Physics [3 Mark Question Answer]

(i) On resistance of wire.
(ii) On strength of current through the wire.
(iii) On time for which the current flows in the wire.

Electric power is the rate of doing work or it is the work done in 1 second.
The S. I. unit of power is ‘Watt’.
If a current of 1 ampere flows through a metallic conductor at a potential difference of 1 volt, the power is said to be 1 watt.
or 1 Watt = 1 Volt × 1 Ampere.

The split ring atts as a commutator in a d.c. motor. With the split ring, the direction of current through the coil is reversed after every half rotation of coil and thus the direction of couple rotating the coil remains unchanged and the coil continues to rotate in the same direction.

We know, Current = Voltage/Resistance
For a given bulb, the filament would have a definite characteristic resistance. Hence
(i) When the bulb is connected to a lower voltage supply, it would draw less current than its ‘proper current’. Hence it would not glow to its full capacity.
(ii) When the bulb is connected to a higher voltage supply, it would draw more current than its ‘proper current’. Hence its filament would get heated to a temperature higher than its proper permitted limit and is, therefore, likely to melt or fuse.

The two main precautions to be observed are:
(i) Never try to find the temperature of water by dipping your fingers in water because some times when water comes in contact with the live or neutral wire, there is a possibility of getting an electric shock.
(ii) The heater must be surrounded by water to the desired level before it is switched on. If not,
so the heat produced in the nichrome wire will not dissipate easily and so the heating element will melt.

Cartidge type fuse are used to provide additional safety to an electric appliance. The fuse wire used must have a lower rating than the maximum current which the appliance or the circuit can withstand. Hence, the fuse blows off just the current nearly approaches the maximum value and thus it protects the appliance from any damage done due to heating.

Under any circumstances, fuse wire always be connected in a live wire and never in neutal wire. The reason is that in the case of over-loading, if fuse wire is connected in neutral wire, it will certainly melt and break, but the electric appliance still be in contact with the live wire, so if any person happens to touch the appliance, he will get a shock which may prove fatal.

Dual switches are double pole type switches. With such switches an appliance, e.g., and electric bulb can be switched off or on from two different places. They are generally used in staircases, long corridors etc.

The earth pin, which is thicker and longer than the other two pins; first connects the appliances to the earth and ensures safety from an electric shock to the user. The live pin on the right and the neutral pin on the left connects the appliance to the live wire and the neutral wire of the circuit at the supply voltage.

A switch should not be touched with wet hands. If water reaches the live wire, it forms a conducting layer between the hand and the live wire of the switch through which the current passes to the hand and the person may get a fatal shock.

If switch is connected in the neutral wire, an electric appliance, e.g., an electric iron will remain connected to the live wire even when the switch is off and so if any one happens to touch the appliances; he of she will get a shock which is sometimes fatal.

A switch is an on-off device for current flowing in a circuit or in an electric appliance. The switch should always be connected in the live wire. Its main function is either to connect or disconnect an electric appliance in an electric circuit.

‘MCB’ stands for ‘miniature circuit breaker’. This is a device that automatically ‘switches off the mains supply if the current drawn Exceeds the prescribed upper safe limit. It can be ‘reset’ with the help of a flip-over switch. It protects the entire circuit and appliances against short circuiting, etc.

Mica is preferred to other insulators in an electric iron because mica is a bad conductor of electricity, but it easily conducts away heat. So when current flows through nichrome wire, the heating element is fully protected against any leakage of electric current, whereas heat is easily conducted.

In an electric oven, the china clay plate is provided either with lateral grooves or grooves in the form of a spiral helix. This type of spiral winding reduces the area of spread of nichrome wire and helps to concentrate heat in a small region.

The leads are much thicker than the filament wire. Thus the resistance of the leads is much smaller than that of filament wire. So with the same current, the filament gets heated while the leads are not.

Kilowatt-hour is a commercial unit of electric energy consumed. It is also called B.O.T. (Board of Trade) unit and is used throughout the world in calculating the amount of electric energy consumed. kWh is defined as the amount of electric energy consumed by an electric appliance of power 1kW for one hour.

