4. Effects of Electric Current

Odd one out: Generator

Fuse wire, bad conductors and rubber gloves are used for safety from electric current. A generator produces electricity.

Odd one out: Thermometer

Voltmeter, ammeter and galvanometer measure electrical quantities and work on the principle of the electric motor. A thermometer measures temperature.

Odd one out: Magnet

A loudspeaker, microphone and electric motor are devices that work on the magnetic effect of electric current and change one form of energy into another. A magnet is not such a device; it only produces a magnetic field.

An electric motor is a device that converts electrical energy into mechanical energy. It works on the principle that a current carrying conductor placed in a magnetic field experiences a force (Fleming’s left hand rule).

Construction:

1. A rectangular loop ABCD of copper wire with resistive coating is placed between the north and south poles of a magnet. Its branches AB and CD are perpendicular to the magnetic field.
2. The two ends of the loop are connected to the two halves X and Y of a split ring. The halves have resistive coating on their inner surfaces and are tightly fitted on the axle.
3. The outer surfaces of X and Y touch two stationary carbon brushes E and F.
4. The brushes are connected to a cell (battery) through a plug key.

Working:

1. When the circuit is completed, current flows in the loop from A to B and from C to D.
2. The magnetic field is from N to S. By Fleming’s left hand rule, a force pushes branch AB down. The current in CD is opposite, so CD is pushed up.
3. So the loop and the axle start rotating anticlockwise.
4. After half a rotation, X and Y exchange their brushes. The current in the loop is reversed (DCBA), so the forces keep turning the loop in the same anticlockwise direction.
5. This repeats every half rotation, and the loop keeps rotating continuously.

An electric generator converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction: when a coil rotates in a magnetic field, a current is induced in it.

Construction:

1. A copper wire coil ABCD is kept between the two pole pieces of a magnet.
2. The two ends of the coil are connected to two separate conducting rings R₁ and R₂ (slip rings). Both rings are fixed on the axle with a resistive coating between the ring and the axle.
3. Two stationary carbon brushes B₁ and B₂ touch the rings. They are connected to a galvanometer, which shows the direction of the current.
4. The axle is rotated from outside by a machine.

Working:

1. When the axle rotates, branch AB goes up and branch CD goes down (the coil rotates clockwise).
2. By Fleming’s right hand rule, current is induced in the direction A → B → C → D. In the external circuit, it flows from B₂ to B₁ through the galvanometer.
3. After half a rotation, AB and CD exchange places. The induced current now flows D → C → B → A.
4. Since each ring always stays with the same brush, the current in the external circuit now flows from B₁ to B₂, i.e. in the opposite direction.
5. The direction reverses after every half rotation. So an alternating current (AC) is produced. A coil of many turns gives a larger current.

Electromagnetic induction means (c) Generation of a current in a coil due to relative motion between the coil and the magnet.

Whenever the number of magnetic lines of force passing through a coil changes, a current is induced in the coil. This is Faraday’s law of induction.

The device used to produce electricity is the (c) Electric Generator (DC).

Principle: Electromagnetic induction. It converts mechanical energy into electrical energy.

Construction:

1. A copper coil ABCD is kept between the north and south poles of a magnet and fixed on an axle.
2. The two ends of the coil are connected to the two halves X and Y of a split ring, like the one in an electric motor.
3. Two stationary carbon brushes E and F touch the split ring and are connected to a galvanometer.

Working:

1. The axle is rotated from outside. As the coil rotates in the magnetic field, a current is induced in it (Fleming’s right hand rule).
2. After every half rotation, the current in the coil reverses. But at the same moment, the halves of the split ring also exchange brushes.
3. So the branch of the coil going upwards is always in contact with one brush, and the branch going downwards is always in contact with the other brush.
4. Hence the current in the external circuit always flows in one direction. This is a direct current, so the device is a DC generator.

Formation:

1. A home electrical connection has ‘live’, ‘neutral’ and ‘earth’ wires. The live and neutral wires have a potential difference of 220 V.
2. If there is a fault in the equipment, or the plastic coating on the live and neutral wires wears away, the two wires come in direct contact.
3. The resistance of the circuit becomes almost zero, so a very large current flows. This is called a short circuit.

