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Wednesday, 1 July 2015

Magnetic effects

Main topics with explanation 

Magnet:
 (i) is an object that attracts objects made of iron, cobalt & nickel.
 (ii)Comes to rest in North-South direction, when suspended freely.

Magnets are used:
 (i) In radio & stereo speakers,
 (ii) In refrigerator doors,
(iii) in audio & video cassettes players,
 (iv) On hard discs & floppies of computers &
 (v) in children‘s toys.

Magnetic field: The area around a magnet where a magnetic force is experienced is called a magnetic field. It is a quantity that has both direction & magnitude.

Magnetic field lines: Magnetic field is represented by field lines. They are lines drawn in a Magnetic field along which a North magnetic pole moves. Magnetic field lines are called as Magnetic lines of force.

Properties of Magnetic field lines:
(i) They do not intersect each other.
(ii) It is taken by convention that magnetic field lines emerge from North pole and merge at the South pole. Inside the magnet, their direction is from South pole to North pole. Therefore magnetic field lines are closed curves.
 Magnetic field lines due to a current through a straight conductor (wire)- consist of series of concentric circles whose direction is given by the Right hand thumb rule.

Right hand thumb rule: If a current carrying straight conductor is held in your right hand such that the thumb points towards the direction of current, then the wrapped fingers show the direction of magnetic field lines.
 Magnetic field lines due to a current through a circular loop
 The strength of the magnetic field at he centre of the loop(coil)depends on:
(i) The radius of the coil- The strength of the magnetic field is inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the centre decreases.
(ii) The number of turns in the coil: As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus the field due to each turn adds up.
(iii) The strength of the current flowing in the coil: as the strength of the current increases, the  strength of the magnetic fields also increases.


Solenoid:
 (i) A coil of many turns of insulated copper wire wrapped in the shape of a cylinder is called a Solenoid.
(ii) Magnetic field produced by a Solenoid is similar to a bar magnet.
(iii) The strength of magnetic field is proportional to the number of turns & magnitude of current.

 Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron core.

Fleming’s Left hand rule: Stretch the thumb, forefinger and middle finger of left hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and centre finger in the direction of current, then the thumb gives the direction of force acting on the conductor.

Electric motor: A device that converts electric energy to mechanical energy.

Principle of Electric motor: When a rectangular coil is placed in a magnetic field and a current is passed through it, force acts on the coil, which rotates it continuously. With the rotation of the coil, the shaft attached to it also rotates.

Electromagnetic induction: Electricity production as a result of magnetism (induced current) is called Electromagnetic induction.

Fleming’s Right hand rule: gives the direction of induced current. Stretch the thumb, forefinger and middle finger of right hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and centre finger in the direction of induced current, then the thumb gives the direction of motion of the conductor.

Electric generator: A devise that converts mechanical energy to electric energy. Electric generator is of two types- 
(i) A.C generator 
(ii) D. C generator

 Principle of Electric generator:
 Electromagnetic induction

 We receive electric supply through mains supported through the poles or cables. In our houses we receive AC electric power of 220V with a frequency of 50Hz.

The 3 wires are as follows-
(i) Live wire- (Red insulated, Positive)
(ii) Neutral wire- (Black insulated, Negative)
(iii) Earth wire- (Green insulated) for safety measure to ensure that any leakage of current to a metallic body does not give any serious shock to a user.

Short circuit: is caused by touching of live wires and neutral wire

Fuse: is a protective device used for protecting the circuits from short circuiting and over loading.

 Important diagrams-
1. Magnetic field lines around a bar magnet.
2. Right hand thumb rule
3. Magnetic field lines through and around a current carrying solenoid.
4. An electromagnet.
5. A simple electric motor
6. Electric generator

HOTS QUESTIONS (SOLVED)

1. On what effect of an electric current does an electromagnet work?
A. Magnetic effect of electric current

2. What is the frequency of AC (Alternating Current) in India?
3. 50 Hz

4. On what effect of an electric current does a fuse work?
A. Heating effect of electric current.

5. An Alternating Current has a frequency of 50 Hz. How many times it changes direction in one second?
A. Since Alternating Current changes its direction twicw in a cycle, it will change its direction 100 times (50x2) in one second as its frequency is 50Hz.

6. What is short circuiting in an electric circuit?
A. Short circuiting in an electric circuit is a situation in which the live wire touches the neutral wire.

7. What kind of quantity is magnetic field?
A. It is a vector quantity as it has both direction & magnitude.

8. How is Solenoid different from a circular coil?
A. Solenoid is different from a circular coil in the sense that the length of the solenoid is much greater than its diameter

9. On what factor the strength of magnetic field of a solenoid depends?
A The strength of magnetic field of a solenoid is proportional to the number of turns of the coil and the magnitude of the current.

10. What kind of motor is used in: 
(i) A fan (ii) A battery operated toy
A. (i) A fan has AC motor 
(ii) A battery operated toy has DC motor.


HOTS QUESTIONS (UNSOLVED)
1. Name the sources of direct current.
2. Why don‘t two magnetic lines intersect each other?
3. What is the role of split ring in an electric motor?
4. What is an earth wire?
5. Why do we use power of two different current ratings at our homes?
6. What would be the frequency of an Alternating Current, it its direction changes after every 0.01 second?
7. Which of the two requires a thin fuse wire to draw: 5A or 15A of current?
8. It is advised to keep magnetsaway form computers, T.V. Give reason
9. In a domestic circuit, MCB (Miniature Circuit Breaker) of one room trips again and
again. What could be the reason for this?
10. What is the application of a solenoid?
11. What is the application of a solenoid?

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