An electric fuse is a safety device which is used to avoid the risk of fire due to overloading and short circuiting. It is also known as safety fuse. An electric fuse consists of a thin wire of pure tin or alloy of copper and tin or lead and tin. The fuse wire is a high resistant wire having low melting point. The electric fuse is always connected to live wire.

 

Test Your Understanding and Answer These Questions:

1. What is safety fuse? What is function of safety fuse?
2. Of what substance is the safety fuse wire made? Why?

 In our homes electricity is brought from electric power station by two insulated wires of aluminium or copper. Out of these two wires, one wire is in red insulation and is called live wire and the other wire is in black insulation and is called neutral wire. The electricity coming in our house is actually alternating current having 220 V potential. Both the live wire and neutral wire enters in a box where a main fuse is put on the live wire.

These wires then pass through electricity meter present in the house. After the electricity meter, these wires enter into main switch which is used to switch off the electricity supply whenever required. After the main switch, these wires are connected to live wires of two separate circuits present in the house to supply electricity. Out of these two circuits, one circuit is of 5A rating which is used to run electric appliances with low power ( such as tube lights, bulbs and fans) and the other circuit is of 15A rating which is used to run electric appliances with high power ( such as air conditioners, refrigerators, electric iron and heaters). It should be noted that all the circuits in house are connected in parallel so that switching off one circuit does not affect the other circuit. One more advantage of parallel connection of circuits is that each electric appliance gets equal voltage.

In house, a third wire called earth wire having green insulation is usually connected to the body of metallic electric appliances. The earth wire sends the current from the body of the appliance to the earth whenever a live wire incidentally touches the body of metallic electric appliances. Thus, the earth wire is a protective measure, which saves us from severe electric shocks. Each electric appliance has a separate switch to ‘on/off’ the flow of current to it. The diagram to show the general scheme of domestic electric circuit is shown below in Figure.

Overloading and Shortcircuiting
The amount of current passing through a wire depends on the power rating of the wire. When the amount of current passing through the wire exceeds the maximum permissible limit, the wires get heated to such an extent that fire may be caused. This is known as overloading.

Shortcircuiting means coming in contact of live wire with neutral wire. This happens when the insulation of wires get damaged. Due to shortcircuiting the resistance of wires become very small which leads to flow of large amount of current through the wires. It results in heating of wires to such an extent that fire may be caused in the building.

Test Your Understanding and Answer These Questions:

1. Explain the domestic electric circuit with the help of a diagram.
2. Why different electrical appliances in a domestic circuit are connected in parallel?
3. What is the function of earth wire? Why is it necessary to earth the metallic appliances?
4. Explain what is short circuiting and overloading in an electric supply.

An electric generator is a device that converts mechanical energy into electrical energy.
Principle of Electric Generator
 An electric generator works on the principle that when a conductor placed in a magnetic field is moved then induced current is produced in the conductor in the direction given by Fleming’s right hand rule.

Construction of Electric Generator
An electric generator consists of following important parts:

1. Armature
Armature is a rectangular coil ABCD of insulated copper wire which is wrapped around a soft iron core and placed between the two poles of a U shaped permanent magnet.

2. Commutators
Commutators are two half rings made up of copper which links the brushes to the coil. In the diagram, commutator rings are represented as C₁ and C₂.

3. Carbon brushes
Electric generator consists of two carbon brush B₁ and B₂ which are pressed lightly against the C₁ and C₂ rings of the commutator respectively. The function of carbon brushes is to supply the current produced in the coil to various electrical appliances.

Working of Electric Generator
Let the coil ABCD be initially in horizontal position. Let the coil be rotated in anticlockwise direction between the poles of a horse shoe magnet so that the side AB of coil move down and side CD move up. When the coil ABCD is rotated, side AB of the coil cuts the magnetic field near the N pole of the magnet and side CD cuts the magnetic field near the S pole of the magnet. This results in production of electric current in the coil. By applying Fleming’s right hand rule, we find that the direction of current in side AB of coil is from point B to A and in side CD is from point D to C.

