean by electrical energy?**

Ans. The loss of electrical potential energy in maintaining current in a circuit is called electrical energy consumed in the circuit. The electrical energy consumed in t second is V I t joules. Its SI unit is Ws or J. But , in practice,it is measured in kilowatt hour(kWh). One kWh energy is consumed when a device converts electrical energy to other forms at a rate of 1kW for a time of 1 hour.

**8. What is electrical power?**

Ans. The rate of conversion of electrical energy into other forms of energy is called electric power.For example, a 100 W bulb converts 100 J of electrical energy into heat and light in one second. Its unit is watt.

** 9. Wen the resistance connected in series with a cell is exactly halved, the current is not exactly doubled. Explain why?**

Ans. When the resistance connected in series with a cell is exactly halved, the current is not exactly doubled;it is due to internal resistance of the cell. The current flowing through the circuit with external resistance R and internal resistance r is given by :

i = E/(R+r)

If R is made R/2, then i will be slightly less than 2 i.

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

Ans. As the motor car is started from the battery the starter takes high current. If r be the internal resistance of the battery, then the potential drop across the battery which causes a considerable reduction in potential difference across the battery. Due to this reason, the light of the motor car becomes slightly dim.

**11. A current is passed through a steel wire heating to red. Then half of the wire is immersed in cold water.Which half of the wire heat up more and why ?**

Ans. The half part of wire immersed in cold water is at lower temperature,so its resistance is less than that of unimmersed part.The net resistance of wire will decrease and so the current will increase. The resistance of exposed part is same as before, therefore the heat produce in exposed part is increased, and hence the exposed part of the wire is heated up still more.

## One Comment

what is the relation between the power and resistance of the bulb in series and parallel circuit?