Calculating Energy Transfer In Appliances (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note
Exam code: 8465
Written by: Ashika
Updated on
Did this video help you?
How to Calculate Energy Transferred
Work is done when charge flows through a circuit
Work done is equal to the energy transferred
The amount of energy transferred by electrical work in a component (or appliance) depends upon:
The current, I
The potential difference, V
The amount of time the component is used for, t
When charge flows through a resistor, for example, the energy transferred is what makes the resistor hot
The energy transferred can be calculated using the equation:
E = P × t
Where:
E = energy transferred in joules (J)
P = power in watts (W)
t = time in seconds (s)
Since P = IV, this equation can also be written as:
E = I × V × t
Where:
I = current in amperes (A)
V = potential difference in volts (V)
The energy transferred electrically also depends on the charge and potential difference:
E = Q × V
Where:
Q = charge in coulombs (C)
V = potential difference in volts (V)
When charge flows around a circuit for a given time, the energy transferred from the cell or battery is equal to the energy transferred to all the components in the circuit
These can be rearranged using the following formula triangles:
Worked Example
Calculate the energy transferred in 1 minute when a current of 0.7 A passes through a potential difference of 4 V.
Answer:
Step 1: Write down the known quantities
Time, t = 1 minute = 60 s
Current, I = 0.7 A
Potential difference, V = 4 V
Step 2: Write down the relevant equation
E = I × V × t
Step 3: Substitute in the values
E = 0.7 × 4 × 60 = 168 J
Examiner Tips and Tricks
'Energy transferred' and 'work done' are often used interchangeably in equations, for example in the previous topic on Power. Always remember that the time t in the above equations must be converted into seconds
Students often find it really confusing that there are so many different equations for energy.
When you begin a calculation, start by writing out the known variables. These are the values you are given in the question. Once you know which variables you have and which variable you are calculating, you can use these to select the correct equation. So if you are given power and time, you would use E = Pt, but if you are given charge and potential difference, you would use E = QV.
Sometimes, there may be more than one variable missing, and you may have calculated a value for it in an earlier part of the question. For harder questions, you may need to use another equation to calculate the missing variable. By listing the known quantities at the start of your calculation, you plan how to get to your answer and choose the most appropriate equations for the task.
Power Ratings
The power of an appliance is the amount of energy it transfers by electrical working every second
Every electrical appliance has a power rating which tells you how much energy it needs to work
The power rating for domestic electrical appliances is normally given on a label. This will include:
The potential difference required to make the device work (eg. 230 V in the UK)
The frequency of the supply (eg. 50 Hz in the UK)
The power rating in Watts (this varies for each device)
The higher the power rating, the faster the energy is transferred
For example, a 2000 W kettle means the kettle transfers 2000 J of energy per second
The different power ratings of various household appliances are listed in the table below as examples:
Applicane | Power in W |
|---|---|
Clock | 10 |
Lamp | 50 |
Drill | 800 |
Iron | 1250 |
Kettle | 2400 |
Hot water heater | 3000 |
Electric oven | 12000 |
Care must be taken not to plug an appliance into a mains supply that has a voltage much higher than stated on the label, for example in another country that has a higher mains voltage
This could cause the appliance to fuse or set on fire and become damaged
Unlock more, it's free!
Join the 100,000+ Students that ❤️ Save My Exams
the (exam) results speak for themselves:
Was this revision note helpful?