MAGNETIC EFFECT OF ELECTRIC CURRENT

 

 MAGNETIC EFFECT:-  The term magnetic effect of electric current means that or electric current flowing in a wire produces in a magnetic field around it. In other words, electric current can produce magnetism.

MAGNET:-  A magnet is an object which attract pieces of iron, steel, nickel and cobalt. Magnets come in various shapes and sizes depending on their in tended use.

BASIC  LAWS OF MAGNETISM:

1] There are two poles namely north pole and south pole.

2] Like poles repel each other, while unlike poles attract each other.

LODE STONE:- The black ore of iron (fe3o) called magnetic capable of attracting  similar pieces of iron is called lode stone.

MAGNETIC SUBSTANCE:-

Those substances which are attracted by magnet are called magnetic substances.

NON MAGNETIC SUBSTANCES:-

Those substances which are not attracted by a magnet are called non magnetic substances.

MAGNETIC FIELD:-

 The space surrounding a magnet in which magnetic force is exerted is called a magnetic field. The magnetic field has both, magnitude as well as direction.] A compass needle placed near a magnet gets deflected due to the magnetic force exerted by a magnet. The north end of the needle of a compass indicates the direction of magnetic field at a point where it is placed. [The S I unit of magnetic field is telsa.

 

MAGNETIC FIELD LINES:-

 A magnetic field is described by drawing the magnetic field lines. (The magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move.) The magnetic field lines are also known as magnetic lines of force. (The magnetic field lines always begin from N-pole of a magnet and end on the S-pole) of a magnet.

Properties of the magnetic field lines:-

(i)           The magnetic field lines originate from the north pole of a magnet and end at its South pole.

(ii)       The magnetic field lines come close to one another near the poles of a magnet but they are widely separated at other places.

(iii)    They exert lateral pressure on each other.

(iv)    They magnetic field lines do not intersect one another. It is because if they do so, it will give two different direction at the same point which is not possible.

 

 

MAGNETIC FIELD DUE TO STRAIGHT CURRENT CARRYING CANDUCTOR:-

 

The magnetic field lines around a straight conductor (straight wire) carrying current are concentric circles whose centers lie on the wire i.e. ,the magnetic field lines are circular in nature. The magnitude of magnetic field produced by a straight current carrying wire at a given point is:-

 

(i)           Directly proportional to the current passing in the wire and

(ii)       Inversely proportional to the distance of that point from the wire.

 

    Direction of a magnetic field produces by straight current carrying conductor:-

 

(Fleming Right Hand Thumb rule)

 

The direction of a magnetic field produced by straight current carrying conductor is given by right hand thumb rule. According to right hand thumb rule.

"Imagine that we are holding the current carrying wire in your right hand so that yours thumb points in the direction of current, than the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire."

MAGNETIC FIELD DUE TO A CIRCULAR LOOP .

It has been found that the magnetic effect of current increases if instead of using a straight wire, the wire is concerted into a circular loop. The pattern of magnetic field due to a current carrying circular loop are circular near the current carrying loop. As we move away, the concentric circles representing magnetic field lines become bigger and bigger. At the centre of the circular loop, the magnetic field lines are straight.

The magnitude of magnetic field produced by current carrying circular loop (or circular wire) at its centre is

(i)           Directly proportional to the current passing through the circular loop or (circular wire), and

(ii)       Inversely proportional to the radius of circular loop (or circular wire)

 

 MAGNETIC FIELD DUE TO A SOLE NOID:-

The solenoid is a long coil containing a large number of close turns of insulated copper wire. The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet. The magnet field lines inside the solenoid are in the form of parallel straight lines. Thus the magnetic field is uniform inside a current carrying solenoid.

1.                        Strength of magnetic field depends upon.

2.                        The strength of current in the solenoid

3.                        The nature of “core material” used in making solenoid.

 Electromagnet:-

 

An electromagnet is a magnet consisting of a long coil of insulted copper wire wrapped around a soft iron core that is magnetized only when electric current is passed trough the coil. The core of an electromagnet must be of soft iron because soft iron loses all of its magnetism when current in the coil is switched off.

Factors affecting the strength of an electromagnet depends on.

