Topic 12 - Electromagnetic induction Flashcards
How can an emf be induced?
By relative motion between a conductor and a magnetic field
Which factors affect the size of the emf induced?
- The speed of the wire
- The strength of the magnetic field
- The length of the wire in the magnetic field
Explain how an emf is induced in a conductor moving in a magnetic field
The magnetic force pushes the free electrons on the conductor to one side, so that side becomes negative. At the same time the other side is left with protons and becomes positive. This charge separation produces a potential difference (emf) across the ends of the conductor
What is required in order an emf to be induced due to relative motion between a conductor and a magnetic field?
The motion needs to be across the magnetic field
(a motion parallel will not induce an emf)

What are four ways to increase the emf induced?
- Faster motion
- Using a magnet with a greater magnetic field strength
- Wrapping a wire into a coil with many turns
- Using magnetic fields and coils with larger areas
What reverses the emf and current?
Reversing the motion or the magnetic field.
Derive the formula for the emf induced in a straight conductor moving in a magnetic field

What is magnetic flux?
The product of the area, A, and the component of the magnetic field strength perpendicular to that area, Bcosθ
Φ = BAcosθ

What is the magnetic flux if the magnetic field is perpendicular to the area?
Because cosθ = 1,
Φ = BA
What is the relationship between field strength, area, and direction and magnetic flux?
- There is greater flux through B than A because the field strength is greater over the same area (more lines)
- There is more flux through C than D because the area is greater for the same field strength
- There is more flux through E than F because the field is perpendicular to the area

What is magnetic flux linkage?
The product of magnetic flux and the number of turns in the circuit (no symbol):
Magnetic flux linkage = NΦ
N = number of turns in a circuit
How is an emf induced by a time-changing magnetic flux?
(see picture)
- Changes in the current in one circuit affect another, separate circuit
- When the switch of A is closed, a current starts to flow around circuit A and this sets up a magnetic field around it
- The magnetic field passes through circuit B
- The sudden change of magnetic field induces an emf and a current
- Only lasts for a moment because the current in A becomes constant after a while
- When the switch is turned off, there is an induced emf/current for a moment in the opposite direction

What are the three ways in which an emf can be induced?
- Part of a circuit can be moved through a stationary magnetic field
- A magnetic field (bar magnet) can be moved through a circuit
- A changing current in one circuit can produce a changing magnetic field which spreads out and passes through another circuit
What is Faraday’s law?
The magnitude of an induced emf is equal to the rate of change of magnetic flux linkage:

What is Lenz’s law?
The direction of an induced emf is such that it will oppose the change that produced it (negative sign in Faraday’s equation)
Describe the emf in a coil rotating at constant speed within a uniform magnetic field
The induced emf is sinusoidal
Explain the function of an AC generator
- The coil of wire rotates in the magnetic field due to an external force
- The flux linkage of the coil changes with time and induces an emf
- The work done rotating the coil generates electrical energy
- Carbon brushes are in contact with slip rings so that induced current can flow into an external circuit

What are the amplitudes of emf when the coil in an AC generator is at different positions?
- The plane of the coil is parallel to the magnetic field, WX and YZ are cutting across the field at the fastest rate → maximum emf
- WX and YZ are moving parallel to the magnetic field → no emf
- Sides are moving in the opposite direction to 1. → reverse maximum emf

What is the effect of changing the frequency of an AC generator on the emf induced?
Slower frequency → smaller rate of change of magnetic flux through it → smaller emf
Halving the frequency will halve the rate of change and therefore halve the induced emf (time period is doubled)
What is root mean square (rms) value?
The rms value given to an alternating current (or voltage) is the same as the value of a direct current (or voltage) that would dissipate power in an ohmic resistor at the same rate

What is the maximum power generated in an AC circuit?
- The power varies but always remains positive because V and I are either both positive or both negative at all times

What is the average power in an AC circuit?

What are effective values of the current and voltage?
The values that give the average power, not the average values, but are less than the maximum values:
(also known as rms value)

What is the resistance in an AC circuit?


