2.4 - Energy Transfers

Question 1

In an electrical circuit, what is power?

Answer 1

The rate at which electrical energy is transfered.

Question 2

What is the unit of power?

Answer 2

Watt (W)

Question 3

What symbol is normally used to represent power in equations?

Answer 3

P

Question 4

How could you calculate power from potential difference and current?

Answer 4

Power = current x potential difference

[P = I V]

Question 5

If you do not know the potential difference in a circuit, how can you calculate power?

Answer 5

Use P = I V and V = I R together:

P = I (I R)

P = I² R

Question 6

How do electrical appliances normally work?

Answer 6

They transfer electrical energy into another form of energy (e.g. heat and kinetic for a hairdryer)

Question 7

What factors affect how much energy an appliance uses?

Answer 7

• The power of the appliance (more powerful appliances use more power each second they are on)
• The length of time the appliance is turned on for

Question 8

In an electrical circuit, when is work done?

Answer 8

Work is done when charge flows in a circuit

Question 9

What is the unit for energy?

Answer 9

Joules (J)

Question 10

What symbol is normally used to represent energy in an equation?

Answer 10

E

Question 11

How would you calculate how much energy is transfered by an appliance?

Answer 11

Energy Transferred = Power × time [E = P t]

Or, we can use P = I V and I = Q / t:

• E = P t
• E = I V t
• E = (Q / t) V t
• E = Q V

Question 12

What is the National Grid?

Answer 12

A system of:

• cables
• transformers which allow electrical power to be transferred from power stations to consumers.

Question 13

What is a step-up transformer?

Answer 13

A component which increases the potential difference along a cable. Current (I) is decreased, so power transferred remains the same (P = I V)

Question 14

What is a step-down transformer?

Answer 14

A component which decreases the potential difference along a cable. Current (I) is increased, so power transferred remains the same (P = I V)

Question 15

Why are transformers used in the National Grid?

Answer 15

• Energy is wasted as heat when current flows through a wire.
• The higher the current, the more heat is lost
• To reduce this loss, the National Grid transmits electricity at a low current
• It uses a step-up transformer to increase the voltage along the wire (and decrease current)
• It uses a step-down transformer to decrease the voltage back to a safe level for use in homes