Alternating Currents

Question 1

What is an alternating electricity supply?

Answer 1

One where the emf changes polarity over time

Question 2

What is meant by rms values?

Answer 2

An rms value is an equivalent d.c. alternative which would produce the same average power dissipation as the a.c. value

Question 3

In a cathode ray oscilloscope (C.R.O.) what is measured on each axis?

Answer 3

› Voltage measured on the y axis

› Time measured on the x axis

Question 4

How do resistors behave in a.c. circuits?

Answer 4

Voltage across a resistor and current through the resistor are always in phase

Question 5

How do capacitors behave in a.c. circuits?

Answer 5

The current through the capacitor leads the voltage across the capacitor by 90° (current is 90° ahead of voltage)

Question 6

How do inductors behave in a.c. circuits?

Answer 6

The current through an inductor lags the voltage across the inductor by 90° (current is 90° behind voltage)

Question 7

How are values for reactance for capacitors and inductors plotted with resistance on a phasor diagram?

Answer 7

› Resistance is always a straight arrow to the left
› Reactance of a capacitor is always a straight line downwards
› Reactance of an inductor is always a straight line upwards

Question 8

What happens at the resonant frequency of a circuit?

Answer 8

› Reactance of the capacitor = reactance of the inductor
› The circuit impedance is at a minimum
› The current flowing is at a maximum

Question 9

How can the resonant frequency be found?

Answer 9

By equating the equations for reactance of a capacitor and reactance of an inductor, and rearranging to find f

Question 10

How is a high Q factor achieved in a circuit?

Answer 10

Resistance is made low

Question 11

What happens to a current-frequency graph as Q factor increases?

Answer 11

› The width of the curve decreases

› The range of frequencies that are amplified gets smaller as Q factor increases