12.1 - Electromagnetic Induction

Remember that electrons moving downwards are like current flowing upwards.

Use Fleming’s Left Hand Rule to predict which way they will move

1. Conductor moving in a magnetic fieldDescribing the Inducing of EMF – learn these steps

The electrons move towards the left side of the conductor shown in this picture.

If a conductor moves at a speed v through a magnetic field, the electrons inside it receive a force due to their Motion in the Magnetic field.

Because the Charges are separated it creates a Potential Difference between the two ends.

The Charges would ‘like’ to move back but can’t because of the Magnetic Field and the Motion of the conductor which are combining to move them apart.

2. An Induced EMF is createdDescribing the Inducing of EMF – learn these steps

If an external circuit is connected they will flow through it to get to the lower potential.

The current that flows makes it’s own force on the conductor.

You can use Fleming’s Left Hand Rule to predict it too.

This force is in the opposite direction to the motion that is creating the current!

3. Conservation of EnergyDescribing the Inducing of EMF – learn these steps

If there wasn’t an opposing Force being made then once you started pushing the Conductor through the field it would

carry on making current for ever with no more force needed. This would contradict the Law of Conservation of Energy

Force creating the Motion

Fleming’s Left-Hand Rule force created by the Current

4. Calculating the Induced EMFDescribing the Inducing of EMF – learn these steps

As the charges separate an Electric Field Force is made (FE)

Eventually it balances the Magnetic Field Force (FB)

𝐹𝐵=𝐵𝑞𝑣Force on charge in B-Field

𝐸=𝐹𝐸

𝑞=VL

Electric Field Strength Definition

𝐵𝑞𝑣=𝑉𝑞𝐿 𝑉=𝐵𝐿𝑣

This is known as the Induced EMF

𝐹𝐵=𝐹𝐸

Flux Density (B) & Flux (Φ)

Magnetic Flux Density (B)This is just a measure of the closeness of the field lines, or the field lines per unit areaFlux (Φ)This is the sum of all the flux density (field lines per unit area) in an Area.

Φ=BA

Most people find the idea of Flux Φ really hard to understand

Going back to: Flux (Φ).... continued

𝑉=𝐵𝐿𝑣 is also the area that the conductor moves through per second

𝑉=𝐵𝐴𝑡

𝑉=𝐵 𝐴𝑡

emf = Rate of Change of flux

Faraday’s LawFaraday’s Law just puts all of these ideas in one simple equation:

Flux is measured in Tesla metre2 (Tm2) Sometimes a small Phi ( is used: ø or φSometimes an E or Ԑ is used for EMF

𝑉=𝐵 𝐴𝑡

Faraday’s Law Simulations

http://phet.colorado.edu/en/simulation/faraday

Faraday’s Law on PhET

http://phet.colorado.edu/en/simulation/faradays-law

Use these to help you understand the ideas behind Faraday’s Law

Fleming’s Right Hand Rule gives the direction of the Induced (made) Current if we know the Motion and the Field.

Extra Facts 1: Fleming’s Right Hand Rule

All of these examples have the B-field at right angles to the motion of the conductor.

If the field is not perpendicular to the direction of motion then we have to use the component of B that is perpendicular.

The equation becomes:

Extra Facts 2 - Non-Perpendicular B-Fields

𝑩𝒔𝒊𝒏θ

Extra Facts 3 - Coils

Faraday’s Law:

Changes for ‘N’ coils and becomes:

What does it all mean? 𝑉=𝑑𝑁𝛷𝑑𝑡

If you want to make lots of EMF then you have to find a way to change the Flux (=BA) near a conductor.

The more you change the Flux near a conductor, the more EMF you make.

So move things fast so that they cut through a large area of field.

Or alternatively have large changes in the B-field.

That’s what happens in Generators. It’s all about moving magnets or coils (conductors) really fast in relation to each other.

Faraday’s Law?