CBSE Class 10 Science Magnetic Effect of Electric Current Notes Set 01

These CBSE Class 10 Science notes for Chapter 12, Magnetic Effects of Electric Current, cover magnetic field and field lines, the field due to a straight wire, a circular loop and a solenoid, the force on a current-carrying conductor, the electric motor, electromagnetic induction, the electric generator, AC and DC, and domestic electric circuits with their safety devices.

Important Notes and Formulas

Magnets and Magnetic Field

A magnet is an object that attracts objects made of iron, cobalt and nickel. When suspended freely, a magnet comes to rest in the North-South direction. The pole that points towards the north is called the north pole (north-seeking) and the pole that points towards the south is the south pole (south-seeking). Like poles repel each other and unlike poles attract each other.

Magnets are used in refrigerator doors, radio and stereo speakers, audio and video players, children's toys and the hard discs of computers.

Magnetic field: The region around a magnet in which its magnetic force can be experienced is called the magnetic field. It is a vector quantity, that is, it has both direction and magnitude. Its SI unit is the tesla (T). The force of a magnet can be detected with a compass needle or with another magnet.

Magnetic Field Lines

The magnetic field is represented by magnetic field lines. These are imaginary lines. When iron filings are sprinkled around a bar magnet, they arrange themselves in a pattern that looks like the field lines. A compass can also be used to trace the lines.

1. Direction: Outside the magnet, the field lines are directed from the north pole to the south pole. Inside the magnet, they are directed from the south pole to the north pole. So the field lines are closed curves.
2. Strength: The closeness of the lines shows the relative strength of the field. Closer lines mean a stronger field. The lines are crowded near the poles, where the field is strongest.
3. No intersection: Two field lines never cross each other. If they did, there would be two directions of the field at the point of crossing, which is not possible.

Magnetic Field due to a Current-Carrying Straight Conductor

Oersted discovered in 1820 that a current-carrying conductor deflects a compass needle, so electricity and magnetism are related. The magnetic field around a straight current-carrying conductor is in the form of concentric circles around the wire.

1. The direction of the field depends on the direction of the current. Reversing the current reverses the field.
2. The field increases with the current (B ∝ I).
3. The field decreases with the distance from the wire (B ∝ 1/r). The circles become larger and farther apart as the distance increases.

Right-hand thumb rule (Maxwell's corkscrew rule): Imagine holding the current-carrying conductor in your right hand with the stretched thumb pointing in the direction of the current. The direction in which the fingers wrap around the conductor gives the direction of the magnetic field. According to the corkscrew rule, if the screw moves forward in the direction of the current, the direction in which the screw is turned gives the direction of the magnetic field.

For a wire carrying current upwards (out of the plane of the page), the field lines are anticlockwise circles when seen from above; for a wire carrying current downwards, they are clockwise.

Magnetic Field due to a Current through a Circular Loop

Every small part of a circular loop produces a field in the form of concentric circles around it, and these circles become larger as we move away from the wire. At the centre of the loop the arcs of these large circles appear as straight lines. The field is strongest at the centre and its direction there is perpendicular to the plane of the loop.

Clock-face rule: A current-carrying loop behaves like a thin disc magnet. If the current appears anticlockwise when looking at one face of the loop, that face is a north pole. If it appears clockwise, that face is a south pole.

The strength of the magnetic field at the centre of the coil depends on:

1. Radius of the coil: The field is inversely proportional to the radius. If the radius increases, the field at the centre decreases.
2. Number of turns: The field is directly proportional to the number of turns (N), because the current in each turn has the same direction and the fields due to the turns add up. For N turns the field is N times that of a single turn.
3. Current: The field increases as the current increases.

So at the centre of a coil, B ∝ N I / r.

Magnetic Field due to a Current in a Solenoid

A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. The magnetic field pattern of a current-carrying solenoid is similar to that of a bar magnet. One end behaves as a north pole and the other end as a south pole.

1. Inside the solenoid the field lines are parallel straight lines, which shows that the magnetic field is the same (uniform) at all points inside the solenoid.
2. The strength of the field depends on the number of turns per unit length, on the current, and on the core material placed inside.
3. A freely suspended current-carrying solenoid rests along the North-South direction, like a bar magnet.

Electromagnet

A strong magnetic field inside a solenoid can magnetise a piece of magnetic material such as soft iron placed inside it. The magnet formed in this way, a long coil of insulated copper wire wrapped on a soft iron core, is called an electromagnet. It is a magnet only while current flows through the coil. Electromagnets are used in electric bells, loudspeakers, motors and cranes for lifting scrap iron.

Force on a Current-Carrying Conductor in a Magnetic Field

A current-carrying conductor placed in a magnetic field experiences a force. This was suggested by André-Marie Ampère, a French physicist, who is considered the founder of electromagnetism. A magnet exerts a force on a current-carrying conductor, and the conductor exerts an equal and opposite force on the magnet.

1. The direction of the force is reversed when the direction of the current is reversed.
2. It is also reversed when the direction of the magnetic field is reversed.
3. The force is largest when the current is at right angles to the magnetic field. The force is zero when the conductor is parallel to the field.
4. The force also depends on the strength of the field, the current and the length of the conductor in the field.

A moving charged particle in a magnetic field also experiences a force perpendicular to both its velocity and the field. There is no force on a charge at rest or one moving along the field.

