Chapter 13 Magnetic effects of Electric Current

Compass needle is a bar magnet when it is brought near a bar magnet, its magnetic field interacts with the magnetic field line of the bar magnet. Therefore deflection occurs in the compass needle.

The region surrounding a magnet in which the force of a magnet can be detected is said magnetic field. On putting magnet filings around magnet the arrange in pattern representing magnetic field line. Magnetic field lines of a bar magnet emerge from the north pole and merge at south pole. Inside the magnet the direction of the field line is from south pole towards north pole.

The properties of magnetic field lines are: 

(a) Magnetic field lines emerge from the north pole.

(b) They merge at the south pole.

(c) The direction of field lines inside the magnet is from the south pole to the north pole.

(d) Magnetic lines do not intersect with each other.

Two magnetic lines never intersect because if two field lines of a magnet intersect, then at the point of intersection, the compass needle points in two different directions which is not possible at same time a needle shows to different directions. Hence, they never interact.

Inside the loop = Pierce inside the table

Outside the loop = Appear to emerge out from the table

For the downward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. Similarly, for the upward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure.

The magnetic field lines inside a current-carrying long straight solenoid are uniform.

(d) The magnetic field inside a long, straight, current-carrying solenoid is uniform. Therefore, it is the same at all points inside the solenoid.

(c) and (d)

When a proton enters an area of magnetic field, it experiences a magnetic force. As a result of the force, the path of the proton becomes circular. Therefore, its velocity and momentum change.

A current-carrying conductor placed in a magnetic field experiences a force. The magnitude of force increases with the amount of current, strength of the magnetic field, and the length of the conductor. Shuffle, the magnetic force exerted on rod AB and its deflection will increase if

i. current in rod AB is increased
i.) a stronger horse-shoe magnet is used
iii. length of rod AB is increased

(d) The direction of the magnetic field can be determined by applying Fleming’s left hand rule. According this rule, if we arrange the thumb, the centre finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the centre finger gives the direction of current, and the forefinger points in the direction of magnetic field. As the direction of a positively charged alpha particle is towards west, the direction of current will be the same i.e., towards the west. If the direction of magnetic force is towards the north. Hence, according to Fleming’s left hand rule, the direction of the magnetic field will be upwards.

Fleming’s left hand rule states that if we arrange the thumb, the centre finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the centre finger gives the direction of current, and the forefinger points in the direction of magnetic field.

The principle of an electric motor is based on the magnetic effect of current. A current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is determined by Fleming's left-hand rule.

The split ring in the electric motor is a commutator. The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this direction of current getting reverse, the coil continues to rotate in the same direction.

The different ways to induce current in a coil are as following:

a. If a magnet is moved relative to a coil, then an electric current is induced in the coil.

b. If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current is induced in the coil.

The principle of an electric generator is electromagnetic induction. It generates electricity by rotating a coil applied in a magnetic field.

DC generator is a source of direct current.

AC generators, power plants are sources that produce alternating current.

(c) When a rectangular coil of copper is rotated in a magnetic field, the direction of the induced current in the coil changes once in each half revolution. Therefore, the direction of current in the coil remains the same.

Two safety measures commonly used in electric circuits and appliances are as follows:

i. Electric fuse. It prevents the flow of excessive current through the circuit. When the current passing through the wire exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current through that circuit, hence protecting the appliances connected to the circuit.

ii. Earthing is a must to prevent electric shocks. Any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.

We can calculate current drawn by electric oven is

Given - Power = 2kW = 2000 W

Potential difference of 220 W by applying the formula P = VI

By putting the value = 

The precautions that should be taken to avoid the overloading of domestic circuits are as follows:

a. Different appliances should not be used at the same time.

b. Different appliances should not be connected to a single socket.

c. Fuse should be connected in the circuit.

d. Faulty appliances should not be connected in the circuit.

(d) The magnetic field lines, produced around a straight current-carrying conductor, are concentric circles. Their centres lie on the wire.

(c) When a straight coil and a magnet are moved relative to each other, a current is induced in the coil. This phenomenon is known as electromagnetic induction.

(a) Generator is the device used to produce electric current.

(d) The main difference is an AC generator has two rings called slip rings. A DC generator has two half rings called commutator.

(c) At the time of short circuit the current increase heavily, when two naked wires of an electric touch each other the amount of current that is flowing in the curcuit increasse.

(a) False

As an electric motor converts electrical energy into mechanical energy.

(b) True

A generator is a device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction.

(c) True

A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel lines.

(d) False

Red insulation covers the live wire whereas green colour.

Three sources of magnetic fields are:

a. Electromagnets

b. Current-carrying conductors

c. Permanent magnets

A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field lines are produced around the solenoid when a current is allowed to flow through it. The field lines produced in a current-carrying solenoid is shown magnetic field line emerges from North pole towards South pole, whereas inside the solenoid magnetic field lines parallel.

