QUESTIONS :
- What is the purpose of Fleming’s left-hand rule?
- A motor converts energy from one form to other .name the two forms in Sequence.
- A generator converts energy from one form to other .name the two forms in Sequence.
- A stream of positively charged particles are moving towards west is deflected towards north by a magnetic field. What is the direction of magnetic field?
ELECTROMAGNETIC INDUCTION :
It is the phenomenon of producing induced current in a Moving conductor or coil in a magnetic field. The current produced by moving a straight wire in a magnetic field is called induced current.
It was discovered by Faraday. The direction of the induced current is given by Fleming’s right hand rule.
The potential difference corresponding to induced current is induced potential difference (Pd) or induced electromotive force (emf). The magnitude of the induced potential difference is directly proportional to the rate of change of magnetic flux.
In figure (a) :
- When a bar magnet is pushed into the coil : The magnetic flux linked with the coil changes i.e increases. As a result of this , an induced current flows in the coil and the galvanometer shows a deflection .
- When a bar magnet is taken from the coil : The magnetic flux linked with the coil changes i.e decreases. As a result of this , an induced current flows in the coil but in a direction opposite to that in case (i) . Obviously , the galvanometer shows a deflection in the opposite direction.
Conclusion :
- Whenever there is a relative motion between a coil and the magnet, induced current flows through the coil.
- Large induced current is produced in the coil if the relative motion between the magnet and the coil is large.
(iii) When the bar magnet is held stationary inside the coil: When the bar magnet is held stationary inside the coil, there will be a magnetic flux in the coil but it will remain constant. Since the magnetic flux does not change, there is no induced current in the coil and the galvanometer shows no deflection.
Conclusion : β Induced current is produced in a coil when varying current flows through a neighboring coil.
The direction of induced current produced in a straight conductor or coil moving in a magnetic field is given by Fleming’s Right Hand Rule.
According to Fleming’s Right Hand and Rule :
Hold the thumb, the forefinger and the centre of your right hand at right angles to one another. Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of the conductor, the direction in which centre finger points, gives the direction of induced current in the conductor.
Direct current: The current which has a constant magnitude and same direction is called direct current (D.C).
The frequency of the D.C current is zero.
Sources : Dry cell , dry cell battery , car battery and DC generator
Alternating current :
The current which changes in magnitude and direction at regular interval of time is called alternating current.
Frequency of AC is the number of cycles per second made by the current.
The frequency of the alternating current in India is 50 HZ.
Sources : House generator and Bicycle dynamo.
Advantages of AC over DC :
- A.C can be transmitted over long distances without much loss of energy.
- A.C can be produced easily and cheaply than D.C.
- A. C voltage can be transformed to any desired value with help of a transformer.
- A.C can be converted into D.C when required.
Disadvantages of A.C over D.C :
- A.C is more dangerous than DC
- A.C cannot be used for electroplating, electrotyping and other electrolytic processes.
ALTERNATING CURRENT (AC) GENERATOR:
It converts mechanical energy into electrical energy.
Principle:
It works on the principle of electro magnetic induction i.e When a coil is rotated in uniform magnetic field , electric current is induced in the Coil.
Construction:
Main components of a.c generator are given below :
- Armature (abcd ): It consists of a large number of turns of insulated copper wire wound over a soft iron core.
- Field Magnet: It produces magnetic field. The armature coil rotates between the pole pieces of the field magnet.
- Slip Rings: The two ends of the armature coil are connected to two hollow metal rings. These rings rotate along with armature coil.
- Brushes or Sliding Contacts: B1 and B2 are flexible metal plates or carbon rods. These are called brushes. The brushes remain fixed while slip rings rotate along with the armature.
Diagram – Refer NCERT TEXT BOOK
Working:
As the armature coil is rotated about an axis , the magnetic flux linked with armature changes. Therefore, an induced current is produced in the armature coil.
(a) Let us suppose that the armature coil ABCD is rotating anti-clockwise so that the arm BA moves inwards and CD moves outwards. Applying Fleming’s Right Hand Rule, we find that the induced current in the armature coil and in the circuit is due to which galvanometer (G) shows deflection towards right.
(b) After the armature coil has turned through 180°, it occupies the position as shown in the fig.. With the armature coil rotating in the same direction, CD moves inwards and BA moves outwards. Thus, again applying Fleming’s Right Hand rule, we find the induced current in the external circuit flows in the opposite direction due to which the direction of deflection in galvanometer is towards left.
Domestic Electric Circuits :
It is a well known fact that the house connections to all the devices are made in parallel, each having independent switch and fuse . Thus ,whenever some fault occurs in circuit of one particular device in one room , devices in other rooms do not suffer.
Live wire (positive) – Red colour
Neutral wire – black colour
Earth wire – Green colour
Earthing : Connecting the metal case of electrical appliance to the earth by means of metal wire is called Earthing.
If the appliance is earthed , its body potential remains zero due to contact with the earth. No electric shock is felt when such an appliance is operated.
Overloading and Short- Circuiting :
Usually there are two separate circuits in a house, the lighting circuit with a 5A fuse and the power circuit with a 15A fuse.
All electrical appliances like bulbs, fans and sockets, etc., are connected tin parallel across the live wire and neutral wire.
An electric current more than the tolerable value will overheat the wire and can cause a fire.
The current may exceed the limit due to two reasons :
(i) Over-loading.
(ii) Short – Circuiting.
(i) Over-loading : The too many electrical appliances of high power rating are switched on at the same time; they draw an extremely large current from the circuit. This is known as overloading the circuit.
Prevention : To avoid over –loading , circuit is divided in different sections having its own fuse in series. Also simultaneous use of high powered appliances must be avoided.
(ii) Short – Circuiting : The touching of the live wire and neutral wire directly is known as short – Circuiting.
When the two wires touch each other, the resistance of the circuit so formed is very, very small. Since the resistance is too small, the current flowing through the wire is very large and heats the wires to a dangerously high temperature and it may lead to fire accident.
This occurs when (a) the insulation of wires is damaged and (ii) there is a fault in the electric appliance.
Prevention : To avoid short-circuiting, good quality wire must be used. Wire used must be coated with PVC.
Electric Fuse : It is a device which is used in series to limit the current in an electric circuit so that it easily melts due to overheating when excessive current passes through it, and hence the circuit gets disconnected.
It is made of a wire of an alloy of lead (75 %) and tin (25%), which melts at around 2000 C.
Few points regarding a fuse are as follows.
1. It is always connected in live wire and not in neutral wire.
2. It is always connected in the beginning of the circuit.
3. Fuses of various current capacities are available. Thicker fuse wire will always have higher current capacity.
