Electricity and Magnetism

(a) False
Correct Statement — A current carrying coil when suspended freely rest in north-south direction.

(b) True

(c) False
Correct Statement — In an electromagnet, the core is made up of soft iron.

(d) True

(e) True

(f) False
Correct Statement — We do not use cell as the source of electricity to run an electric immersion rod.

(g) True

(h) False
Correct Statement — Wool is an insulator of electricity.

(i) False
Correct Statement — Silver is a conductor of electricity.

(j) True

(k) True

(l) True

(m) True

(n) True

(a) Two

(b) repel, attract

(c) iron

(d) soft iron

(e) dry cell

(f) mains

(g) battery

(h) Conductors

(i) Insulators

north-south direction

Reason — The earth has a magnetic field of its own and its magnetic South pole is in geographic north and its North pole in geographic south. Since unlike poles attract so when a magnet is suspended freely, it rests in north-south direction.

soft iron

Reason — Electromagnets are temporary magnets made up of soft iron because they can easily be magnetized on passing electric current and gets de-magnetized when flow of electric current is stopped.

electric bell

Reason — An electromagnet is used in electric bell as it is a temporary magnet.

to make and break the circuit

Reason — The movement of armature towards and away from the electromagnet in an electric bell makes and breaks the circuit.

the cell

Reason — Dry cell is used as a source of electricity in a torch.

copper wire

Reason — Copper is a good conductor of electricity.

rubber

Reason — Rubber is an insulator, it does not allow the flow of electricity through it.

electricity may pass through our body

Reason — We should not touch the switch with wet hands as tap water conducts electricity. Electricity will pass through our body and we can get electric shock.

The two properties of a bar magnet are:

1. It attracts small pieces of iron i.e. it has attractive property.
2. It always rests in the north-south direction when suspended freely i.e. it has directive property.

Suspend the given rod such that it swings freely and note the direction in which the rod rests. Repeat the process two to three times. If the rod rests in a particular direction pointing north and south in each case then the rod is a magnet. But if it rests in any random direction, then it is not a magnet.

1. Take the rod and place it on a table.
2. Take a magnet and touch the rod at one end, then at another end and then in middle. If the rod sticks to the magnet at both the ends and also at the middle, then the rod is made up of iron.

Let the two identical bars be bar A and bar B.

1. Take the bar A and place it on a table.
2. Take the bar B and touch it to one end of bar A, then at another end and then in middle. If the bar B sticks to the bar A at both the ends and also at the middle, then the bar A is made up of soft iron.
3. Repeat the above process by placing the bar B on the table and touch the bar A on one end, then on another end and finally on middle of bar B. We will find that the bar A will be attracted towards both the ends of bar B, but not to the middle which shows that bar B is a magnet.

A magnet when suspended freely, always rests in geographic north-south direction, so we can easily find the north south direction of the place by seeing the position of magnet.

1. Take two bar magnets A and B.
2. Suspend one magnet A with a silk thread from a support so that it is free to swing.
3. The magnet A will come to rest in the north-south direction. The north pole of the magnet A is in north direction and its south pole is in the south direction.
4. Now hold the other magnet B in your hand and bring its north pole near the north pole of the suspended magnet A without touching the magnet.

We will observe that two poles repel each other.

5. Now bring the south pole of the magnet B near the north pole of the suspended magnet A without touching it.

We will observe that the two poles attract each other.

The above experiment shows that like poles repel and unlike poles attract each other.

If a bar magnet is broken at the middle in two parts, each part is found to be a magnet. Each part has the property to attract small iron pieces. Each part when suspended freely, rests in north-south direction. This shows that new poles are formed at each broken parts. If these parts are broken again and again, still each part will be a complete magnet with north and south poles. This shows that poles exists in pairs.

A magnetic compass is a small magnetic needle pivoted at the centre of a small brass box which has a glass top. It is used to locate directions at a place.

The space around a magnet in which the magnetic needle of the magnetic compass gets influenced is called its magnetic field.

An electromagnet is a temporary magnet made by winding wire around a soft iron core and on passing electric current it creates a magnetic field around itself and behaves like a magnet.

Soft iron

1. Take a soft iron bar PQ and wind a thin insulated copper wire around the bar.
2. Connect a cell or a battery and a key K in series between the ends of a coil as shown in the below figure.
3. When key is closed current passes through the winding of the coil and the bar becomes a magnet.
4. But when key is opened the current stops flowing in the coil and bar gets demagnetized.

