Chapter 12: Magnetic Effects of Electric Current
Magnetic field and field lines
Oersted showed that an electric current produces a magnetic field. Field lines are closed curves, run from north to south outside a magnet, never cross each other, and the field is stronger where they are closer together.
Magnetic field of a current-carrying conductor
Right-hand thumb rule: if the thumb points along the current, the curled fingers show the direction of the field. Inside a solenoid the field is uniform and strong, and outside it looks like that of a bar magnet. A soft iron core inside a solenoid makes an electromagnet.
Force in a magnetic field and the motor
A current-carrying conductor experiences a force in a field. Its direction is given by Fleming's left-hand rule (thumb force, forefinger field, middle finger current). The electric motor works on this and converts electrical energy into mechanical energy. The split ring reverses the current every half rotation.
Electromagnetic induction, generator and domestic circuits
Producing a current in a coil by changing the magnetic field is electromagnetic induction (right-hand rule). A generator converts mechanical energy into electrical energy. AC reverses its direction every 1/100 s (50 Hz). A domestic circuit has a live (red), a neutral (black) and an earth (green) wire. Fuses and earthing give safety.
Important questions
Q1. What did Oersted observe?
A compass needle placed near a wire carrying current got deflected, which shows that an electric current produces a magnetic field.
Q2. What are magnetic field lines?
Imaginary lines along which a free north pole would move. Outside a magnet their direction is from the north pole to the south pole.
Q3. State the properties of magnetic field lines.
They are closed curves, never cross each other, and the field is stronger where they are closer together.
Q4. Why can two magnetic field lines never cross each other?
If they did, a compass needle at the point of crossing would point in two directions, which is impossible.
Q5. State the right-hand thumb rule.
Hold the wire in your right hand with the thumb along the direction of current. The curled fingers show the direction of the magnetic field.
Q6. How does the magnetic field around a straight wire change with current and distance?
The field increases when the current increases and decreases as we move away from the wire.
Q7. What is the magnetic field inside a solenoid like?
Uniform and strong, in the form of parallel straight lines.
Q8. What is an electromagnet?
A magnet made by placing a soft iron core inside a solenoid. It behaves as a magnet only while current flows through it.
Q9. On what does the direction of the force on a current-carrying conductor in a magnetic field depend?
On the direction of the current and the direction of the field. The force is greatest when they are perpendicular.
Q10. State Fleming's left-hand rule.
Stretch the thumb, forefinger and middle finger of the left hand perpendicular to each other. If the forefinger points along the field and the middle finger along the current, the thumb shows the direction of the force.
Q11. What is the function of the split ring (commutator) in a motor?
It reverses the direction of the current in the coil after every half rotation, so that the coil keeps rotating in the same direction.
Q12. State the principle and function of an electric motor.
A current-carrying conductor placed in a magnetic field experiences a force. A motor converts electrical energy into mechanical energy.
Q13. What is electromagnetic induction?
The production of an electric current in a coil due to a change in the magnetic field or relative motion between the coil and a magnet.
Q14. State Fleming's right-hand rule.
Stretch the thumb, forefinger and middle finger of the right hand perpendicular to each other. If the thumb shows the motion of the conductor and the forefinger the field, the middle finger shows the direction of the induced current.
Q15. What are the principle and function of an electric generator?
The principle is electromagnetic induction. A generator converts mechanical energy into electrical energy.
Q16. Differentiate between AC and DC.
AC reverses its direction periodically (50 Hz in India), while DC always flows in one direction.
Q17. What is the advantage of AC over DC?
AC can be transmitted over long distances with less loss of energy.
Q18. What are the colours of the live, neutral and earth wires in a domestic circuit?
Live wire: red. Neutral wire: black. Earth wire: green.
Q19. What is the function of the earth wire?
The earth wire carries leaked current to the ground, so the metal body of an appliance is not dangerous to touch.
Q20. Why is a fuse used?
The thin, low-melting-point wire of a fuse melts when too much current flows (short circuit or overloading), breaking the circuit and protecting the appliances.
Quiz
1. Who discovered the magnetic effect of current?
Oersted discovered it from the deflection of a compass needle.
2. What energy change does a motor bring about?
A motor converts electrical energy into mechanical energy.
3. On which principle does a generator work?
A generator works on electromagnetic induction.
4. What is the colour of the earth wire?
In India the earth wire is green.
5. What kind of wire is a fuse wire?
It melts and breaks the circuit when too much current flows.