Past Paper Questions - Dec 2016

Question 1

With the aid of a sketch, explain why the current lags the voltage by 90 degrees in an ideal inductor.

Answer 1

In inductive loads there is a phase difference between the current and the voltage, (current lags voltage by 90 degrees) so indicates a lagging power factor, with the reactive power Q being positive. Sketch magnitude in Y, time in X and plots of voltage and current, 90 degrees out of phase.

Question 2

Electricity is often described as a “high grade” energy source. Very briefly, explain what this means.

Answer 2

High grade means that electricity energy is concentrated in a small space. The molecules or particles that store these forms of energy are highly ordered and compact and thus considered as high grade energy.

Question 3

Briefly explain why the efficiency of many large thermal power stations is below 40%.

Answer 3

The majority of the energy is rejected as heat from the condenser. The heat is sent to a cooling tower or nearby water.

Question 4

List four key polluting emissions of electricity generation using fossil fuels and for each briefly describe their environmental impact.

Answer 4

Coal Emissions

CO2 - Carbon Dioxide: contribution to greenhouse effect and global warming (climate change). Can be stored, not ideal.

NOx - Nitrogen Oxide: contribution to acid rain
staged combustion helps to reduce emissions.

SOx - Sulfur Oxide:
contribution to acid rain
Can be processed to remove impurities such as sulphates.

Particulates: Air quality and health are effected by particulates. Removed by electrostatic precipitation.