So, soft iron bar behaves as an electromagnet when current passes through the coil.

Circuit diagram to make a horse shoe electromagnet is given below:

The two factors on which the strength of magnetic field of an electromagnet depends are:

1. Number of turns of the coil.
2. Strength of current passing through the coil.

The two ways by which the strength of magnetic field of an electromagnet can be increased are:

1. By increasing the total number of turns of the coil.
2. By increasing the strength of current passing through the coil.

The two common uses of electromagnets are:

1. For loading furnaces with iron.
2. It is used in electrical gadgets like electric bell, electric tram, microphone, loudspeaker, electric motor etc.

Electric bell

Labelled diagram of the electric bell is shown below:

Working of electric bell:

1. When the switch (bell push) is pressed, current flows through the coil of electromagnet.
2. The electromagnet gets magnetized and attracts the armature. As the armature moves the hammer attached to it moves and strikes the gong so the bell rings.
3. During this process the contact between the metallic springy strip and adjusting screw breaks so the circuit becomes incomplete.
4. The current stops flowing through the coil of electromagnet and it gets demagnetized.
5. As the electromagnet gets demagnetized, the armature comes back to its original position due to spring action of the metallic springy strip and comes in contact with the adjusting screw.
6. The circuit becomes complete, if the bell push is pressed again, current will flows through the coils of electromagnet and it gets magnetized.
7. It attracts the armature and hammer attached with it strikes the gong again and the bell rings.
8. This process of making and breaking the circuit continues and the bell rings as long as the bell push remains pressed.

The complete and labelled diagram of the electric bell is shown below:

Magnetic declination is the angle of the horizontal plane between the magnetic north and the geographic north (or true north).

Current is defined as the rate of flow of charge i.e., the amount of charge flowing in one second.

The four appliances which work using electricity are:

1. Electric fan
2. Electric heater
3. Television
4. Electric iron

The two sources of electricity are:

1. Electric cell and battery
2. Generator

A battery is a group of two or more cells and it is used where we require more electricity.

The path along which the electricity flows is called the electric circuit.

1. Take about 1 metre long insulated metal wire and cut it into two halves by a blade.
2. Remove the plastic covering at the ends of each wire.
3. Take a torch bulb and with help of cello tape fix one end of each wire to each terminal of the bulb.
4. Take a cell.
5. Fix the other ends of the wires to the terminals X and Y of the cell.

We will see the bulb will glow.

Electricity flows from terminal X of the cell to the bulb through wire 1, then through filament of the bulb to wire 2 then to terminal Y of the cell. As the circuit is complete or closed the electricity flows along the circuit and the bulb glows.

Now remove the wire 1 from the bulb connected to terminal X of the cell as shown below.

We will see bulb will not glow.

As wire 1 is removed so circuit is incomplete or open, so electricity does not flow, hence bulb will not glow.

The above experiment show that electricity flows only if the circuit is complete and it does not flow if the circuit is incomplete.

Diagram showing a complete circuit to light the bulb is given below:

The bulb will glow in third case i.e. One terminal of the cell is joined to one terminal of the bulb and other terminal of the cell to the other terminal of the bulb.
In this case the circuit is complete allowing the flow of electricity so bulb will glow.

The bulb will glow in circuit A as both the wires i.e. copper and aluminium are good conductors of electricity. But in other circuits B, C and D silk, cotton etc. are used which are insulators i.e. they do not allow the flow of electricity through them. So bulbs do not glow in circuits B, C and D.

The direction of current is shown in the below diagram:

(i) Switch

Symbol:

Function:
A Switch is used to put the current on and off in a circuit.

(ii) Cell

Symbol:

Function:
A cell acts as a source of direct current for the circuit.

Circuit diagram for a bulb connected to a cell with a switch with direction of flow of current indicated by arrows is shown below:

Appliances are connected in parallel arrangements in the electric circuit of our homes because of the following reasons:

1. Each appliance works independently.
2. If one appliance is out of order, other appliances are not affected.
3. Each appliance works on its own without interruption of other appliances.

The two precautions that must be taken when switching on an electric circuit are:

1. Do not touch the switch or any component with wet hands.
2. Connecting wires should not be naked, they should be properly insulated everywhere.