Question of Exercise 2

# Question An electric iron draws a current 4A when connected to a 220 V mains. Its resistance must be

Option 1 1000

Option 2 55

Option 3 44

Option 4 55/4

Explain the following

(a) Why is tungsten used almost exclusively for filament of electric lamps?

(b) Why are the conductors of electric heating devices, such as bread - toasters and electric irons, made of an alloy rather than a pure metal?

(c) Why is the series arrangement not used for domestic circuits?

(d) How does the resistance of a wire vary with its area of cross-section?

(e) Why are copper and aluminum wires usually employed for electricity transmission?

Solution:

Explanation:

(a) Tungsten has a very high melting point due to which the filament made up of tungsten does not melt at very high temperatures.

(b) Alloy has more resistance than conductors. As electrical heating devices are required to produce a lot of heat they need to be made up of elements having high resistance. High resistance elements release a lot of heat as the loss in voltage due to the resistance of alloy gets converted to heat energy.

(c) The series arrangement is not used for domestic circuits because there is a loss in potential difference across the elements in a series arrangement due to which the power in the circuit will be less than the applied power and the farther the element is connected in the circuit, lesser the power supplied to that element.

(d) The resistance of a wire varies with its area of cross-section because according to the formula of resistance,R=Pl/A resistance is inversely proportional to the area of cross-section of the conductor.

(e) Copper and aluminum wires are usually employed for electricity transmission because they have low resistivity due to which there is less loss of power while transmitting the electricity.

(a) Tungsten is used almost exclusively for filament of electric lamps because it does not get destroyed at high temperatures.

(b) The conductors of electric heating devices, such as bread - toasters and electric irons are made of an alloy rather than a pure metal because alloys have more resistance.

(c) The series arrangement is not used for domestic circuits because uneven power is supplied in a series arrangement.

(d) The resistance of a wire varies with its area of cross-section because it is inversely proportional to the area of cross-section of the wire.

(e) Copper and aluminium wires are usually employed for electricity transmission because they have low resistivity.

An electric motor works on the principle

A: of electromagnetic induction

B: when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously.

C: both A and B

D: None of the above

Solution:

Explanation:

In an electric motor the electrical energy is converted into the mechanical energy.

And the motor used the magnetic effect of the current.

It is saying that when a rectangular coil is placed in a magnetic field and current is passing through it then a force is exerted in the coil which rotates the coil.

The option(B) is correct.

State joules law of heating

Solution:

Explanation:

It is used to calculate the rate at which electric energy is converting into the heat energy due to electrical resistance.

The formula of joule’s law of heating is Q = I2RT

Here, Q is the heat energy, I is the electric current, R is electric resistance and T is the time interval.

Hence, the  joule's law of heating is = I2RT.

Write Joules law of heating

Solution:

Explanation:

• A conductor becomes heated when an electric current flows through it for a long time. Joule's law of heating describes

the heating of a conductor caused by the flow of electric current across it.

Heat geretated

H=i2Rt

Here, i is the current running through the conductor, R is thHe conductor's resistance, and t is the length of time the current has flown.

Hence, Joule’s law of heating is given by H=i2Rt.

Name some devices in which electric motors are used

Solution:

Explanation:

• In contrast to an electric generator, an electric motor is a device that converts electricity into mechanical energy.
• They work on the basis of electromagnetism, which states that when an electric current is present in a magnetic field, a force is applied.
• Motors feature a lot of moving elements that allow them to spin and provide power as needed.

Electric motors that are used in certain devices are:

1. Pumps for water
2. Fans with electricity
3. Mixers with electricity
4. Machines for washing