Chapter (11)

Electric cookers convert the electrical energy into a _____ energy.

• heat

• light

• mechanical

Answer explanation

Electric cookers use heating elements to convert electrical energy into heat energy.

Lamps used for street lighting are connected in _____.

• series

• parallel

• none of these

Answer explanation

Street lights are always connected in parallel because if it was in series then due to fault in any of the bulb, the whole line will be switched off then and till the fault is not cleared it'll be dark.

Electric fuses are so devised as to limit the _____.

• current

• resistance

• voltage

Answer explanation

The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

The consumption of electrical energy in the household is measured in terms of _____.

• kilowatt hour

• volt

• joule

Answer explanation

1kWh=1unit of electricity

Cost of electricity per month must be paid by the product of cost of electricity per unit and units used.

A fuse wire is usually made of _____ alloy.

• copper-tin

• iron-lead

• tin-lead

Answer explanation

The fuse wire is made up of Sn (tin) and Pb (lead) alloy because of its high resistance and low melting.

The electric ______ is the rate of transfer of electrical energy.

• current

• power

• charge

Answer explanation

The rate of electrical energy means W/t.

P= W/t

If the potential difference across a resistor is doubled the power used by it is ____ increased.

• 2 times

• 8 times

• 4 times

Answer explanation

$P=\frac{V^2}{R}$
P is directly proportional to the square of V.
If V is doubled, P is increased by 4 times.

The work done is related to the amount of heat by the equation _______.

• W=JH

• W=Fs

• W=Pt

Answer explanation

By Joule’s law,

W=JH

If two unequal resistors are connected in parallel the _____ resistor develops heat at the greater rate.

• smaller

• larger

• none of these

Answer explanation

Rate of heat means electrical power.

$P=\frac{V^2}{R}$

P is inversely proportional to R.
So, if two unequal resistors are connected in parallel the smaller resistor develops heat at the greater rate.

The power rating of an electric iron which draws a current 5A when operated at 240V is ______.

• 48W

• 1200W

• 120W

Answer explanation

P=VI

P=240x5=1200W