Magnetism and electricity are deeply connected phenomena. In 1820, Hans Christian Oersted discovered that an electric current flowing through a wire deflects a magnetic compass needle placed near it, proving that an electric current produces a magnetic field. This discovery opened the door to electromagnetism, which today powers electric motors, generators, electromagnets, electric bells, transformers and countless other devices that form the backbone of modern technology.
A magnetic field is the region around a magnet or a current-carrying conductor in which a magnetic force can be detected. It is represented by magnetic field lines, which show the direction and strength of the magnetic field. The magnetic field produced by a current-carrying conductor can be mapped using a compass needle, and its direction is determined by the right-hand thumb rule.
In this chapter, we will study magnetic fields and field lines, the magnetic field around a straight conductor, a circular loop and a solenoid, the force on a current-carrying conductor in a magnetic field, the working of an electric motor, electromagnetic induction, the working of a generator, domestic electric circuits and the safety precautions associated with electricity.
A magnetic field is the space around a magnet or current-carrying conductor in which its influence can be detected. Magnetic field lines are the imaginary lines used to represent the magnetic field.
When current flows through a straight wire, a magnetic field is produced around it in concentric circles. The direction of the field lines depends on the direction of the current and is given by the right-hand thumb rule:
If the current is held in the right hand with the thumb pointing in the direction of the current, the direction in which the fingers curl gives the direction of the magnetic field lines.
A circular loop of wire carrying current produces a magnetic field similar to that of a short bar magnet. The strength of the magnetic field depends on the current and the number of turns of the coil.
A solenoid is a coil of many turns of insulated copper wire wound in the shape of a cylinder. When current flows through a solenoid, it behaves like a bar magnet, with a north pole at one end and a south pole at the other. The magnetic field inside the solenoid is strong and uniform. The strength of the solenoid's field increases with the number of turns and the current. An electromagnet is made by inserting a soft iron core inside a solenoid; when the current is switched off, the iron core loses its magnetism.
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is because the magnetic field of the conductor interacts with the external magnetic field. The direction of this force is given by Fleming's left-hand rule:
If we hold the thumb, forefinger and middle finger of the left hand mutually perpendicular, with the forefinger pointing in the direction of the magnetic field (from N to S) and the middle finger in the direction of the current, then the thumb points in the direction of the force (motion) on the conductor.
The magnitude of the force increases with the strength of the magnetic field, the amount of current, and the length of the conductor in the 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.
A motor consists of:
When current flows through the coil placed in the magnetic field, the two sides of the coil experience forces in opposite directions (by Fleming's left-hand rule), producing a couple that rotates the coil. After half a rotation, the commutator reverses the current direction so that the rotation continues in the same direction. Thus, the coil keeps rotating as long as current flows. Electric motors are used in fans, washing machines, mixers, refrigerators and electric vehicles.
The production of electricity from magnetism is called electromagnetic induction, discovered by Michael Faraday. When there is a relative motion between a conductor and a magnetic field, or when the magnetic field lines passing through a coil change, an electric current is induced in the coil. The induced current flows only as long as the magnetic field is changing.
The potential difference induced in a conductor moving perpendicular to a magnetic field is given by:
$$\varepsilon = Bvl$$
Where B is the magnetic field strength, v is the velocity of the conductor, and l is the length of the conductor in the field.
An electric generator is a device that converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction.
A generator consists of a coil that is rotated in a magnetic field, with slip rings and brushes connected to the coil. As the coil rotates, the magnetic flux linked with the coil changes continuously, inducing a current in the coil. The induced current changes direction with each half rotation, producing alternating current (AC). A generator that produces AC is called an AC generator, while a generator that produces direct current (DC) uses a split ring commutator and is called a DC generator.
In our homes, electricity is distributed through two main wires: the live wire (red/brown, at high potential) and the neutral wire (black/blue, at low potential). In addition, there is an earth wire (green), which provides a path for current in case of a fault.
| Feature | AC | DC |
|---|---|---|
| Direction of current | Changes periodically | Constant |
| Frequency in India | 50 Hz | Zero |
| Generation | AC generator | DC generator, battery, cell |
| Transmission | Can be transmitted over long distances | Difficult to transmit over long distances |
| Source example | Mains supply | Battery, cell |
| Rule/Device | Purpose |
|---|---|
| Right-hand thumb rule | Direction of magnetic field around a current-carrying conductor |
| Fleming's left-hand rule | Direction of force on a current-carrying conductor in a magnetic field |
| Fleming's right-hand rule | Direction of induced current in a generator |
| Electric motor | Converts electrical energy into mechanical energy |
| Electric generator | Converts mechanical energy into electrical energy |
| Electromagnet | Temporary magnet made by a solenoid with an iron core |
The magnetic effects of electric current reveal the deep and elegant connection between electricity and magnetism. Oersted's discovery that current creates a magnetic field, and Faraday's discovery of electromagnetic induction, together unlocked the twin technologies that power the modern world: the electric motor and the electric generator. Magnetic field lines, the right-hand thumb rule, and Fleming's rules give us a clear, systematic way to determine directions in these phenomena. The solenoid and electromagnet demonstrate how a simple coil can become a powerful tool for lifting, switching and controlling. In the home, the principles of the magnetic effect of current are applied in fans and appliances, while safety devices like fuses and earth wires protect us from the dangers of overloading and short-circuiting. This chapter is both conceptually beautiful and practically essential, preparing you for the examination and for a deeper understanding of electromagnetism in higher classes.