After half rotation, sides AB and CD interchange their positions so that AB go to right and CD go to left. Now, side AB starts to move up and CD moves down. So the direction of induced current produced in each side gets reversed after half revolution. Also the split rings C₁ and C₂ of commutator interchange their contacts with carbon brushes. Due to this, the direction of current in outer circuit remains same.

 If the turning coil of an electric generator has a commutator then the connection will be reversed each half turn, so the current produced will always flow in the same direction. Such a current which always flow in the same direction is called direct current (D.C.). On the other hand, if the turning coil of an electric generator is without a commutator then the current produced will reverse the direction every half turn. Such a current which changes direction continuously with time is called alternating current (A.C.).

Test Your Understanding and Answer These Questions:

1. What is an electric generator?
2. Explain the principle of an electric generator?
3. Explain the construction of an electric generator?
4. Explain the working of an electric generator?
5. Define alternate current.
6. Define direct current.
7. What is the difference between an electric generator and electric motor?
8. Give differences between alternate current and direct current.

The phenomenon of electromagnetic induction was discovered by Michael Faraday in 1831. He discovered that moving a wire in a magnetic field generates an electric current in the wire. This phenomenon is known as electromagnetic induction and the current so produced is called induced current. Thus, electromagnetic induction may be defined as the phenomenon of moving a wire in a magnetic field to produce electric current.

Experiment to Demonstrate Electromagnetic Induction
Take a long coil of insulated copper wire having a number of turns. Connect it to a sensitive galvanometer as shown in Fig. 14. Hold a bar magnet above the coil in such a manner that its north pole points towards the coil.

Push the north pole of the magnet in the coil in such a manner that magnet does not touch the walls of the coil. Now, you will observe that the galvanometer has suddenly deflected to the right side. This deflection indicates the production of current in the coil. Now, pull the north pole of the magnet out of the coil. In this case, you will observe that the galvanometer has deflected to the left side.

This shows that moving a wire in a magnetic field produces electric current in the wire. Similar results can be obtained if the magnet is kept stationary and the coil is moved.

Determination of Direction of Induced Current
The direction of induced current produced in a wire moving in a magnetic filed can be determined by using Fleming’s right hand rule.

Fleming’s Right Hand Rule
According to this rule: stretch the thumb, forefinger and central fingers of your left hand mutually perpendicular to each other so that the thumb points in the direction of motion and forefinger in the direction of magnetic field then the direction in which the central finger points gives the direction of induced current produced in the conductor.

Test Your Understanding and Answer These Questions:

1. Define electromagnetic induction.
2. What is Fleming’s right hand rule?
3. Which rule is used to determine the direction of induced current?
4. Name the scientist who discovered the phenomenon of electromagnetic induction.
5. Give an experiment of demonstrate electromagnetic induction.
6. A coil of copper wire is connected to a galvanometer. What would happen if a bar magnet is (a) pushed into the coil with its north pole entering first? (b) pulled out of the of the coil? (c) held stationary inside the coil?

When electric current is passed through a conductor placed in a magnetic field, it experiences a mechanical force. The force due to magnetic field acting on a conductor can be demonstrated by Kicking Wire Experiment.

Kicking Wire Experiment

Take a flexible copper wire AB and suspend it from a rigid support with the help of a flexible joint. Place a U shaped magnet around the lower end B of the wire. In the centre of the U shaped magnet place a vessel filled with mercury in such a manner that the lower end B of copper wire touches the surface of mercury. Now, connect the upper end A of wire with the positive terminal of battery and dip another wire from the negative terminal of the battery in the vessel filled with mercury. In this way the circuit gets completed. Now, insert the key. When the key is inserted, the current flows through the wire from end A to B perpendicular to the magnetic field and the wire is kicked outward. This happens because when current passes through a wire placed in a magnetic field, it experiences a force which jerks it in outward direction. It should be noted that the direction of force acting on a current carrying wire placed in a magnetic field is always perpendicular to both the direction of current and magnetic field.

You will also observe that when the direction of current flowing through wire AB is reversed, the wire will move in inward direction.

