Introduction
What is a Magnet?
Magnet is defined as a material that can produce its own magnetic field.
There are three types of magnets:
1. Permanent magnet
2. Temporary magnet
3. Electromagnet
What is a Compass Needle?
A compass needle is a small magnet.
Compass needle gets deflected on passing current through a metallic conductor.
It’s one end, which points towards north is called North Pole, and the other end, which points towards south, is called a South Pole.
Magnetic Field and Field Lines:
A magnetic field is a force field that is created by magnetic dipoles and moving electric charges, and it exerts a force on other nearby moving charges and magnetic dipoles.
Magnetic Field is a vector quantity because it has both magnitude and direction.
A magnetic field line or lines of forces shows the strength of a magnet and the direction of a magnet’s force.
Direction of magnetic field: Magnetic field lines are directed from the north pole to south pole outside the magnet and from the south pole to north pole inside the magnet. (Refer the figure below.)
Strength of magnetic field lines: The relative strength of magnetic field is shown by the degree of closeness of the field lines.
Thus, more the field lines are crowded, stronger is the magnetic field.
Properties of magnetic field:
1. The magnetic field lines are closed curves.
2. The field lines do not intersect each other.
Magnetic field due to a current through a straight conductor
A thick copper wire is inserted through the centre, normal to the plane of a rectangular cardboard and connected vertically between points X and Y in series with the battery, rheostat, ammeter, a plug and a key. Sprinkle some iron fillings uniformly on the cardboard. Keep the variable of the rheostat at a fixed position.
On applying the current, the iron fillings will align in a pattern of concentric circles indicating magnetic field lines around a straight conducting wire. (ensure that the key is closed and copper wire is placed between points X and Y remains vertically straight.).
Properties of magnetic field:
The direction of magnetic field through a current carrying conductor depends upon the direction of flow and intensity of electric current.
The magnetic field produced by a current carrying through straight wire depends inversely on the distance from it.
Magnetic field lines are always parallel to each other.
No two field lines cross each other.
Right-Hand Thumb Rule:
Rule: Hold a current-carrying straight conductor in the right hand (vertically) such that the thumb points towards the direction of current and wrap the fingers around the conductor in the direction of the magnetic field lines.
The Right-Hand Thumb rule is also known as Maxwell’s corkscrew rule.
The current flows through the wire in north-south direction. Applying the right-hand thumb rule, we get that the magnetic field turns clockwise in a plane perpendicular to the wire when the current flows north to south and anticlockwise, when the current flow south to north.
Magnetic field due to a current through circular loop
Suppose a straight wire is bent in the form of a circular loop and a current is passed through it.
At every point of a current carrying circular loop, the concentric circles representing the magnetic field around it would become larger and larger as we move away from the wire.
At the centre of circular loop, the arcs of these big circles would appear as straight lines.
On applying right-hand rule, it can be concluded that every section of the wire contributes to the magnetic field lines in the same direction within the loop.
The magnetic field produced by a current carrying wire at a given point depends directly on the current passing through it.
Therefore, if the circular coil having n turns, the field produced is n times as large as that produced by a single turn. This is because the current in each circular turn has the same direction and field due to each turn gets add up.
Circular coil turns α magnetic field produced.
Magnetic field due to a current in a Solenoid
A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
The magnetic field lines of solenoid is similar to that of a bar magnet, i.e., the field lines emerges from north pole and merges at south pole.
One end of the solenoid behaves as a magnetic north pole, while the other behave as the south pole. The field lines inside the solenoid are in the form of parallel straight lines.
This indicates that the magnetic field is uniform inside a solenoid.
A strong magnetic field produced inside a solenoid (coil) can be used to magnetize a piece of magnetic material, like soft iron. Such a magnet is called an Electromagnet.
Force on a current-carrying conductor in a magnetic field
An electric current flowing through a conductor produces a magnetic field. The field so produced exerts a force on a magnet placed in the vicinity of the conductor.
French scientist, Andre Marie Ampere suggested that the magnet must also exert an equal and opposite force on current carrying conductor.
The direction of force over the conductor gets reversed with the change in direction of flow of electric current. It is observed that the magnitude of force is highest when the direction of current is at right angles to the magnetic field.
Fleming’s Left Hand Rule: Rule states that if we arrange our thumb, forefinger and middle finger of the left-hand perpendicular to each other, then the thumb points towards the direction of the magnetic force, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the current. |  |
Electric motor
Principle:
An electric motor is a rotating device that converts electrical energy into mechanical energy.
Construction:
An electric motor (figure) consists of a rectangular coil ABCD of insulated copper wire.
