Calculating Energy Transfers Flashcards

All 6 cards in this deck

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.