Fleming’s Left Hand Rule
The direction of force acting on a current carrying conductor can be easily determined by using Fleming’s left hand rule. According to this rule: stretch the thumb, forefinger and central fingers of your left hand mutually perpendicular to each other so that the forefinger points in the direction of magnetic field and central finger in the direction of current then the direction in which the thumb points gives the direction of force acting on the conductor.

Test Your Understanding and Answer These Questions:

1. What is Fleming’s left hand rule?
2. Explian kicking wire experiment with the help of a diagram.
3. Prove that when a current carrying conductor is placed in magnetic field it experieces a force.

 An electromagnet is a temporary magnet in which a long insulated copper wire is wrapped on a soft iron core in the form of a helix. Thus, an electromagnet is a solenoid having a core of soft iron core at its centre. When electric current in passed through it, it behaves as a magnet because a strong magnetic field is produced in it. An electromagnet is a temporary magnet because it behaves as magnet as long as the current is passed through it. Both the ends of electromagnet behave as north and south pole respectively. The polarity of an electromagnet can be reversed by reversing the flow of current through the coil. When the supply of current is stopped, the magnetism of electromagnet also disappears.

Factors Effecting Strength of Magnetic Field Produced in an Electromagnet
The strength of magnetic field produced in a solenoid depends upon following factors:

1. Strength of Current
The strength of magnetic field produced in a solenoid is directly proportional to the magnitude of current passing through it. Thus, strength of magnetic field increases if the magnitude of current passing through the solenoid is increased.

2. Number of Turns of Wire
Magnetic field can be increased by increasing the number of turns of copper wire in the solenoid.

3. Nature of Core
Magnetic field of a solenoid can be increased by using soft iron as core in the centre of solenoid.

Use of Electromagnets

       Following are the uses of electromagnets:

1. Electromagnets are used to load and transport scrap iron, steel bars and machine parts.
2. Electromagnets are used in electric bells.
3. Electromagnets are used in telegraphs to send telegrams.

Differences between a Permanent Magnet and an Electromagnet

Test Your Understanding and Answer These Questions:

1. What is an electromagnet?
2. Write various uses of an electromagnet.
3. Describe the construction and working of an electromagnet with the help of a diagram.
4. On what factors does the strength of magnetism produced by an electromagnet depend?
5. Give differences between an electromagnet and a permanent magnet.

A solenoid is a long insulted copper wire wound in the form of a helix. When an electric current in passed through a solenoid, magnetic field is produced around it. The pattern of magnetic field produced in a solenoid when an electric current is passed through it is shown in figure. It should be noted that magnetic field produced in the solenoid resembles with that of a bar magnet.

 

Factors effecting strength of magnetic field produced in a solenoid
The strength of magnetic field produced in a solenoid depends upon following factors:

1. Strength of Current
The strength of magnetic field produced in a solenoid is directly proportional to the magnitude of current passing through it. Thus, strength of magnetic field increases if the magnitude of current passing through the solenoid is increased.

2. Number of Turns of Wire
Magnetic field can be increased by increasing the number of turns of copper wire in the solenoid.

Test Your Understanding and Answer These Questions:

1. Define a solenoid.
2. Draw the pattern of magnetic lines of force due to current carrying solenoid.
3. On what factors does the strength of magnetic field produced by a solenoid depend?

To find out magnetic field due to a circular wire loop carrying electric current take a circular wire and pass it through the centre of a horizontal sheet of cardboard. Connect the ends of wire to a battery. Sprinkle some iron fillings on the cardboard sheet and switch on the circuit to pass electric current through the wire. Now, tap the cardboard gently. On tapping, you will observe that the iron fillings arrange themselves around the wire in concentric circles.

On careful observation you will find that the magnetic lines of force produced at every point of the wire are circular near the wire and straight at the centre of the coil. The strength of magnetic field is more at the centre of the coil because all the lines of force aid each other at the centre of the coil.

Test Your Understanding and Answer These Questions:

1. Describe the formation of magnetic field due to a circular wire carrying current.
2. Draw a sketch of the pattern of magnetic field due to a current carrying circular wire.