Alternating Current and Voltage in Power Systems

What is one of the main advantages of using alternating current (a.c.)?

• It produces less arcing and sparking on contacts.

• It is more efficient than d.c.

• It requires less maintenance.

• It is more environmentally friendly.

Answer explanation

One of the main advantages of using alternating current (a.c.) is that it produces less arcing and sparking on contacts compared to direct current (d.c.), which enhances the longevity and reliability of electrical components.

What happens to the electromotive force (EMF) in alternating current (a.c.)?

• It periodically reverses its direction.

• It remains constant.

• It increases over time.

• It decreases over time.

Answer explanation

In alternating current (a.c.), the electromotive force (EMF) periodically reverses its direction, which is a defining characteristic of a.c. Unlike direct current (d.c.), where the flow is constant, a.c. changes direction cyclically.

What device is used to transform voltages in an a.c. system?

• Transformer

• Rectifier

• Inverter

• Converter

Answer explanation

A transformer is the device specifically designed to change voltage levels in an alternating current (a.c.) system, making it the correct choice. Rectifiers, inverters, and converters serve different functions.

What is the voltage stepped up to for transmission across the UK?

• 400,000 volts (400kV)

• 300,000 volts (300kV)

• 500,000 volts (500kV)

• 600,000 volts (600kV)

Answer explanation

The voltage for transmission across the UK is stepped up to 400,000 volts (400kV) to minimize energy loss over long distances, making it the correct choice among the options provided.

At what voltage is electricity typically supplied to domestic users in the UK?

• 230V

• 110V

• 400V

• 500V

Answer explanation

In the UK, domestic electricity is typically supplied at a voltage of 230V. This standard voltage is used for most household appliances, making it the correct choice among the options provided.

What is the purpose of using a step-up transformer in power stations?

• To increase voltage for long-distance transmission.

• To decrease voltage for local distribution.

• To stabilise voltage fluctuations.

• To convert a.c. to d.c.

Answer explanation

A step-up transformer increases voltage, which reduces current for long-distance transmission. This minimizes energy loss due to resistance in the wires, making it efficient for transporting electricity over vast distances.

What is the main reason for not using direct current (d.c.) for transmission and distribution?

• Transformers do not work with d.c.

• D.c. is more expensive.

• D.c. is less efficient.

• D.c. is less reliable.

Answer explanation

The main reason for not using direct current (d.c.) in transmission is that transformers, which are essential for voltage conversion, do not operate with d.c. This limitation makes it impractical for efficient long-distance power distribution.

What is the voltage level for heavy industry supply in the UK?

• 33kV or 11kV

• 50kV or 25kV

• 66kV or 33kV

• 100kV or 50kV

Answer explanation

In the UK, heavy industry typically uses supply voltages of 33kV or 11kV. These levels are standard for industrial applications, making this the correct choice among the options provided.

What type of transformer is used to increase the voltage from 25kV to 275kV?

• Step-down transformer

• Step-up transformer

• Isolation transformer

• Auto transformer

Answer explanation

A step-up transformer is designed to increase voltage levels. In this case, it raises the voltage from 25kV to 275kV, making it the correct choice for this application.

What is the voltage level after the final step-down transformer before reaching residential areas?

• 11kV

• 33kV

• 230V

• 400V

Answer explanation

The final step-down transformer reduces voltage to 230V for residential use, making it safe and suitable for household appliances. Other options like 11kV, 33kV, and 400V are higher voltages not used directly in homes.