Past Paper Questions - Jan 2015

Question 1

How do you convert from waveform to phasor?

Answer 1

Divide the magnitude by root 2, and angle is in the brackets.

Question 2

What is the cosine rule?

Answer 2

CosACosB = 0.5*[Cos(A-B) + Cos(A+B)]

Question 3

What is the sine rule?

Answer 3

SinASinB = 0.5*[Cos(A-B) - Cos(A+B)]

Question 4

Explain why the electricity transmission network operates at extremely high voltage levels?

Answer 4

High voltage allows large quantities of power to be transmitted with low current flow. As power losses are proportional to i squared, cutting down losses improves the systems efficiency.

Question 5

State the relationship between the time varying

current and voltage waveforms in a pure inductor.

Answer 5

In an inductor, voltage is proportional to the change in current over the change in time, current lags the voltage by 90 degrees. (Think of the graph)

Question 6

State 3 differences between an inductor and a resistor.

Answer 6

1. Voltage and current are in phase for a resistor, current lags the voltage in an inductor.
2. Power is dissipated in a resistor, power is not dissipated in an inductor.
3. No energy storage in a resistor, whereas energy is stored and discharged in an inductor.