Topic 11 - Magnetism

Magnetic flux

The normal component of the magnetic field through a closed loop of wire times the area of the loop.

Faraday’s law

The induced emf in a closed loop is proportional to the rate of change of magnetic flux through the loop.

Lenz’ law

A current will be induced in a conductor to oppose any change in magnetic flux across the conductor

Time Constant

The time it takes for the potential difference/charge/current of a capacitor to drop to e-1 = 37% of its initial value while discharging (or increase to 63% of its maximum value while charging)

Magnetic Flux Linkage

Total magnetic flux across a coil with multiple hoops/turns. (Magnetic Flux * Number of turns)

Capacitance

The ratio between the charge of one plate and the potential difference between both plates (a capacitance ability to store charge)

Capacitor

Two conductors with an insulator in between to store energy in the form of electric charge

Explain, using Faraday’s law of induction, how the transformer steps down the voltage

• varying voltage/current in primary coil produces a varying magnetic field

• this produces a change in flux linkage / change in magnetic field in the secondary coil

• a «varying» emf is induced/produced/generated in the secondary coil

• voltage is stepped down as there are more turns on the primary than the secondary

Outline how energy losses are reduced in the core of a practical transformer

• laminated core reduces eddy currents

• less thermal energy is transferred to the surroundings

Step-up transformers are used in power stations to increase the voltage at which the electricity is transmitted. Explain why this is done

• for a certain power to be transmitted, large V means low I

• less thermal energy loss as P = RI^2 / joule heating

• (step up voltage → step down current → R decreases → reduce power loss due to resistance)