QCEphysics

Electromagnetic induction

define the terms magnetic flux, magnetic flux destiny, electromagnetic induction, electromotive force (EMF), Faraday's law & Lenz's law

- Magnetic flux is a measurement of the total magnetic field that passes through a given area

• A measure of the number of magnetic field lines passing through the given area
• Weber - Wb

- Magnetic flux density is the strength of a magnetic field or the number of magnetic field lines per unit area

• Magnetic field strength = flux density
• Wbm-2 = T

- Electromagnetic induction is the production of an electromotive force (EMF) or voltage across an electrical conductor due to its dynamic interaction with a magnetic field

- Electromotive force (EMF) is the difference in potential that tends to give rise to an electric current

• Voltage induced in the coil that produces a current

- Faraday's Law states that when the magnetic flux linking a circuit changes, an electromotive force is induced in the circuit proportional to the rate of change of the flux linkage

- Lenz's Law states that the direction of an induced electric current is such that it produces a current whose magnetic field opposes the change in the circuit or the magnetic field that produces it

solve problems involving the magnetic flux in an electric current-carrying loop

- The angle measured between the normal to the loop and the direction of the field

• The question needs to say 'the perpendicular to the loop' or 'the normal'

• If it says 'the plane of the loop'- convert so that the angle is the normal

▪ 0= 90 - angle (of the plane of the loop) 

describe the process of inducing an EMF across a moving conductor in a magnetic field

- When the flux threading a loop changes, the electrons in the wire that are free to move will experience a force along the length of the wire

• As electrons shift to one end of the wire, there is a net excess of negative charge at that end, and a net excess of positive charge at the other

• Leads to potential difference or EMF, and current will flow in the circuit

solve problems involving Faraday's Law and Lenz's Law

- For Farday's Law, if the question asks for the magnitude of the EMF, do not consider the negative sign as it indicates direction

- For Lenz's Law, Fleming's right-hand rule can be used

- When an induced current flows through the solenoid, the magnetic field produced by the solenoid has a polarity that repels the incoming permanent magnet pole

• Repels the magnet and forces you to do work

▪ Without having to do work, electricity could be generated with no work - would violate the Law of Conservation of energy

• Generators - mechanical energy to electrical energy

North pole approaching - coil produces a north pole at the top end to repel the incoming pole.

As the magnet moves away, the coil produces a south pole to try to keep the magnet there.

To generate a north pole, current has to flow anticlockwise

To generate south pole, induced current has to flow clockwise

explain how transformers work in terms of Faraday's Law and electromagnetic induction.

- Transformers work through mutual induction - the production of an EMF in a circuit by a change in the current in an adjacent circuit that is linked to the first by the flux lines of a magnetic field

- How to produce continuously expanding and collapsing magnetic fields?

- When switch is closed in left-hand circuit, expanding magnetic flux is produced that cuts the right-hand circuit.

- This induces a voltage pulse across the ends of the right-hand coil and opposes the expanding flux (according Lenz's Law)

• No voltage is induced when the current in the left-hand coil is constant

• If the switch is then opened, a collapsing magnetic flux cuts the right-hand coil and voltage is induced, opposite in direction to the original pulse.

- Use a primary circuit driven by an alternating current (AC) - similar to produce by magnet moving in and out of coil

- Transformer is a device that transfers an alternating current from one circuit to another, usually with an increase or decrease in voltage

• If np > ns - step-down transformer - reduces alternating voltage.

  If np < ns - step-up transformer - increases alternating voltage

• In transformers, primary and secondary coils are wound on a soft iron core

▪ Core concentrates magnetic flux lines threading both coils

▪ Transformers do not work with batteries, as the voltage is not changing

Electromagnetic radiation

define and explain electromagnetic radiation in terms of electric fields and magnetic fields. Students should be able to recall the properties of waves (Unit 2 Topic 2: Waves) 

- Electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, light, ultraviolet, X-rays, and gamma rays

○ James Maxwell proposed that if a changing electric field is produced, this changing electric field will produce a magnetic field at right angles. The changing magnetic field would, in turn, produce a changing electric field and the cycle would be repeated.

• Resulting wave is a transverse wave (wave where direction of oscillation of particles is perpendicular to direction of energy transfer)

• Oscillations of EF and MF are in phase

• Energy of EF = Energy of MF (as seem by equal amplitude)

• Electromagnetic waves travel at the speed of light and do not need a medium for propagation (can travel in a vacuum)

• When the source is turned off, the waves still keep travelling.

Students should be able to recall the properties of gamma radiation (Unit 1 Topic 2: Ionising radiation and nuclear reactions).

- Gamma radiation is not made up of charged particles and is not deflected in electric or magnetic fields

• High energy electromagnetic radiation (short wavelength)

- Due to no charge, they have very high penetrating powers