Power Equations
Brook Edgar & Hannah Shuter
Teachers
Contents
Explainer Video
Power Equations
Power is defined as the rate of energy transfer. The unit of power is Watts. It can be calculated using the following equation first seen in Topic :
Formula:
If we want to find the power of an electrical appliance, we can either use this equation, or two other equations.
Formula:
or
Formula:
Which equation we use is decided by what quantities we are given in the question.
Example: If we wanted to work out the power of a washing machine which is plugged into the UK mains and carrying a current of , we would use the equation since we know the potential difference () of the mains in the UK is and we are given the current () in the question. We can not use the other two equations as we are not given the energy nor the resistance.
Example: My air fryer has a broken fuse, and I want to know whether to replace it with a or a fuse. I can read from the label that the air fryer has a power of and a resistance of .
I know the power (), the resistance () and I am looking for the current (). The only equation that has all three of these variables is . I must also remember to convert into by multiplying by :
Therefore, I should use a fuse, as it allows the of current needed to flow but will break if any higher current is drawn. The fuse allows too much current to flow before breaking, so I would likely get electrocuted before the wire in the fuse would melt.
Example: I want to know how long my oven needs to be on to cook a chicken. The oven has a power of and needs to transfer of energy to the chicken to cook it.
I know the power (), the energy transferred () and we are looking for the time taken (). The equation with all three of these variables is :
*Remember, if you have forgotten how to rearrange equations, go back to the first few pages of notes in Topic for detailed steps.
Worked Example:
A toaster draws from the UK mains. Calculate its resistance and the power it uses.
Answer:
The toaster is plugged into the UK mains, so the potential difference is .
Using :
We could use either or to find the power; however, in an exam, it is good practice to only use values given in the question rather than calculated ones in case they are wrong, so we will use :
Worked Example:
An electric car has a power consumption of and a battery potential difference of . Calculate the current in the motor.
Answer:
First we need to convert by multiplying by so becomes .
Energy Transfers in Everyday Appliances
Work is done when charge flows in a circuit. This means that energy is transferred. Appliances are designed to transfer energy from one form into another. The amount of energy an appliance transfers depends on how long it is switched on and its power rating.
These types of questions usually involve two equations, so they are worth more marks.
Worked Example:
A lightbulb plugged into the UK mains transfers of energy in minutes. Calculate the current in the lightbulb.
Answer:
This question will require us to apply two equations and is normally worth five or more marks in an exam.
We know the pd is as the question states that the UK mains is used.
To calculate the current in the light bulb using the equation we first need to calculate the power using the equation, .
*Remember we need to convert minutes into seconds first.
Now we can calculate the current in the bulb:
Practice Questions
A watch battery lasts around years. Its charge content is about and it has a potential difference of .
Convert years to seconds, stating your answer to significant figures.
Calculate the average current supplied by the watch battery.
Estimate the power consumption of the watch.
-> Check out Hannah's video explanation for more help.
Answer:
A electric radiator draws power through a cable with a resistance of . When it is switched on, the current in the wire is .
What is the potential difference between the two ends of the cable?
What is the resistance of the radiator?
What is the power wasted by heating in the cable?
-> Check out Hannah's video explanation for more help.
Answer: