Energy Transfers Flashcards

All 34 cards in this deck

What is the equation linking power, potential difference and current?

$\text{power} = \text{potential difference} \times \text{current}$

$P = VI$

What is the equation linking power, current and resistance?

$\text{power} = (\text{current})^2 \times \text{resistance}$

$P = I^2R$

What is 1 watt equal to in joules and seconds?

1 watt = 1 joule of energy transferred per second.

What are the steps to find the current in a device from its power and potential difference?

e.g. a 2300 W heater on a 230 V supply

1. Use $P = VI$: $2300 = 230 \times I$
2. Rearrange: $I = P \div V = 2300 \div 230$
3. $I = 10$ A

What are the steps to find the resistance of a component from its power and current?

e.g. a component dissipating 8 W with a current of 2 A

1. Use $P = I^2R$: $8 = 2^2 \times R$
2. Rearrange: $R = P \div I^2 = 8 \div 4$
3. $R = 2\ \Omega$

True or false? Doubling the current in a fixed resistor doubles the power dissipated.

False. Power quadruples, because $P = I^2R$ and the current is squared.

What is the equation linking energy transferred, charge flow and potential difference?

$\text{energy transferred} = \text{charge flow} \times \text{potential difference}$

$E = QV$

What happens, in terms of work, when charge flows in a circuit?

Work is done, so energy is transferred from the power supply to the circuit components.

What are the units of energy transferred, charge flow and potential difference in $E = QV$?

Energy in joules (J), charge in coulombs (C), potential difference in volts (V).

In an electric kettle, energy is transferred from which store to which store?

From the ac mains, by electrical work, to the thermal energy store of the water.

What are the steps to find the energy transferred from the charge flow and the potential difference?

e.g. 20 C of charge flows through a 12 V supply

1. Use $E = QV$
2. Substitute: $E = 20 \times 12$
3. $E = 240$ J

What is the equation linking energy transferred, power and time?

$\text{energy transferred} = \text{power} \times \text{time}$

$E = Pt$

How many seconds are there in one hour?

3600 s ($60 \times 60$).

What are the steps to calculate the energy transferred by an appliance in a given number of minutes?

e.g. a 100 W lamp switched on for 2 minutes

1. Convert time to seconds: $2 \times 60 = 120$ s
2. Use $E = Pt$: $E = 100 \times 120$
3. $E = 12\,000$ J

What are the steps to find the time taken from the energy transferred and the power?

e.g. 3.6 kJ transferred by a 120 W device

1. Convert energy to joules: $3.6$ kJ $= 3600$ J
2. Use $E = Pt$: $3600 = 120 \times t$
3. $t = 3600 \div 120 = 30$ s

What are the steps to find the energy transferred from the current, potential difference and time?

e.g. 2 A at 12 V for 10 s

1. Find power with $P = VI$: $P = 12 \times 2 = 24$ W
2. Use $E = Pt$: $E = 24 \times 10$
3. $E = 240$ J

True or false? Time must be converted into seconds in $E = Pt$ if the energy is to come out in joules.

True. Watts are joules per second, so minutes and hours must be converted to seconds.

What does the power rating of a domestic appliance tell you?

The energy it transfers each second when in use, measured in watts (or kilowatts).

The amount of energy an appliance transfers depends on which two things?

The power of the appliance and how long it is switched on for.

True or false? An appliance with a higher power rating always transfers more energy than one with a lower rating.

False. It also depends on time — a low-power appliance left on for longer can transfer more energy.

True or false? If two appliances are used for the same length of time, the one with the higher power rating transfers more energy.

True. $E = Pt$, so with equal times the greater power transfers more energy.

Two chargers work at the same potential difference, but charger B has double the current of charger A. How do their charging times for the same battery compare?

Charger B takes half the time — double the current means double the power, so the energy is transferred in half the time.

What is the National Grid?

A system of cables and transformers linking power stations to consumers.

What two types of transformer are used in the National Grid?

Step-up transformers and step-down transformers.

What does a step-up transformer do in the National Grid, and where is it?

It increases the potential difference between the power station and the transmission cables.

What does a step-down transformer do in the National Grid?

It decreases the potential difference to a much lower value for domestic use.

True or false? Electricity is transmitted through National Grid cables at the same potential difference as the mains supply in homes.

False. Transmission is at a much higher potential difference, which is stepped down for domestic use.

What is transferred from power stations to consumers by the National Grid?

Electrical power (energy transferred by electrical work).

What is the advantage of transmitting electricity at a very high potential difference?

The current is very low, so less energy is transferred to the surroundings by heating in the cables, making transmission more efficient.

What effect does a step-up transformer have on the current in the transmission cables?

It decreases the current (as it increases the potential difference).

Which equation explains why a low current in the transmission cables reduces energy losses?

$P = I^2R$ — power lost as heating in the cables depends on the square of the current.

True or false? Electricity is transmitted at high potential difference because energy travels faster along the cables.

False. It is to keep the current low so less energy is wasted heating the cables.

True or false? Step-up transformers increase the amount of energy supplied to the National Grid.

False. They increase the potential difference and decrease the current; they do not create energy.

What would be the effect of transmitting electricity along the National Grid at a low potential difference?

The current would be high, so much more energy would be wasted heating the cables and transmission would be less efficient.