AC Circuits

Question 1

An alternating current is

Answer 1

The current that waves periodically with time in direction

Question 2

What is the root mean square of an alternating current or voltage?

Answer 2

The value of the steady current/voltage that would dissipate heat as the alternating current/voltage in a resistive load

Question 3

The transformer is a device that..

Answer 3

Is used to change the alternating voltage to a larger or smaller value

Question 4

Rectification is

Answer 4

The process of converting an alternating current to a direct current

Question 5

Define the period of an alternating current

Answer 5

The time taken for the completion of one cycle of the a.c.

Question 6

Define frequency of an alternating current.

Answer 6

Number of complete cycles on the a.c. per unit time

Question 7

Define the alternating frequency of a current

Answer 7

The frequency in hertz multiplied by 2π radians per cycle

Question 8

Define the peak value of an alternating current

Answer 8

The maximum value (in terms of magnitude) of the a.c. for one cycle.

Question 9

Describe the use of a laminated core in a transformer

Answer 9

To reduce energy loss due to heat by reducing the size of induced/eddy currents in the core, thus reducing power loss due to heat dissipated.

Question 10

Describe the use of a soft iron core in a transformer

Answer 10

To strongly concentrate the magnetic field in the core to increase flux linkage and minimize flux LEAKAGE

Question 11

How does a transformer work?

Answer 11

• an alternating e.m.f. applied across the primary coil produces an alternating current in the primary coil.
• this sets up a varying magnetic field which cuts the secondary coil due to the presence of the soft iron core.
• there will be a change in magnetic flux linkage in the secondary coil, and by Faraday’s law of electromagnetic induction, an alternating e.m.f. is induced across the secondary coil.
• as the rate of change of magnetic flux linkage alternates due to the primary current, the e.m.f. induced in the secondary coil will be alternating.

Question 12

What is an ideal transformer?

Answer 12

A transformer that is 100% efficient and has no power losses.

Question 13

Define direct current

Answer 13

A current with one direction only since the polarity of the d.c. supply remains constant with time.

Question 14

Why is there a heating effect on a coil connected to alternating current even though the average current in the heating coil is 0?

Answer 14

For a sinusoidal a.c., although the average current is zero over one cycle, current would still pass through the heating coil, independent of the currents direction, causing electrical energy to be converted to thermal energy throughout the cycle.

Hence a heating effect is produced in the coil which depends on the r.m.s. current rather than the average current.