PHYSICS [2 Mark Question Answer]

Modern appliances like refrigerators make use of significantly less energy than older appliances as they have star rating according to their efficient use of electricity. Higher the star rating, higher is the efficiency.

By properly insulating a home, it is possible to maintain a comfortable temperature inside. It will reduce the cost of heating devices in winter and cooling devices in summer.

Two ways to save energy:

1. Instead of fossil fuels, other renewable sources of energy such as the biogas prepared from animal dung should be used.
2. The use of hydroelectric energy, wind energy etc. should be given priority.

The S.I. unit of resistance is 'ohm' (volt per ampere).
The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it when the potential difference across it is 1 volt.

A metal wire has free electrons which move in random directions. When the ends of the wire are connected to a cell, the electrons start moving from the negative terminal of the cell to its positive terminal through the metal wire. During their movement, they collide with the free electrons and fixed ions of the wire. This causes them to lose their speed and change their direction. As a result, the electrons slow down and slowly drift towards the positive terminal. Thus, the wire offers resistance to the flow of current (or electrons) through it.

Potential difference between two points is 1 volt; it means 1 joule of work is done in transferring 1 coulomb of charge from one point to the other point.

S.I. unit of potential difference is volt (joule per coulomb). Potential difference between two points is said to be 1 volt if work done in transferring 1 coulomb of charge from one point to the other point is 1 joule.

I = 0.2 A R = 15 V According to Ohm's law , V = IR ∴ V = 0.2 × 15 = 3.0 V

Potential difference: The amount of work done in moving a unit positive charge from one point to another point in an electric field is called potential difference.

Electric potential difference between two conductors is equal to the work done in transferring a unit positive charge from one conductor to other conductor.

When both the conductors are joined by a metal wire:
(i) Electrons will flow from A to B.
(ii) Current will flow from B to A.

(a) Resistance of a wire is directly proportional to the length of a wire; so if the length is doubled, resistance is also doubled.
(b) Resistance of a wire is inversely proportional to the area of cross-section the wire. Thus, if radius is doubled, area increases four times and hence the resistance becomes one-fourth.

Resistance of a wire depends upon:
(i) Length of wire: Resistance is directly proportional to the length of a wire.
(ii) Area of cross-section of wire: Resistance is inversely proportional to the area of cross-section the wire.
(iii) The temperature of wire: Resistance of a wire is directly proportional to the temperature of the wire.

Ohm's law states that the electric current flowing through a metallic wire is directly proportional to the potential difference V across its ends provided its temperature remains the same.

Secondary cells are cells which provide current as a result of reversible chemical reactions. It converts electrical energy into chemical energy when current is passed in it (i.e. during charging), while it converts chemical energy into electrical energy when current is drawn from it (i.e., during discharging). Example: Lead (or acid) accumulator.

Primary cells are cells which provide current as a result of irreversible chemical current. Examples: Simple Voltaic cell and Leclanche cell.

Two kinds of cells:
(a) Primary cell: e.g. Leclanche cell
(b) Secondary cell: Lead (or acid) accumulator

Conductors have a large number of free electrons and they offer a very small resistance in the path of current but insulators have no free electrons and they offer a very high resistance in the path of current.

The substances which allow electric current to flow through them easily are called conductors. Examples: Impure water and metals.
The substances which do not allow the electric current to flow through them are called insulators. Examples: Rubber and wood.

(a) (b)
(c) A rheostat is used to control the current in the circuit.

Voltmeter : ;
It is used to measure the potential difference between two points of a circuit.

Ammeter : ; It is used to measure the current flowing through the circuit.

Rheostat symbol : It is used to control the current in the circuit.

Cell symbol : ; It acts as a source of direct current for the circuit.

Key symbol : ; It is used to switch the current on or off in the circuit.