Alternative Current

Question 1

Root mean square definition

Answer 1

The root mean square value of the alternating current or voltage is that value of the direct current or voltage that would produce thermal energy at the same rate in a resistor.

Question 2

What is the function of an alternating current

Answer 2

Uses electromagnetic induction to step up or step down an alternative voltage

Question 3

How does a transformer work

Answer 3

When an alternative current flows through the primary coil, it sets up a varying magnetic field in the iron core which links the primary coil with the secondary coil.

By faraday’s law of emi, the varying magnetic field induces an alternating emf across each turn of wire in both the primary and secondary coil.

Question 4

Defition of period

Answer 4

Time taken for one complete cycle

Question 5

Frequency definition AC

Answer 5

Number of complete cycles per unit time

Question 6

Peak current I。

Answer 6

Amplitude of the current

Question 7

Features of an ideal transformer

Answer 7

Resistances of primary and secondary cool zero

No flux leakage. Both could have the same magnetic flux through them

It is 100% efficient (power o/p = power I/p)

Question 8

4 causes of power loss in transformer

Answer 8

Heating effect of the current in copper wires of the coils

Heating effect due to eddy currents induced in the iron core

Energy is used to the process of magnetising the iron and reversing the magnetisation every time current
reverses, heating up the core

Some of the magnetic field lines produced by the primary coil donnit link with the secondary coil, reducing emf induced in secondary coil

Question 9

4 design features to reduce power loss

Answer 9

Thick copper wire of Low resistance is used, particularly for coil carrying high current at Low voltage

The iron core is made of laminated sheets, cutting across the paths of the eddy currents

The iron core is made of soft iron, which can be easily magnetised and de magnet used by magnetic field of primary coil

Flux leaked can be minimised by having a E-I shaped iron core, the secondary coil is wound on TOP of the primary coil and the iron core forms a closed loop

Question 10

Advantages of using Ac

Answer 10

Can be stepped up and down easily compared to dc for high voltage transmission

High voltage transmission of electrical power is more efficient due to power losses in the cables

Question 11

Definition of rectification

Answer 11

Rectification is the process of converting an alternative current to a direct current

Question 12

Rms of sinusoidal waveform

Answer 12

I。/ root 2

Average power p。/2

Question 13

Rms of half wave rectificied

Answer 13

I。/2

Average power P。/4

Question 14

Rms of full wave rectified

Answer 14

I。/root 2

Average power P。/2

Question 15

Rms rectangle waveform

Answer 15

I。

Average power P。

Question 16

what is the purpose of the iron core

Answer 16

To confine magnetic field lines to Ensure max magnetic flux linkage between primary and secondary coils.