(i)           The number of turns in the coil.

(ii)       The current flowing in the coil.

(iii)    The length of air gap between its poles.

 

Difference between a permanent (bar) magnet and electromagnet.

 

Permanent(Bar) Magnet                                                 (i)It is permanent magnet                                                       

(ii)       It produces a weak magnetic force.

(iii)     It strength can not be changed.

(iv)    The North-South polarity of a permanent magnet if fixed.

 

 Electromagnet

(i)            It is a temporary magnet

(ii)       It produces a strong magnetic force

(iii)    The strength of an electromagnet can be changed

(iv)    The North-South polarity of an electromagnet can be changed by changing the direction of current in its spoil

 

 

Magnetism in human beings:-

 

 Extremely weak electric currents are produces in the human body. The two main organs of the human body where the magnetic field produced is quite significant are the heart and the brain.  

Force on current carrying conductor placed 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 on a current carrying conductor placed in a magnetic field can be reversed by reversing the direction of current flowing in the conductor.

 

Direction of force (Fleming’s left hand rule)

 

According to Fleming’s left-hand rule. Hold the fore finger, the centre finger and the thumb of your left hand at right angles to one another. Adjust our hand in such a way that the forefinger points in the direction of magnetic field and the centre finger points in the direction of current, then the direction in which thumb points gives the direction of force acting on the conductor.

Electric motor:-

A motor is a device which converts electrical energy into mechanical energy electric motor is used in electric fans, washing machines, refrigerators, mixer and grinder, electric cars and many other appliances.+

 

Principle of a motor:- A motor works on the principle that when a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates continuously. The rotational coil is given by fleming left hand rule.

 

Construction and working of electric motor:

 

Electric Motor

Construction:- An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and O of a spilt ring. The external conducting stationary brushes X and Y.

Working of electric motor:- Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through Y. The current in arm AB of the coil flowing from A to B. In arm CD it flows from C to D. The force acting on arm AB pushes it downwards while the force acting on  arm CD pushes it upwards. At half rotation, Q makes contact with brush X and P with brush Y. Therefore, the current in the coil gets reversed and flows along with path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. The reversing of the current in repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Electromagnet Induction:-

 

 The production of electricity from magnetism is called electromagnetic induction. When a straight wire is moved up and down rapidly between the two poles of a horseshoe magnet, then an electric current is produced in the wire. This is an example of electromagnetic induction has led to the construction of generators for producing electricity at power stations. The phenomenon of electromagnetic induction was discovered by a british scientist Michael Faraday and an American scientist Joseph Henry independently in 1831. 

 

Fleming (Right hand rule):-

 

According to Fleming’s right hand rule. Hold the thumb, the forefinger and the centre finger of our right hand at right angles to one another. Adjust our hand in such a way that forefinger points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.

DIAGRAM.

 

 

Direct current and Alternate current

 Direct current:- If the current flows in one direction only, it is called a direct current. The positive (+) and negative (-) polarity of a direct current is fixed.

Alternate current:- If the current flows in reverse direction after equal intervals of time, it is called alternating current. The positive (+) and negative (-)  polarity of an alternating current is nit fixed.

 Electric Generator:-

  The electric generator converts mechanical energy into electrical energy.

Principle of electric generator:-

The electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor. The direction of the current is given by the fleming right hand rule.

 

CONSTRUCTION AND WORKING OF ELECTRIC MOTOR:

Construction:- An electric generator consists of a rotating coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two ring R1 and R2. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2 respectively. The two rings R1 and R2 are internally attached to the axle.

Working:- When the axle attached to the two rings is rotated such that the arm AB moves up and the arm CD moves down in the magnetic field produced by the permanent magnet. The coil ABCD is rotated clockwise in the arrangement. By applying fleming’s right hand rule, the induced currents are setup in these arms and the current flows in the direction ABCD. After a rotation, arm CD starts moving up and AB moving down. As a result, the direction of the induced currents in both the arms change, giving no rise to the net induced current in the direction DCBA.

 

 

DIAGRAM

 

 

 

 

 

 

 

 

 

 

 

 

 

Earthing:- Earthing means to connect the metal case of electrical appliances to the earth (at zero potential) by means of a metal wire called “ earth wire”. Earthing saves the electrical appliance from damage due to excessive current.

