ELECTROMAGNET AND PERMANENT MAGNET
Electromagnet :-
An electric current can be used for making temporary magnets known as electromagnets. It works on the magnetic effect of current. It consists of a long coil of insulated copper wire wound on a soft iron core. To make an electromagnet all that we have to do is to take a rod NS of soft iron and wind a coil C of insulated copper wire round it. When the two ends of the copper coil are connected to a battery, an electromagnet is formed. It should be noted that the solenoid containing soft iron core in it acts as a magnet only as long as the current is flowing through the solenoid. If we switch off the current in the solenoid, it will not behave as a magnet. All the magnetism of the soft iron core disappears as soon as the current in the coil is switched off. A very important point to be noted is that it is the iron piece inside the coil which becomes a strong electromagnet on passing the current.
Factors affecting the strength of Electromagnet :-
The strength of an electromagnet depends on :
- The number of turns in the coil. If we increase the number of turns in the coil, the strength of the electromagnet increases.
- The current flowing in the coil. If the current in the coil is increased, the strength of electromagnet increases.
- The length of air gap between the poles. If we reduce the length of air gap between the poles of an electromagnet, then its strength increases.
Uses of Electromagnets :
- Electromagnets are used in electrical devices such as an electric bell, an electric fan, telegraph, an electric train, an electric motor, generator, etc.
- For lifting and transporting large masses of iron in the form of girders.
- In medical practice for removing pieces of iron from wounds.
Permanent Magnets :
Permanent magnets are usually made of alloys such as: Carbon steel, Chromium steel, Cobalt steel, Tungsten steel, and Alnico (Alnico is an alloy of Aluminium, Nickel, Cobalt and Iron). Permanent magnets of these alloys are much stronger than those made of ordinary steel.
Uses of Permanent Magnets :
- Electric meters (galvanometers, ammeters, voltmeters, speedometers, etc.)
- Microphones, Loudspeakers
- Electric clocks
S.No Permanent bar magnet Electromagnet :
- It is a permanent magnet It is a temporary magnet. Its magnetism is only for that duration till the current flows through it.
- It produces a weak magnetic field It produces a strong magnetic field.
- Its strength cannot be changed Its strength can be changed.
- The north – south polarity of a permanent magnet is fixed. The north- south polarity of an electromagnet can be changed by changing the direction of current in the coil.
Force acting on a current carrying conductor in a magnetic field :
When a current carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which can make the conductor move.
The direction of force acting on a current carrying wire placed in a magnetic field is:
- Perpendicular to the direction of current
- Perpendicular to the direction of the magnetic field.
It should be noted that the maximum force is exerted on a current carrying conductor only when it is perpendicular to the direction of the magnetic field.
The direction of force on a current carrying conductor placed in a magnetic field can be reversed by reversing the direction of current flowing in the conductor. The direction of force acting on the current-carrying conductor can be found using Fleming’s left-hand rule.
According to Fleming’s left-hand rule :
Hold the forefinger, the centre finger and the thumb of your left hand at right angles to one another. Adjust your hand in such a way that the forefinger points in the direction of the magnetic field and the centre finger points in the direction of current, the direction in which thumb points, gives the direction of force acting on the conductor.
Magnitude of Force: F=I L B
Where
B= magnitude of magnetic field
I = current flowing in the wire
L= length of the current-carrying wire placed in the magnetic
ELECTRIC MOTOR :
It converts electrical energy into mechanical energy.
Principle of a motor :
When a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously.
Construction :
Main components of electric motor are given below :
- Armature: It consists of a large number of turns of insulated copper wire wound over a soft iron core.
- Field Magnet: It produces magnetic field
- Split-Ring or Commutator: These are two halves of the same metallic ring. The ends of the armature coil are connected to these halves which also rotate with the armature.
- Brushes or Sliding Contacts: These are two flexible metal plates or carbon rods which are so fixed that they constantly touch the revolving Commutator.
- Battery: It is connected across the brushes. This battery supplies the current to the coil
β Diagram Refer NCERT TEXT BOOK
Working :
- Let us suppose that the battery sends current to the armature in the direction South (S) to North (N). Applying Fleming’s Left-Hand Rule, we find that the arm BA experiences a force which is acting outwards and perpendicular to it and arm CD experiences a force which is acting inwards and perpendicular to it. These two forces form a couple, makes the armature rotates in the anti-clockwise direction.
- After the armature has completed half a revolution the direction of current in the arm BA and CD is reversed. Now arm CD experiences an outward force and arm BA experiences an inward force. The armature thus continues to rotate about its axis in the same anti-clockwise direction.
The speed rotation of the motor can be increased by
- Increasing the current through the armature.
- Increasing the number of turns in the coil of armature.
- increasing the area of the coil
- Increasing the strength of the magnetic field.
Uses of Electric Motors :
- They are used in electric fans for cooling and ventilation.
- They are used for pumping water.
- They are used in electric locomotives, electric cars, electric cranes and electric lifts.
- Small motors are used in various toys.
- Used in Washing machine.
