An electric current-carrying wire behaves like a magnet.
Electricity and magnetism are linked to each other.
Compass needle is deflected passing an electric current through a metallic conductor because the electric current through the copper wire has produced a magnetic effect.
MAGNETIC FIELD AND FIELD LINES
- The ends of the compass needle point approximately towards north and south directions.
- The end pointing towards north is called north seeking or north pole. The other end that points towards south is called south seeking or south pole.
- Like poles repel, while unlike poles of magnets attract each other.
Magnetic field :- It is region around a magnetic material or a moving electric charge where the magnetic force can be experienced.
Field line of magnet:- The imaginary lines of magnetic field drawn around a magnet are called field line or field line of magnet.
- Magnetic field is a quantity that has both direction and magnitude.
- The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it.
- Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole.
- Inside the magnet, the direction of field lines is from its south pole to its north pole.Outside the magnet, the direction of magnetic field line is taken from north pole to South Pole. Thus the magnetic field lines are closed curves.
- The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded
- No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR
A simple electric circuit in which a straight copper wire is placed parallel to and over a compass needle. The deflection in the needle becomes opposite when the direction of the current is reversed.
- Magnetic Field due to a Current through a Straight Conductor
- A current carrying straight conductor has magnetic field in the form of concentric circles around it.
- If the current in the copper wire is changed the deflection in the needle also changes.
- If the current is increased, the deflection also increases. It indicates that the magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
- If the compass is moved from the copper wire but the current through the wire remains the same then the deflection in the needle decreases. Thus the magnetic field produced by a given current in the conductor decreases as the distance from it increases.
The concentric circles representing the magnetic field around a current-carrying straight wire become larger and larger as we move away from it.
Right-Hand Thumb Rule
It is a convenient way of finding the direction of magnetic field associated with a current-carrying conductor.
Right-hand thumb rule:- Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field. This is known as the right-hand thumb rule.
- It is also known as Maxwell’s Corkscrew Rule.
Magnetic Field due to a Current through aCircular Loop
In case of a circular current carrying conductor, the magnetic field is produced in the same manner like that of a straight current carrying conductor.
- 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 .
- By the time we reach at the centre of the circular loop, the arcs of these bigcircles would appear as straight lines.
- Every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the center of the loop.
- By applying the right hand rule, it is easy to check that every section of the wire contributes to the magnetic field lines in the same direction within the loop.
Magnetic Field due to a Current in a Solenoid
Solenoid:- A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
- A current carrying solenoid produces similar pattern of magnetic field as a bar magnet.
- one end of the solenoid behaves as a magnetic north pole, while the other behaves 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 the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.
A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material, like soft iron, when placed inside the coil . The magnet so formed is called an electromagnet.
Electromagnet :- a soft metal core made into a magnet by the passage of electric current through a coil surrounding it.
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 (1775–1836) suggested that the magnet must also exert an equal and opposite force on the 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.
- we considered the direction of the current and that of the magnetic field perpendicular to each other and found that the force is perpendicular to both of them.
- The three directions can be illustrated through a simple rule, called Fleming’s left-hand rule.
- According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular . If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.
- Devices that use current-carrying conductors and magnetic fields include electric motor, electric generator, loudspeakers, microphones and measuring instruments.
An electric motor is a rotating device that converts electrical energy to mechanical energy. Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players etc.
In an electric motor, a rectangular coil is suspended between the two poles of a magnetic field. The electric supply to the coil is connected with a commutator. Commutator is a device which reverses the direction of flow of electric current through a circuit.
When electric current is supplied to the coil of electric motor, it gets deflected because of magnetic field. As it reaches the half way, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of current reverses the direction of forces acting on the coil.
The change in direction of force pushes the coil; and it moves another half turn. Thus, the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation.
The commercial motors use :-
- an electromagnet in place of permanent magnet.
- large number of turns of the conducting wire in the current- carrying coil; and
- a soft iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus the coils, is called an armature. This enhances the power of the motor.
Electromagnetic induction: process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.
In 1831, Faraday made an important breakthrough by discovering how a moving magnet can be used to generate electric currents.
- A galvanometer is an instrument that can detect the presence of a current in a circuit.
Motion of a magnet with respect to the coil produces an induced potential difference, which sets up an induced electric current in the circuit.
We can induce current in a coil either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient in most situations to move the coil in a magnetic field.
The induced current is found to be the highest when the direction of motion of the coil is at right angles to the magnetic field.
Fleming’s left hand thumb rule:-
To know the direction of the induced current.
Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other . If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current. This simple rule is called Fleming’s right-hand rule.
Principle: An electric generator works on the principle of electromagnetic induction phenomenon. According to it, whenever a coil is rotated between the poles of a magnet, an induced current is set up in the coil, whose direction is given by Fleming’s right hand rule.
In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.
AC AND DC CURRENT
- The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.
- Most power stations constructed these days produce AC. In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz.
- An important advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy.
DOMESTIC ELECTRIC CIRCUITS
In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One of the wires in this supply is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The potential difference between the two is 220 V.
The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth. It is used as a safety measure to ensure that any leakage of current to a metallic body does not give any severe shock to a user.
- In our homes, we receive supply of electric power through a main supply (also called mains), either supported through overhead electric poles or by underground cables.
- One of the wires in this supply, usually with red insulation cover, is called live wire (or positive). Another wire, with black insulation, is called neutral wire (or negative).
- In our country, the potential difference between the two is 220 V.
- The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house.
- This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc.
- The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current.
- Thus, it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.
- A fuse in a circuit prevents damage to the appliances and the circuit due to overloading.
- Overloading can occur when the live wire and the neutral wire come into direct contact. (This occurs when the insulation of wires is damaged or there is a fault in the appliance.)
- In such a situation, the current in the circuit abruptly increases. This is called short-circuiting.
- The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current.
The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current.
The Joule heating that takes place in the fuse melts it to break the electric circuit. Overloading can also occur due to an accidental hike in the supply voltage. Sometimes overloading is caused by connecting too many appliances to a single socket.