Fleming's left-hand rule: Stretch the forefinger, the middle finger and the thumb of the left hand so that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, then the thumb points in the direction of the force (motion) on the conductor. The three directions are like the x, y and z axes.

Devices that work on this rule include the electric motor and the loudspeaker.

Electric Motor

An electric motor is a device that converts electrical energy into mechanical energy. It may be a DC motor or an AC motor.

Principle: When a rectangular coil is placed in a magnetic field and a current is passed through it, a force acts on each of its two sides in opposite directions, and the coil rotates. The direction of the force is given by Fleming's left-hand rule.

Construction:

1. Armature: the rectangular coil ABCD placed between the two poles of the magnet.
2. Split ring (commutator): two halves of a metallic ring connected to the ends of the coil. It reverses the direction of the current in the coil after every half rotation.
3. Magnet: a permanent magnet whose poles N and S supply the magnetic field.
4. Brushes: two fixed contacts X and Y that press against the split ring and carry the current from the battery.
5. Battery: a source of current, a few cells connected together.

Working: When the current flows in the coil, the forces on arms AB and CD are equal and opposite and they turn the coil. As the coil reaches the half-way (vertical) position, the split ring reverses the direction of the current in the coil. This reverses the forces on the arms, so the coil continues to rotate in the same direction. This process keeps the motor rotating.

In commercial motors, an electromagnet is used in place of a permanent magnet, and the armature is a soft iron core with a large number of turns of wire. This produces a much stronger field. Motors are used in electric fans, water pumps, washing machines, mixers and toys. The speed of a motor can be increased by increasing the current, the number of turns, the area of the coil or the strength of the magnet.

Electromagnetic Induction

Michael Faraday, an English physicist, studied the production of electric current using a magnetic field and a conductor. The production of an electric current in a conductor by a changing magnetic field is called electromagnetic induction, and the current so produced is called the induced current.

1. When a magnet is moved towards or away from a coil, or the coil is moved relative to the magnet, a current is induced in the coil. A galvanometer, which detects current, shows a deflection.
2. The deflection is in opposite directions when the magnet is moved in and out. There is no deflection when the magnet is at rest relative to the coil.
3. A current can also be induced in a coil by changing the current in a nearby coil, for example by closing or opening the key of the circuit that contains it.
4. The induced current is maximum when the motion of the conductor is at right angles to the magnetic field.

This is the opposite of what happens in a motor: in a motor a current produces motion in a field, while here motion in a field produces a current. Electromagnetic induction converts mechanical (kinetic) energy into electrical energy.

Fleming's right-hand rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular. If the forefinger points in the direction of the magnetic field and the thumb points in the direction of motion of the conductor, then the middle finger shows the direction of the induced current.

Electric Generator

A device that converts mechanical energy into electrical energy is called an electric generator. Generators are of two types, AC generators and DC generators.

Principle: An electric generator works on the principle of electromagnetic induction.

Construction and working: The structure is similar to that of a motor. A rectangular armature coil is placed between the poles of a permanent magnet and is rotated about an axle. As the coil moves in the field, a current is induced in it. After every half rotation the direction of the induced current reverses, so the current changes its direction twice in every rotation. Therefore the generator produces alternating current (AC). The ends of the coil are connected to two slip rings, which touch brushes connected to the external circuit. If a split-ring commutator is used instead of the slip rings, the output is a direct current (DC).

AC and DC

Alternating current (AC): A current whose direction changes periodically is called alternating current. In India the AC supply has a frequency of 50 Hz, that is, the current changes direction after every 1/100 second (100 times per second). Direct current (DC): A current that flows in one direction only is called direct current. Electrochemical cells produce DC.

Advantages of AC over DC:

1. AC can be transmitted over long distances without much loss of energy.
2. The cost of generating AC is less than that of DC.
3. AC can easily be converted into DC.
4. AC can be controlled by a choke with little loss of power, whereas DC is controlled by resistances, which waste energy.

AC cannot be used for electrolysis because it keeps reversing its polarity.

Domestic Electric Circuits

We receive electric power through mains supported on poles or underground cables. In India the domestic supply is AC of 220 V and frequency 50 Hz. Three wires are used:

1. Live wire (red insulation): carries the current from the supply at high potential.
2. Neutral wire (black insulation): returns the current to the supply; its potential is close to zero.
3. Earth wire (green insulation): connected to a metal plate deep in the earth and to the metal bodies of appliances. It is a safety measure that ensures that any leakage of current to the metal body does not give a serious shock to the user.

Appliances are connected in parallel so that each gets the same voltage and can be operated independently.

Short circuit: When the live wire and the neutral wire touch each other (because of damaged insulation or a fault in an appliance), the resistance of the circuit becomes very small and a very large current suddenly flows.

Overloading: When too many appliances are connected to a circuit, or a high-power appliance is used, the current drawn exceeds the safe value. The wires overheat and may cause a fire.

Electric fuse: A fuse is a protective device that protects the circuit from short-circuiting and overloading. It is a piece of wire of a material with a low melting point.

1. A fuse is always connected in series with the live wire at the beginning of the circuit.
2. It works on the heating effect of the current: when the current exceeds the safe value, the wire melts and breaks the circuit.
3. A fuse of 5 A is used for lights and fans and a fuse of 15 A for appliances of 2 kW or more.
4. A burnt-out fuse must be replaced by a fuse of the same rating.