Magnetic flux & Electromagnetic Induction : Magnetic Fields

What is magnetic flux density?

• It is the number of flux lines (flux) per square metre.

What is magnetic flux?

• The number of flux lines that pass through (or link) with each turn on a coil.

How do you calculate magnetic flux of a coil with a single turn? When is this formula used?

• Magnetic Flux = BA

• This equation is used where B is normal to A.

What is the unit for magnetic flux?

Wb (webers)

When does a coil cut through the maximum number of flux lines?

• When B is normal (parallel) to A - aka magnetic filed lines are parallel to the area vector.

What happens when a coil of wire is at an angle (or being rotated through an angle)?

It cuts through less flux.

.. would not get a maximum flux linkage calculated value.

How do you calculate the magnetic flux of a coil that is at an angle (or being rotated through an angle)?

• Magnetic flux = BAcos(0)

• where cos (0) is the angle between B & A ( the magnetic field & the area vector).

When the angle between the magnetic field lines & the area vector increases, what happens?

• Increasing the angle, reduces the magnetic flux (as the coil is cutting through less flux), which reduces the emf induced.

How do you calculate the magnetic flux (linkage) of a coil that has multiple turns?

• (N x magnetic flux) = BAN

• aka BAN

How do you calculate the magnetic flux of a coil with multiple turns that is being rotated (or positioned at an angle)?

• (N x magnetic flux) = BANcos(0)

• where cos(0) is again the angle between B & A.

Draw graph which shows how the magnetic flux of a coil rotating in a field changes, as it rotates from 0 to 360 degrees?

What is Faraday’s Law?

• The rate of change of flux linkage (just quote the equation).

How does electromagnetic induction work?

• When a wire experiences a change in flux linkage.

• This generates an emf

• .. which will generate a current (in specific an eddy current).

When a coil is parallel to the magnetic field or not moving, how does this affect flux linkage & induced emf??

• The coil would cut through maximum flux linkage.

• However, would not induce an emf, as it is not experiencing a change in flux linkage.

Explain how electricity would be generated with a coil & magnetic field:

• There are two ways:

• one where a coil rotates about a magnetic field (to experience a change in flux linkage & hence induce a current)

• Another where a magnetic field (e.g. a magnet) moves about a coil (to change flux linkage)

What is Lenz’s law?

• The direction of the induced current will oppose the direction of the change that is causing it (the change in magnetic field).

How does Lenz's law relate to Farday’s Law?

• When relating to faraday’s law:

• e = - change(N X magnetic flux)/ change in time

• direction of induced emf (would generate a current to create magnetic field that) opposes the change in magnetic field that caused it (hence why value is a negative).

Draw a graph to show how the magnetic flux linkage varies as a coil rotates in a uniform magnetic field: What is the gradient of this graph?

The gradient of the graph is the induced emf (faraday’s law equation), but this is a negative gradient due to Lenz’s law.

Draw an emf/ time graph as a coil is rotated through a magnetic field:

How can calculate the induced emf from an emf/ time graph?

• Use the equation NBAwsin(wt)

• IMPORTANT NOTE: “NBAw” is the maximum emf of the graph.

• You can find omege by findinding the time period of the graph.