Alternating current and transformers

What is an alternating current?

A current or voltage that changes direction with time

Example of an AC producer

An alternator is a generator of AC

How do alternators work

• convert KE into electrical energy - induce electric current by rotating a coil in a magnetic field

• has slip ring commutators to connect coil to circuit

• output voltage and current will change direction every half rotation

• creates AC

How do transformers work?

1. Alternating current flows through the primary coil

2. Creates an alternating magnetic field (enhanced by the iron core)

3. The alternating magnetic field passes through the secondary coil

4. The change in magnetic flux linkage induces an emf in the secondary coil that alternates at the same frequency as in the input voltage

5. The ratio of the number of turns on each coil and the voltage across the primary coil determines the size fo the emf induced on the secondary

Limitations of transformers

Only works using AC as a change in flux is needed to induce an emf

They are not 100% efficient

What is the purpose of the iron core?

To enhance the magnetic field of the primary coil to maximise energy transfer

Transformer equation

number of turns of secondary/primary = voltage of secondary/primary = current of primary/secondary

Transformer power equation in ideal transformer

Power of primary = power of secondary

Combination of transformers and faradays law

voltage of p = -(number of turns in the primary coil x change in flux)/time

voltage of s = -(number of turns in the secondary coil x change in flux)/time

How to find the root mean square of voltage using an oscilloscope?

Vrms = V0/root 2

V0 = peak voltage

What is the point of finding rms of V?

To compare ac to dc supply

How do you find the average power of an ac supply?

Find Vrms and Irms

P = IrmsVrms

How do you find the root mean square of current using an oscilloscope?

Irm = I0/ root 2