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

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Question

Explain the following:

(a)Why is the 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?

Solution

(a) For filament of electric lamp we require a strong metal with high melting point.

Tungsten is used exclusively for filament of electric lamps because its melting point is extremely high.

(b) Conductors of electric heating devices are made of an alloy rather than a pure metal due to high resistivity than pure metal and high melting point to avoid getting oxidized at high temperature.

(c) Series arrangement is not used for domestic circuits as current to all appliances remain same in spite of different resistance and every appliance cannot be switched on! Off independently.

Question

A 4 resistance wire is doubled on it. Calculate the new resistance of the wire.

Solution

We consider wire is doubled on it means to fold the length of wire. It means its length will get half and area of cross section will get doubled.

Let the resistance of the wire originally ‘R’ of length ‘L’ and area of cross-section ‘A’ with resistivity of material is ‘p’,

Then Now, for new arrangement,

Thus,

Question

Two identical resistor each of resistance 10 are connected in

i) Series and then

ii) in parallel, in line to a battery of 6 volts.

Calculate the ratio of power consumed in the combination of resistor ¡n the two case

Solution

Net resistance in series = 2O and power

Net resistance in parallel and power

Question

An electric kettle of 2 kW works for 2 H daily. Calculate the

(i) Energy consumed in SI and commercial unit,

(ii) Cost of running it in the month of June at the rate of Rs. 3 per unit

Solution

(i) Energy consumed

(ii) Cost Rs 3 per unit

Question

A torch bulb is rated 5V and 500 mA. Calculate (i) 1t5 power, (ii) its resistance and (iii) the energy consumed if this bulb is lighted for 4 hours

Solution

(i) Power P VI 2.5 W

(ii) Resistance

(iii) Energy Consumed

Question

Two resistors, with resistances 5 and 10 respectively are to be connected to a battery of emf 6 V so as to obtain:

(i) Minimum current flowing

(ii) maximum current flowing

(a) How will you connect the resistances in each case?

(b) Calculate the strength of the total current in the circuit in the two cases

Solution

Here

(a) (i) For minimum current flowing in the circuit resistances and should be connected in series

(ii) For maximum current flowing in the circuit resistance and should be connected in parallel.

(b) (i) In series combination

(ii) In parallel combination

Question

Two wires of equal length, one of copper and the other of manganin (an alloy) having the same thickness. Which one can be used for (i) electric transmission lines (ii) electrical heating devices? Why?

Solution

(i) Copper is used for electric transmission lines because its resistivity and consequently resistance is small and power loss in transmission lines is small

(ii) Manganin alloy is used for electrical heating devices because its resistivity is high and it does not oxidise easily on heating.

Question

(i) Why are metals good conductors of electricity whereas glass is a bad conductor of electricity? Give reason.

(ii) Why are alloys commonly used in electrical heating devices?

Give reason.

(iii) List the factor on which resistivity depends.

Solution

  • Metals are good conductors as they have large number of free electrons and have low resistivity.

  • Whereas, Glass ¡s a bad conductor because ¡t has no free electron and its resistivity is higher.

  • Alloys are commonly used in electrical heating devices due to their high resistivity and high melting point which produces more heat.

  • Factors on which resistivity depends are:

    • Nature of material

    • Temperature

Question

Why is parallel arrangement used in domestic wiring?

Solution

When devices are connected in parallel, following benefits are observed:

(i) All appliances can be operated at same voltage as electric supply.

(ii) If one appliance stops working, or goes out of order, then all other appliances keep on working.

(iii) All devices can be operated independently with separate switches.

Question

Two bulbs, one rated loo w at 220 V and other 200 w at 220 V are connected

In series

In parallel to the electric main supply of 220 V.

Find the current drawn in each case.

Solution

Resistance R of each lamp from given power P and operating voltage V is obtained from the relation,

Hence resistance of

Resistance of 200 W, 220 V lamp

Series: Equivalent resistance

Current drawn by lamps that are connected in series

Parallel: Current drawn by 100W lamp

Current drawn by 200 W lamp

Total current drawn by lamps when they are connected in parallel

Question

The following questions consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below:

(i) Both A and R are true and R is correct explanation of A.

(ii) Both A and R are true and R is not correct explanation of A.

(iii) A is true but R is false.

(iv) A is false but R is true

Assertion (A): Electric appliances with metallic body have three connections, whereas an electric bulb has a two pin connection.

Reason(R): Three pin connections reduce heating of connecting wires.

Solution

(C), Here the third pin is the earth wire which helps in passing extra charges to the earth hence prevent electrical appliances from shock. It has no relation to the heating effect.

Question

The following questions consists of two statements – Assertion (A) and Reason (R). Answer these questions selecting the appropriate option given below:

(i) Both A and R are true and R is correct explanation of A.

(ii) Both A and R are true and R is not correct explanation of A.

(iii) A is true but R is false.

(iv) A is false but R is true

Assertion (A): The conducting wires are made of copper.

Reason(R): The electrical conductivity of copper is very high.

Solution

(a), Copper conducts current due to high conductivity.

Question

(a) A current of 1A flows in a series circuit containing an electric lamp and a conductor of 5 when connected to a 10V battery.

Calculate the resistance of the electric lamp.

(b) Now if a resistance of 10 is connected in parallel with this series combination, what change (if any) in current flowing through 5 conductor and potential across the lamp will take place? Give reason.

Solution

(a)Total current in the circuit,

Since 50 and lamp are in series,

P.d across lamp=I

(1/2)

(b) When 100 resistance is connected in parallel with R

Total resistance (R’) in the circuit is given by

Current through the circuit,

(1/2)

Since 10 and ) are in parallel, current through R i.e

There is no change in current through conductor of.

Father, P.d across the lampi.e., there is no change in p/d across the lamp.

(1)

Question

In a given ammeter, a student sees that needle indicates 17 divisions in ammeter while performing an experiment to verify Ohm’s law. If ammeter has lo divisions between 0 and 0.5 A, then what is the value corresponding to 17 divisions?

Solution

An ammeter has lo divisions between O to 0.5 A. So, 1 division