# Illustrations

Given the following data, calculate the electrostatic force and gravitational force between two electrons. Show that the electrostatic force is much greater than the gravitational force for the electrons separated by a certain distance.

Charge on the electron = 1.6 Ã— 10^{â€“19} C

Mass of the electron = 9.1 Ã— 10^{â€“31 }kg

Gravitational constant = 6.67 Ã— 10^{â€“11} Nm^{2} kg^{â€“2}

Let the distance between two electrons be *d*.

Then the gravitational force of attraction between them is given by .

The electrostatic force of repulsion between them is .

Calculate the electrostatic force of attraction between a proton and an electron in a hydrogen atom. Given: the radius of the electron orbit is 0.05 nm and charge on the electron is 1.6 Ã— 10^{â€“9} C.

From Coulombâ€™s inverse square law:

The magnitude of charge on electron and proton is the same = 1.6 Ã— 10^{â€“19} C.

Therefore, electrostatic force is given by

= 0.9216 Ã— 10^{â€“29} Ã— 10^{+22}

= 0.9216 Ã— 10^{â€“7} N

= 9.216 Ã— 10^{â€“8} N.

Calculate the electric current in the circuit shown below.

Any current that passes through the 10 Î© resistor also passes through 5 Î©, i.e., 10 Î© and 5 Î© are connected in series. Their equivalent resistance is R_{eq} = 10 Î© + 5 Î© = 15 Î©.

The equivalent circuit is shown in the above figure.

The current .

Two resistors of resistances 10 Î© and 20 Î© are connected in parallel. A battery supplies 6 A of current to the combination. Calculate the current in each resistor.

The electricity tariff in a town is â‚¹ 3.00 per unit. Calculate the cost of running an 80 W fan for 10 h a day for the month of June.

Total time for which the fan is used = (10 hours) Ã— (30 days) = 300 hours.

Energy consumed = (80 W) Ã— (300 hours)

= 24,000 Wh = 24 kWh.

But 1 kWh = 1 unit.

Therefore, 24 kWh = 24 units.

The cost of this power = 24 units Ã— â‚¹ 3 = â‚¹ 72.00.

When an electric iron is connected to 230 V mains, 750 W power is consumed.

- What is the resistance of its heating element?
- What current is passed through it?

Working voltage, V = 230 V

Power consumed, P = 750 W

- Let R be the resistance of the heating element
- Current through the iron,

A transformer lowers emf from 220 V to 12 V. If the number of turns in primary coil is 8800, how many turns are there in the secondary coil?

*E*_{p} = 220 V; *E*_{s} = 12 V; *N*_{p} = 8800; *N*_{s} = ?

Hence, the number of turns in secondary coil is 480.

An emf applied to a primary coil is 210 V. If the number of turns in primary coil is 200 and that in the secondary coil is 20, then find out the output voltage. Name the type of the transformer.

*E*_{p} = 210 V; *N*_{p} = 200; *N*_{s} = 20 *E*_{s} = ?

Since *E*_{s} < *E*_{p}, it is a step-down transformer.