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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 × 10–19 C


Mass of the electron = 9.1 × 10–31 kg


Gravitational constant = 6.67 × 10–11 Nm2 kg–2


Let the distance between two electrons be d.


Then the gravitational force of attraction between them is given by Description: 92054.png.


The electrostatic force of repulsion between them is Description: 92065.png.


Description: 92075.png


Description: 92083.png


Description: 92092.png


Description: 92100.png



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 × 10–9 C.


From Coulomb’s inverse square law: Description: 92110.png


The magnitude of charge on electron and proton is the same = 1.6 × 10–19 C.


Therefore, electrostatic force is given by


Description: 92120.png


Description: 92134.png


Description: 92144.png


= 0.9216 × 10–29 × 10+22


= 0.9216 × 10–7 N


= 9.216 × 10–8 N.



Calculate the electric current in the circuit shown below.


Description: 100125.png


Any current that passes through the 10 Ω resistor also passes through 5 Ω, i.e., 10 Ω and 5 Ω are connected in series. Their equivalent resistance is Req = 10 Ω + 5 Ω = 15 Ω.


Description: 100135.png

The equivalent circuit is shown in the above figure.


The current Description: 92169.png.



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


Description: 97125.png

Description: 100158.png
The current in the 10 Ω resistance is
Description: 92172.png
The current in the 20 Ω resistor is
Description: 92191.png




The electricity tariff in a town is ₹ 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) × (30 days) = 300 hours.


Energy consumed = (80 W) × (300 hours)


= 24,000 Wh = 24 kWh.


But 1 kWh = 1 unit.


Therefore, 24 kWh = 24 units.


The cost of this power = 24 units ×  3 =  72.00.



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

  1. What is the resistance of its heating element?
  2. What current is passed through it?

Working voltage, V = 230 V


Power consumed, P = 750 W

  1. Let R be the resistance of the heating element
    Description: 92210.png
    Description: 92217.png
  2. Current through the iron, Description: 92229.png
    Therefore, the current that is passed through the iron is 3.26 A.
    Resistance of the heating element of the iron is 70.55 Ω.



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?


Ep = 220 V; Es = 12 V; Np = 8800; Ns = ?


Description: 92246.png


Description: 92256.png


Description: 92265.png


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.


Ep = 210 V; Np = 200; Ns = 20 Es = ?
Description: 92275.png


Description: 92283.png


Description: 92293.png


Since Es < Ep, it is a step-down transformer.

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