Electromagnetism: Class 10 Physics

Key points

A magnet has a north and a south pole. Like poles repel and unlike poles attract. The space around a magnet where its force can be felt is the magnetic field. Field lines leave the north pole and enter the south pole outside the magnet. They never cross, and they are closer together where the field is stronger. The strength of a magnetic field is called magnetic flux density B. Its unit is the tesla (T).

A current carrying conductor produces a magnetic field. This was discovered by Oersted. Around a long straight wire the field lines are circles centred on the wire. The direction is given by the right hand grip rule: hold the wire in your right hand with the thumb pointing along the current, and the curled fingers show the direction of the field. The field is stronger when the current is larger and weaker as the distance from the wire grows.

A solenoid is a long coil of wire. When current passes through it, the field inside is strong and almost uniform, and the field outside looks like that of a bar magnet. Curl the fingers of the right hand in the direction of the current and the thumb points to the north pole. The field of a solenoid is made stronger by more turns, a larger current, and by placing a soft iron core inside. A solenoid with an iron core is an electromagnet. It is used in electric bells, cranes for lifting scrap and relays.

A current carrying conductor placed in a magnetic field feels a force. The force is largest when the wire is at right angles to the field. The magnitude is F = B I L, where B is the flux density, I the current and L the length of the wire in the field. When the wire is parallel to the field there is no force. In general F = B I L sin θ, where θ is the angle between the wire and the field.

The direction of the force is given by Fleming's left hand rule. Stretch the thumb, the first finger and the second finger of the left hand at right angles to each other. If the First finger points along the Field and the seCond finger along the Current, then the Thumb points in the direction of the force (Thrust or motion).

A moving coil galvanometer uses this force. A coil is pivoted between the poles of a strong magnet. Current in the coil makes the two sides feel forces in opposite directions, which turns the coil. A spring provides a restoring force, so the deflection is proportional to the current. A pointer on a scale shows the current. A galvanometer can be changed into an ammeter by connecting a small resistance (a shunt) in parallel, and into a voltmeter by connecting a large resistance in series.

A DC motor changes electrical energy into mechanical energy. A rectangular coil sits between the poles of a magnet and is joined to a split ring commutator, which touches two carbon brushes. The forces on the two sides of the coil are in opposite directions, so the coil rotates. The split ring reverses the current every half turn so the coil keeps turning in the same direction. The speed can be raised by increasing the current, the number of turns or the field strength.

Electromagnetic induction is the production of an e.m.f. in a conductor when the magnetic flux linked with it changes. This was discovered by Faraday. An e.m.f. appears when a magnet is moved into or out of a coil, or when a conductor cuts magnetic field lines. If the circuit is closed, a current flows. No e.m.f. is induced when there is no relative motion.

Laws of induction:

• Faraday's law: the induced e.m.f. is proportional to the rate of change of magnetic flux linkage. For a coil of N turns, ε = N ΔΦ / Δt, where Φ is the magnetic flux in weber (Wb).
• Lenz's law: the induced current flows in a direction such that it opposes the change that produced it. This follows from conservation of energy. Pushing the north pole of a magnet toward a coil makes the near end of the coil a north pole that repels the magnet.
• For a straight conductor of length L moving at speed v at right angles to a field, ε = B L v.

An AC generator (dynamo) changes mechanical energy into electrical energy. A coil is rotated in a magnetic field. The flux linked with the coil changes continuously, so an alternating e.m.f. is induced. The coil ends are connected to two slip rings, which keep contact with the brushes, so the output reverses every half turn. The e.m.f. is increased by rotating the coil faster, using more turns, a stronger magnet or a larger coil.

A transformer changes the voltage of an a.c. supply. It has a primary coil and a secondary coil wound on a soft iron core. The changing current in the primary produces a changing flux in the core, which induces an e.m.f. in the secondary (mutual induction). Vs / Vp = Ns / Np. A step up transformer has more turns on the secondary and raises the voltage. A step down transformer has fewer turns on the secondary and lowers it. For an ideal transformer, power in equals power out, so Vp Ip = Vs Is. A transformer works only with a.c. In power transmission, the voltage is raised to cut the current and so reduce the heat loss (I² R) in the lines.

Study tip: Practise drawing the motor and the generator with labelled parts, because both are repeated board questions. Keep the two hand rules separate by remembering that the motor uses the left hand and gives a force from a current. In transformer numericals, check that the power out is not greater than the power in.

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Common mistakes

• Mixing up the left hand rule (for motors, force on a current) with the right hand rule (for generators and field direction).
• Forgetting that there is no induced e.m.f. when the magnet and coil are at rest relative to each other.
• Using the transformer turns ratio upside down. The secondary voltage is Vp × Ns / Np.
• Saying a transformer can be used with a battery. It needs a.c.
• Writing the Lenz's law direction as helping the change. It always opposes it.

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