Effect:

1. The large current produces a lot of heat in the wires.
2. If inflammable material like wood, cloth or plastic is nearby, it can catch fire. Buildings may catch fire and appliances get damaged.
3. To prevent this, a fuse wire or MCB is used. It breaks the circuit as soon as a high current flows.

1. The coil in a bulb must get very hot to emit light. It is heated to nearly 3400 °C.
2. Tungsten has a very high melting point (about 3400 °C) and high resistivity, so it gets very hot and glows without melting.
3. A solenoid type coil fits a long wire in a small space, so more heat and light are produced.

1. Heat produced is H = I²Rt, so a material with high resistance produces more heat.
2. Nichrome has a much higher resistivity than pure metals, and a high melting point.
3. It also does not oxidise (burn) easily when red hot. Pure metals have low resistivity and oxidise quickly at high temperature. So nichrome is used.

1. Copper and aluminium have very low resistivity. They are good conductors of electricity.
2. So very little electrical energy is lost as heat during transmission.
3. Aluminium is also light and cheap, which suits long overhead lines.

1. The joule is a very small unit of energy.
2. The energy used in homes and industries is very large. In joules, it would be a huge number that is difficult to write and read.
3. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J. So kWh (1 unit) is a convenient unit, and it is used in electricity bills.

The correct statement is (d) The magnetic lines of force are in concentric circles with the wire as the center, in a plane perpendicular to the conductor.

This can be seen with iron filings spread on a cardboard around the wire. Their direction is given by the right hand thumb rule.

Solenoid: When a copper wire with resistive coating is wound in a chain of loops (like a spring), it is called a solenoid.

Comparison:

So the magnetic field of a solenoid is very similar to the field of a bar magnet.

Diagram (a) shows Fleming’s right hand rule.

Stretch the thumb, index finger and middle finger of the right hand so that they are perpendicular to each other. Then:

• the thumb shows the direction of motion of the conductor,
• the index finger shows the direction of the magnetic field,
• the middle finger shows the direction of the induced current.

It is used to find the direction of the induced current, e.g. in an electric generator.

Diagram (b) shows Fleming’s left hand rule.

Stretch the thumb, index finger and middle finger of the left hand so that they are perpendicular to each other. Then:

• the index finger shows the direction of the magnetic field,
• the middle finger shows the direction of the current,
• the thumb shows the direction of the force on the conductor.

It is used to find the direction of the force on a current carrying conductor, e.g. in an electric motor.

Figure (a) shows fuses. A fuse wire melts when a large current flows in the circuit, due to a short circuit or overloading. This breaks the circuit and prevents fire and damage to appliances.

Figure (b) shows a miniature circuit breaker (MCB). It is a switch used in homes. When the current in the circuit suddenly increases, it opens automatically and the current stops. It can be switched on again after the fault is removed.

Figure (c) shows an electric generator (DC generator). A coil rotates between the poles of a magnet and is connected to a galvanometer. It is used to convert mechanical energy into electrical energy, i.e. to produce electricity. (See the diagram and explanation in Q5.)

Given: P = 100 W, I = 3 A. To find: R

The resistance is 11 Ω.

Given: P₁ = 100 W, P₂ = 60 W, V = 220 V. To find: total current I

The current in the main conductor is 0.72 A.

250 Wh > 200 Wh, so the TV set will spend more electrical energy.

Given: P = 1100 W = 1.1 kW, time = 2 h per day, April has 30 days, rate = Rs 5 per unit.

The electricity expense for April is Rs 330.

A simple model of a generator can be made under the guidance of your teacher:

1. Wind about 200 turns of thin enamelled copper wire around a small cardboard box, leaving the middle open.
2. Push a nail through the box as an axle, and fix a strong bar magnet (or two ring magnets) on the nail inside the box.
3. Scrape the enamel off the two ends of the wire and connect them to a small LED.
4. Spin the nail fast by hand or with a thread. The moving magnet changes the magnetic field through the coil, a current is induced, and the LED flickers.