Electromagnetic InductionCambridge IGCSE Physics: Revision notes

Electromagnetic induction is the process by which an e.m.f. (electromotive force) is induced in a conductor when it moves through a magnetic field or when the magnetic flux through it changes.

An e.m.f. is induced because:

• A moving conductor cuts through magnetic field lines
• A changing magnetic field links with a stationary conductor
• The change in magnetic flux triggers the induction of e.m.f.

This is a key principle that explains how electrical generators work and is central to energy conversion from mechanical to electrical form.

The most common experimental demonstration uses a moving magnet and a coil:

1. A coil of wire is connected to a galvanometer (or sensitive ammeter) to detect induced current
2. A bar magnet is moved towards or away from the coil
3. As the magnet moves, the galvanometer needle deflects, showing that current has been induced
4. The direction of deflection reverses when the magnet moves in the opposite direction

Alternatively, a conductor moving through a magnetic field can be demonstrated:

1. A straight wire is placed perpendicular to a magnetic field (between magnet poles)
2. The wire is moved at right angles to both the field and itself
3. An e.m.f. is induced, which can be detected across the ends of the wire

Key observations:

• An e.m.f. is only induced when there is relative motion or change in flux
• No e.m.f. is induced if the magnet is stationary relative to the coil
• The faster the change, the greater the induced e.m.f.

The magnitude (size) of the induced e.m.f. depends on three key factors:

For a conductor moving perpendicular to a magnetic field:

e.m.f. = B × L × v

Additional factors affecting induced e.m.f. in coils:

• Number of turns (N): More turns → larger e.m.f. (e.m.f. is proportional to the number of turns)
• Rate of change of flux: Faster change → larger e.m.f.
• Angle between motion and field: Maximum e.m.f. when motion is perpendicular to the field; zero when parallel

This relationship shows that inducing a large e.m.f. requires strong fields, rapid motion, and/or long conductors.

Lenz's law states that the direction of an induced e.m.f. (and the resulting induced current) opposes the change that causes it.

What this means in practice:

• When a magnet is pushed towards a coil, the induced current flows in a direction to create a repelling magnetic field
• When a magnet is pulled away from a coil, the induced current flows in a direction to create an attracting magnetic field
• The induced effect always resists the original change

Consequences of Lenz's law:

• Energy must be supplied to move the conductor or magnet (you must do work against the magnetic force)
• The induced current creates its own magnetic field that opposes the change in flux
• This principle ensures conservation of energy: mechanical work is converted to electrical energy

Why it works:

• The induced current cannot make things easier for the change causing it—this would violate energy conservation
• Instead, the induced current always works to maintain the status quo, opposing any disturbance

Practical consequence: When generating electricity, you must continuously push or pull to maintain motion, which is why generators require mechanical energy input.

The right-hand rule connects the directions of magnetic field, current, and force:

For a current-carrying conductor in a magnetic field:

1. Point your thumb in the direction of current (I)
2. Point your fingers in the direction of the magnetic field (B)
3. Your palm pushes in the direction of the force (F)

This gives the relationship: F, B, and I are mutually perpendicular

Alternatively, for induced e.m.f. and current direction (when a conductor moves through a field):

1. Point your fingers in the direction of the magnetic field (B)
2. Point your thumb in the direction of motion/velocity (v)
3. Your palm shows the direction of the force on positive charges (and thus the induced e.m.f. and current)

Key principle:

• The force on moving charges in the conductor determines the direction of induced e.m.f.
• Positive charges experience a force perpendicular to both the field and their motion
• This force drives the induced current
• Negative charges (electrons) experience a force in the opposite direction, but move opposite to the force, creating the same current direction

Practical application:

• In a motor, current and field directions determine the direction of rotation
• In a generator, the motion direction and field direction determine the current direction
• Reversing either the field or the motion reverses the current direction