# NCERT Science Class 10 Chapter 16 Electricity CBSE Board Sample Problems Long Answer

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## Question

**(a) Define electrical energy with S.L unit?**

**(b)A house hold uses the following electric appliance;**

(i)Refrigerator of rating 400w for ten hour each day.

(ii)Two electric fans of rating 80w each for twelve hours each day.

(iii)Six electric tubes of rating 18w each for 6hours each day.

Calculate the electricity bill of the household for the month of June if the cost per unit of electric energy is Rs. 3.00.

### Solution

(a) The work done by a source of electricity to maintain current in a circuit is known as electrical energy. Its S. I. unit is joule.

(b)(i) Electricity consumed by refrigerator ¡n one day = power time

(ii)Electricity consumed by 2 electric fans in 1 day = power

Time

(iii)Electricity consumed by 6 electric tubes in 1 day

Total energy consumed in one day

Total energy consumed in one month

Cost of 1 unit (kwh) = Rs 3.00

Cost of 197.04 kwh

Electricity bill =Rs 591.12

## Question

**When we say the resistors are in parallel? Write the expression for the resistors in parallel. How many** **resistors (in parallel) are required to carry 5 A on a 220V line?**

### Solution

When we can say that resistors are in parallel if same potential difference is applied across them.

For three resistors, if we connect them in parallel, the equivalent resistance is R

For x number of resistors of resistance 1 76C2, the equivalent resistance of the resistors connected in parallel is given by

Ohm’s law as

Where,

Supply voltage, V = 220 V

Current, I = 5 A

Equivalent resistance of the combination = R,

Given as,

From Ohm’s law,

## Question

**State and derive joule’s law. An electric iron consumes energy at rate of 420w when heating is at maximum rate and 180 w when heating is at minimum. The voltage is 220V .What is the current and resistant in each case?**

### Solution

Joules Law states that the heat produced in the conductor ¡s always directly proportional to:

1) The square of the current flowing through the conductor.

2) Resistance of the conductor.

3) Time for which the current flows through the conductor.

Deriving Joules Law

Potential difference

Now

By ohm’s law

Therefore

But as all the work done to move unit charge offered resistance gets converted into heat energy

Now 1) In the first case

Energy rating means Power.

Now Power = 420 W

Potential difference= 220 V

Now

Amperes

Now,

Or, R = 220*11/21 ohms

2) In the second case

Power = 180 W

Potential difference= 220 V

Amperes

Now,

ohms.

## Question

**What is heating effect of electric current? Find an expression for amount of heat produced. Name some appliances based on heating effect of current.**

### Solution

When a current flows through a conducting wire, heat is developed and temperature of wire rises. It is known as the heating effect of electric current.

Work done for flow of Q charge

This work done is converted into heat hence, heat produced (joule’s Law of Heating)

Electric Iron, Electric Stove, Toaster, Geyser, Electric room heater, are the appliances based on the heating effect of electric current

## Question

**(i) Why are electric bulbs filled with chemically inactive nitrogen or argon?**

(ii) What is meant by the statement that the rating of a fuse in a circuit is 5 A?

(iii) Derive relation between 1 kWh and SI unit of energy.

(iv)Define heating effects of current.

(V) An electric bulb with 60 W rating operates for lo hours and an electric fan with 100 w can run for 7 hours. Calculate the total units consumed in one day.

### Solution

(i) To prevent oxidizing the filament due to high temperature or to prolong the life of the filament

(ii) The current in the fuse wire must not exceed 5 A otherwise it will melt.

(iii) Relation between kWh and joules:

(iii) When electric current passes through a high resistance wire, the wire becomes and produces heat. This is called heating effect of current. According to joule’s Law of Heating,

Heat produced in a resistor is

(a) Directly proportional to the square of current for a given resistor.

(b) Directly proportional to resistance of a given resistor.

(C) Directly proportional to time for which current flows through the resistor.

(iv) We know

**Electric Bulb:** Energy consumed = power x time

**Electric fan:** Energy consumed

Total energy consumed in 1 day kWh

= 1.3 units (since 1 unit = 1 kWh)

## Question

**Define electric power. Derive an expression for power in terms of current and resistance. An electric bulb is connected to a 200 V generator. The current is 0.5 A. What is the power of the bulb?**

### Solution

Electric power is the rate at which electrical energy is consumed is called electric power.

Power

Power