When the direction of current is perpendicular to the direction of magnetic field. The force experienced by the current carrying conductor is maximum.

The direction of the magnetic field is determined by Fleming’s left hand rule. Magnetic field inside the chamber will be perpendicular to the direction of current and direction of deflection either upward or downward. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from the back wall to the front wall. The direction of magnetic force is rightward. By using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.

An electric motor converts electrical energy into mechanical energy.

Electric motor is based on the principle of the magnetic effect of current. A current-carrying coil rotates in a magnetic field.

When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating anti-clockwise. This happens because a downward force acts on length MN and at the same time, an upward force acts on length ST. As a result, the coil rotates anti-clockwise.

Current in the length MN flows from M to N and the magnetic field acts from left to right, normal to length MN. Therefore, according to Fleming’s left hand rule, a downward force acts on the length MN. Similarly, current in the length ST flows from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an upward force acts on the length ST. These two forces cause the coil to rotate anti-clockwise.

After half a rotation, the position of MN and ST interchange. The half-ring D comes in contact with brush A and half-ring C comes in contact with brush B. Hence, the direction of current in the coil MNST gets reversed.

The current flows through the coil in the direction TSNM. The reversal of current through the coil MNST repeats after each half rotation. As a result, the coil rotates unidirectional. The split rings help to reverse the direction of current in the circuit. These are called the commutator.

The devices in which electric motors are used are as follows:

a. Electric mixers
b. Water pumps
c. Washing machines
d. Electric fans

A current induces in a solenoid if a bar magnet is moved relative to it. This is the principle of electromagnetic induction.

(i) When a bar magnet is pushed into a coil of insulated copper wire, a current is induced in the coil. As a result of which the needle of the galvanometer deflects direction of current.

(ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire, a current is again induced in the coil in the opposite direction. As a result, the needle of the galvanometer deflects in the opposite direction.

(iii) When a bar magnet is held stationary inside the coil, no current will be induced in the coil. So, no deflection in the galvanometer.

Two circular coils A and B are placed close to each other. When the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This is called electromagnetic induction.

(i) Right hand thumb rule, is applied to determine the magnetic field produced around a straight conductor carrying current.

(ii) Fleming’s left hand rule, determine the direction of force experienced by a current-carrying straight conduct placed in a magnetic field which is perpendicular to it.

(iii) Fleming’s right hand rule, determine the direction of current induced in a coil due to its rotation in a magnetic field.

An electric generator is an electric device used to convert mechanical energy (kinetic energy) into electrical energy (electricity).

Principle: Electric generator based on the principle of an electromagnetic induction. When the coil of an electric generator rotates in a magnetic field it induces the current flows in the circuit connected with the coil.

There are two types of electric generator:

i. AC generator
ii. DC generator

AC generator: AC generator is a device that converts mechanical energy into electrical energy in the form of alternating current or AC.

DC generator: DC generator is a device that converts mechanical energy into electrical energy in the form of direct current or AC.

AC Generator Construction: The components of AC generator are:

1. Armature: Large number of turns of insulated copper wire wound over a soft iron core form  armature coil (ABCD).

2. Strong field magnet: A strong permanent magnet or an electromagnet whose poles are cylindrical in shape is a field magnet. The armature coil rotates between the poles. The uniform magnetic field is provided by the field magnet perpendicular to the axis of rotation of the coil.

3. Slip Rings: Two brass slip rings R1 and R2 are connected to the two ends of the armature coil. Rings rotate with the armature coil. Rings R1 and R2 are at different heights.

4. Brushes: These brushes are connected to the external circuit across which the output is obtained. Two carbon brushes (B1 and B2), are pressed against the slip rings. The brushes are fixed while slip rings rotate along with the” armature.

Working: The armature coil ABCD when rotates in the magnetic field that is provided by the strong field magnet, it cuts the magnetic field lines.therefore, the change in  magnetic field produces induced current in the coil. The direction  of the induced current in the coil is determined by Fleming's right hand rule.

The current flows out through the brush B1 and brush B2 is in the opposite direction. The brush B1 in one direction in the first half of the revolution and by the brush B2 in the next half revolution in the opposite direction. This process is repeated several times. Hence, induced current produced is of alternating current.

DC generator or Dynamo Construction:

When neutral wire and live wire touch each other that they come in direct contact, the resistance of the circuit becomes small and a large amount of current flows through it. As a result of which, a large amount of heat is produced and the circuit catches fire.

Earth wire is a safety measure that prevents short circuits and shock. When a live wire touches the metallic case of an electric gadget, the electric current flows from the casing of the appliance to the earth through the copper wire. As the earth offers very low or almost no resistance to the flow of current, so large current passes through the copper wire instead of the human body. Due to this large current heat is produced in the circuit and hence the fuse in the circuit melts. So, the circuit is switched off automatically and the electric appliance is saved from burning and no electric shock to the human body.