Earthing of an electrical appliance meant the outer metallic case of appliance is connected with earth, wire which is welded to the end of a copper rod and it is burned in the ground.
It save the appliance during short-circuiting by passing Excessive current to earth.

We would prefer the parallel wiring. This is done for the following reasons:
(i) In parallel wiring, each lamp can have its independent switch and the disconnection (putting off) of one of them will not effect the other lamps.
(ii) All lamps have the same potential difference (equal to that of the mains) across the terminals and are thus able to work as per their specified rating.

In this position, when current passes through the nichrome wire, it becomes red hot and the concave reflector helps in spreading the heat radiation in the room because it forms a divergent beam on account of the heating element being placed between the pole and the principal focus of the reflector.

Nichrome and manganin are the common alloys that are mostly used for making heating elements.
Nichrome is: 60% nickel + 25% iron + 15% chromium.
Manganin is: 83% manganese + 13% copper + 4% nickel.

(i) They are used in light houses.
(ii) They are used in search lights and cinema projection appliances.
(iii) They are used in electric furnace and electric welding.

The three main characteristics of an heating element are:
(i) It should have a high resistance so that a major amount of electric energy is converted into heat energy.
(ii) It should not get oxidised easily up to a temperature of 1000°C.
(iii) It should have a high melting point and a low thermal Expansion so that it does not get loosened on getting heated.

Watt hour is a smaller commercial unit of electric energy, which is spent by an electric appliance of 1 watt power in 1 hour, i.e.
1 watt hour = 1 watt × hour = 1 watt × (60 × 60)s
= 3600 joule.

The three advantages of ring system are:
(i) Length of wiring used is much less and so less costly, since a single ring feeds the whole house.
(ii) In case of short-circuit, only the fuse of that particular appliance blows off, diconnecting it from the electric circuit; while the circuits of other appliances remain unaffected.

The three main disadvantages are:
(i) Longer length of wire is required for wiring different rooms in a house. Since all circuits start from the main distribution board and so it is comparatively more Expensive.
(ii) If fuse blows off in any part of the building, it disconnects all the appliances in the circuit.

The two main systems of wiring are:
(i) the tree system, and
(ii) the ring system.
Out of these systems, the ring system is better. The reason is that this system of wiring is cheaper because a low rating wire can be used for connection of appliances.

The metallic wire gets heated up on passing an electric current through it due to collisions of free electrons and energy is transferred to the positive ions which begin to vibrate more violently, causing to produce heat. It is possible to melt the wire by passing a heavy current. The wire will melt when its temperature due to heating reaches its melting point.

Given: E = 6V, r = 2Ω, i = 0.5A, t = 10 minute = 10 × 60s = 600s.

The total energy supplied by the battery = Eit

= 6 × 0.5 × 600 = 1800 J

Step-up transformer. Step-up transformer works on the principle of electromagnetic induction.

No. The wire used in set B must be thicker and of better insulation than that used in set A. The reason is that in set B, the appliances will draw heavy current (= 15A) when they are used, whereas in set A, the appliances will draw ordinary current (= 5A) when they are in use. Therefore, the resistance of wire used in set B must be less than that of wire used in set A. A thick wire has less resistance than a thin wire.

The earth pin being thicker can never enter live or neutral sockets thus a given electric appliance gets connected only in one proper electrical position, where the live wire, neutral and earthing are connected correctly.
The earth pins being longer, get connected to earth terminal first, thus ensuring that the user will not get an electric shock, because the current will melt the fuse in case the electrical appliance is short-circuited.

The labelled diagram of the circuit is given below.

The different appliances have been connected to the mains in parallel.

The labelled diagram of a three-pin plug top is shown below.

The pin marked as E is connected to the outer metallic case of the appliance. The fuse is connected to the live line L.

A three-pin socket-outlet is shown in the figure.

E—Earth
N—Neutral
L—Live wire.