Overloading:- Overloading means to draw extremely a large current from the mains. Overloading can happen when we switch on so many appliances together simultaneously.

Short-circuiting:- When the live wire comes in direct contact with neutral wire due to defective or damaged wiring, the resistance of the circuit becomes suddenly zero and the current in the circuit increases abruptly. This is called short-circuiting.

Electric- Fuse:- An electric fuse is a safety device used to protect an electric circuit. A fuse is a wire of material having low melting point. It is always connected to live wire in the circuit. A good fuse is made of tin, but cheaper wire is made up of an alloy of tin and copper.

 NCERT

 

Page no. 62

Q.1.  A compass needle get deflected, when brought near a bar magnet because magnetic force existed by the bar magnet acts on the poles of compass needle, which is itself a tiny magnet.

Page no. 65

Q.1.  diagram

Q.2. see back

Q.3. The magnetic field lines do not intersect one another. It is because if they do so, it will give two different direction at the same point which is not possible.

Page no. 67

Q.1. DIAGRAM

Q.2. The magnetic field lines due to a magnet producing a uniform magnetic field are straight parallel lines.

 

 

 

 

Page no. 71

Q.1. See back

Q.2. See back

Q.3. In electric motor, the split ring act as a commutator. The role of the split ring in an electric motor is to reverse the direction of current flowing through the armature coil after every half rotation.

Page no. 72

Q.1. A current can be induced in a coil by-

(i) Moving a magnet towards or away from the coil or vice-versa.

(ii) Changing current in the conductor placed near the coil.

(iii)Relative motion between the coil and the conductor carrying current.

 

Page no. 73

Q.1. See back

Q.2. Dry cell, electric battery used in automobiles, electrical inventors and solar cells.

Q.3Some sources of alternating current are:-

(i) Hydro electric generators.

(ii) Thermal power generators.

(iii) Nuclear power generators.

 

Page no. 76

Q. 1. Forth wire and fuse

Q.2. Power of electric oven=2KW=2000W

Voltage=220V

The current drawn by the electric oven I=P/V=2000/220=9A (approx)

The current drawn by oven is 9A, which is more than the current rating i.e., 5A. Therefore, the circuit will be damaged due to overloading.

Q.3. (i) two many appliances should not be connected to a single socket.

(ii) Use of fuses to protect circuit from overloading.

(iii)Higher power appliances should not be connected simultaneously.

(iv)All electric circuit should be provided with fuse of proper rating.

 

Text Book Exercise

Q.6.(i) Bar magnet

(ii) Electromagnet

(iii)Earth’s magnetism

 

Q.7. The solenoid is a long coil containing a large number of close turns of insulated copper wire. The magnetic field produced by a current carrying is similar to that of magnetic field produced by bar magnet.

 

 

 

 

 

 

 

 

 

 

 

Bring the bar magnet with known north pole near to one end of the solenoid. If the solenoid moves towards the bar magnet, the end of the solenoid is a south pole. In case the solenoid moves away from the magnet then that end of the solenoid is a north pole.

 

Q.8. According to Fleming’s left hand rule, force experienced by a current carrying conductor placed in a magnetic field is perpendicular to current carrying conductor.

 

Q.9. As the electrons move from back wall towards the front and we know that the direction of current is opposite to the direction of motion of electrons. Hence the direction of current is from front wall towards the back. It is indicated by central finger. Force on the electron is towards right, which is indicated by thumb than according to Fleming’s left hand rule magnetic field should be directed downwards.

Q.10. See back

Q.11. See back

 

 

 

Q.12. When the North pole (N) of the magnet is pushed into the coil, the galvanometer deflected toward the right.

B) When the North pole (N) of the magnet is with drawn from the coil, the galvanometer deflected toward the left

Q.13. Yes as the current in the coil. A changes, the magnetic field associated with it also changes. Thus the magnetic field lines around the coil B, also change. Therefore, the change in magnetic field lines associated with the coil B is the cause of induced electric current in it.

Q.14. See back

Q.15. See back

Q.16 See back