Short Answer -Class 10 -Science- Physics-Chapter 12:Electricity

Class 10 -Science-Short Answer – Physics-Chapter 12:Electricity

Q1. What is meant by “electrical resistance” of a conductor? State how resistance of a conductor is affected when

a. a low current passes through it for a short duration;

b. a heavy current passes through it for about 30 seconds.

 Ans : Electrical resistance is the property of a conductor by virtue of which it opposes the flow of current through it. It is equal to the ratio of the potential difference applied across its ends to the current flowing through it. R = (V/I)

a. When a low current is passed for a short duration, through a conductor, heat produced is almost negligible and hence no appreciable change in its resistance.

b. When heavy current is passed through the conductor for 30 s. Conductor may be get heated and its resistance and resistivity change.

Q2. (a) List the factors on which the resistance of a conductor in the shape of a wire depends.

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

(c) Why are alloys commonly used in electrical heating devices? Give reason.

Ans : a. Factors on which resistance of a wire depends:

i. Resistance is directly proportional to length.

ii. Resistance is inversely proportional to area of cross-section.

        R ∞ l, R ∞ 1/A

        R ∞ l/A or R = ρ (l/A)

b. Metal are good conductor due to having large number of free electrons and their low resistivity. Glass is a bad conductor because it has no free electrons and its resistivity is higher.

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

Q3. (a) Explain why a conductor offers resistance to the flow of current.

(b) Differentiate between conductor, resistor and resistance.

Ans : a. When a current is passed through a conductor, the atoms or molecule of the conductor produce an hindrance in the path of flow of electron. This hindrance in the path of flow of charge is called resistance of the conductor.

b. A substance which allow to pass the charges through them easily is called a conductor.

Resistor : A conductor having some value of resistance is called a resistor. 

Resistance : It is the property of any conductor by virtue of which it opposes the flow of charge through it.

Q4. A wire of length l and resistance R is stretched so that the length is doubled and area of cross-section is halved. How will its:

 a. Resistance change?

b. Resistivity change?

Ans :

 a. Let initial length, area of cross-section and resistance of wire are l, A and R respectively. When length is stretched two times, let its area of cross-section becomes A’. Its initial volume of wire = final volume of the wire.

Al = A’ 2l

A’/A = ½ or A’ = A/2

R’ = ρ ((2l) /A’) = ρ ((2l x 2)/ A)

= 4ρ (l/A)

= 4 R

Resistivity does not change because it is property of the material of a conductor.

Q5. (a) Why are copper or aluminium wires generally used for electrical transmission and distribution purposes?

(b) Two wires, one of copper and other of manganin, have equal lengths and equal resistances. Which wire is thicker? Given that resistivity of copper is lower than that of manganin.

Ans :  a. Copper or aluminium wires are used for transmission and distribution of electricity due to their low resistivity and high conductivity.

b. we know that R =  ρ (l/A)

               ρ  ∞ A

 Thicker the wire, more the resistivity. The resistivity of manganin is more than copper. So manganin wire is thicker than copper.

Q6. What do you mean by heating effect of electric current? Explain the production of heat in a resistor by flow of electric current through it. Name two devices based on heating effect of current.

Ans : When current is passed through a conductor, heat is produced. Production of heat in a conductor on passing the current is called heating effect of current. When a current is passed in a conductor by applying a potential difference, electrons get accelerated and collide with the atoms of the conductor. During the collision there is a loss of kinetic energy. This loss in K.E. appears in the form of heat energy in the conductor 1. Electric heater, 2. Bulb.

Q7. (a) Why is the series arrangement not used for domestic circuits?

(b) Why is the tungsten used almost exclusively for filament of electric lamps?

(c) Why are the conductors of electric heating devices such as bread toasters and electric irons made of an alloy rather than a pure metal?

(d) Why are copper and aluminium wires usually employed for electricity transmission?

(e) Why does the cord of an electric heater not glow while the heating element does?

Ans :  a. In the series arrangement if any device fails to work, circuit will be broken and other devices stop working. In series combination current remains same. Where as different devices requires different current.

b. Tungsten is used due to its high resistivity and high melting point.

c. Alloys have higher resistivity than their pure constituent and hence produces more heat.

d. Copper and aluminium are good conductors and have low values of resistivity so these are used for electric transmission.

e. Cords core made up of good conductor with low resistivity where as heating elements are made up of alloys of higher resistivity to produce more heat