The coil is placed between 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 Q of a split ring.
The inner sides of these halves are insulated and attached to an axle.
The external conducting edges of P and Q touch two conducting stationary brushes X and Y respectively.
Working:
The current in the coil ABCD enters from the source battery through conducting brush X and flow back to the battery through brush Y.
The current in the arm AB of the coil flows from A to B. In arm CD it flows from C to D, i.e., opposite to the direction of the current through arm AB.
On applying Fleming’s Left-Hand rule for the direction of force on a current carrying conductor in a magnetic field.
The force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus, the coil and the axle O, mounted free to turn about an axis, rotate anticlockwise.
At half rotation, Q makes contact with the brush X and P with brush Y. therefore, the current in the coil gets reversed and flows along the path DCBA.
A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motor, split rings act as a commutator.
The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus, the arm AB of coil that was earlier pushed down, is now pushed up and the arm CD previously pushed up is now pushed down.
Therefore, the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of coil and to the axle.
Uses:
Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players, etc.
Electromagnetic induction
- Michael Faraday studied, that moving magnet can generate electric current.
- The change in magnetic field in a conductor induces a current in another conductor is called electromagnetic induction.
Or
Electricity production as a result of magnetism (induced current) is called Electromagnetic Induction.
- When a conductor is set to move inside a magnetic field or a magnetic field is set to be changing around a conductor, electric current is induced in the conductor. This is just opposite to the exertion of force by a current carrying conductor inside a magnetic field. In other words, when a conductor is brought in relative motion vis-à-vis a magnetic field, a potential difference is induced in it. This is known as electromagnetic induction.
Electromagnetic induction by one coil.
Electromagnetic induction by two coil (changing position of one coil)
Fleming’s Right Hand Rule: Rule states that if we arrange our thumb, forefinger and middle finger of the right-hand perpendicular to each other, then the thumb points towards the direction of the magnetic force, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the current. |  |
Galvanometer | An instrument that can detect the presence of current in a circuit. It can be either zero or deflect to left or right depending upon the direction of current. |
Ammeter | An ammeter is an instrument used to measure the current value in a circuit, in amperes (A). |
Electric generator
Principle:
In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.
Construction:
An electric generator consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet.
The two-ends of this coil are connected to two rings R1 and R2. The inner side of these rings are insulated.
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 an axle. The axle maybe mechanically rotated from outside to rotate the coil inside the magnetic field.
Outer ends of the two brushes are connected to the galvanometer.
Working:
When the axle attached to the two rings is rotated the arm AB moves up and the arm CD moves down.
If the coil is rotated clockwise and on applying Fleming’s Right-Hand rule, the induced currents are set up in these arms along the directions of AB and CD. Thus, an induced current flow in the direction of ABCD.
If there are large numbers of turns in the coil, the current generated in each turn adds up to give a large current through coil. This means that the current in the external circuit flows from B2 to B1.
After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction of DCBA. The current in the external circuit flows from B1 to B2.
Thus, after every half rotation the polarity of the current in respective arm changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (AC current). The device is called AC generator.
To get a direct current (DC, which does not change its direction with time), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with arm moving up while other with arm moving down. Thus, unidirectional current is produced. The generator is DC generator.
Difference between AC and DC Current
AC (Alternating Current) | DC (Direct Current) |
Current in which direction is changed periodically is called Alternate Current. | Current that flows in one direction only is called Direct current |
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A.C is transmitted upto a long distance without much loss of energy is advantage of A.C. over D.C. | D.C. can not be transmitted over long disance |
More efficiency | Less efficiency |
Power consumption less | Power consumption is more |
Domestic electric circuits
- The households receive electric power through a main supply (also called mains), either supported through overhead electric poles or the underground cables. In our country, the potential difference between the two is 220V.
- Three cable connection:
1. red insulation cover, is called live wire (or positive)
2. black insulation, is called neutral wire (or negative).
3. green insulation cover is called earth wire, is connected to a metal plate deep in the earth near the house. This is used for a safety measure (to prevent from electric shock), especially for those appliances that have a metallic body, for example, electric press, toaster, table fans, refrigerator, etc.
- Each appliances have a separate switch to ON/OFF the flow of current through it. If each appliance has equal potential difference, they are connected parallel to each other.
- Electric fuse is an important component of the circuit.
A fuse in a circuit prevents from damage to the appliance and the circuits due to overloading.
- Overloading: It can occur when live and neutral wire come into direct contact.
- Short-circuiting: When current in circuit abruptly increases. (This occurs when the insulation wire gets damaged or there is fault in the line.)