# Understanding Current, Potential Difference and Resistance

In this worksheet, students will revise the role of current, resistance and potential difference in circuits as well as apply the fundamental equation V = IR.

Key stage:  KS 4

Difficulty level:

### QUESTION 1 of 10

You need to have a secure understanding of current before you try this activity. If not, we recommend that you go back and try out the activity called 'Understand Current and Charge'.

Did you know that when you use an electrical device, it gets hot? You will already know this if you've previously done the activity on energy transfers, but you might have noticed this effect for yourself when using your phone or laptop. Especially when you are on Wi-Fi, you’ll find that it might get hot to the touch. This is because of an electrical effect called resistance. In this worksheet, we will be looking at resistance, current and potential difference and how all three of these concepts are related.

First of all, let’s look at what some of these terms mean.

1   Resistance  This is how much the electrons are being slowed down. In order to make a current you have to move electrons, but there is stuff in the way of those electrons. This means that the electrons are being slowed down when they bash into that stuff, reducing the current. More resistance means less current.

2    Potential difference This is the difference in the charge between two places. In order to make a current you need a positive and a negative charge. Well, we know how big these charges are by measuring the potential difference between them (measured in volts). If you have a bigger potential difference, then the electrons will be moving quicker. This means that you will have a bigger current.

3    Current  You should already know what this means, but did you know that there are two different ways that you can increase or decrease the current? The first way we have just spoken about – you can make them go faster. The second way is by forcing more electrons to move. Imagine if it is just you walking down a corridor, should be easy right? Now imagine there are a hundred of you trying to walk down that same corridor – well, that’s a lot more difficult, isn’t it. This means there is more resistance if there is more current.

You should be able to see now how all three of these ideas are linked and we can see this in the most famous of all of the electrical equations:

V = IR

V = potential difference (measured in volts (V)) – also known as voltage.
I = current (measured in amperes or amps (A))
R = resistance (measured in ohms (Ω))

Now let’s take a look at that equation in action:

Question:  An electrical circuit has a current of 10 A and a total resistance of 3 Ω. Calculate the potential difference needed by this circuit.

Step 1   Highlight all of the numbers in the equation:

An electrical circuit has a current of 10 A and a total resistance of 3 Ω. Calculate the potential difference needed by this circuit.

Step 2   Write out the numbers next to their symbols:

V = ?
I = 10 A
R = 3 Ω

Step 3    Put the numbers into the equation:

V = 10 x 3

V = 30 V

Let’s try some questions on this!

Match the terms to their units.

## Column B

Resistance
A
Current
V
Potential Difference
Ω

What could you add to a circuit to reduce the current?

Describe the function of the potential difference in a circuit.

[2]

Write the equation for current, potential difference and resistance.

[1]

Which of the following things could be done to reduce the current in a circuit?

Increase resistance

Increase potential difference

Decrease resistance

Decrease potential difference

A circuit has a resistance of 6 Ω. The batteries are providing a current of 1.5 A.

Calculate the voltage in the circuit.

Sam has a circuit that runs with a current of 3 A and a resistance of 100 Ω.

Calculate the voltage supplied by the power supply.

Sam finds that his circuit doesn't work correctly because he doesn't have enough potential difference to power all of the components. He needs to reduce the voltage of the whole circuit to 200 V. He decides to take out three components, reducing his resistance to 50 Ω and increasing his current to 4 A.

Will his circuit work now?

Yes

No

Brooke has just bought a car that runs on electric motors. Each motor has a battery and is independently run. The motors need a minimum current of 10 A to run and have an operating resistance of 35 kΩ.

Calculate the voltage needed by each motor.

Yes

No

Describe how to work out the following to someone who has never done a maths equation question before!

James has a circuit with a current of 0.1 A and a resistance of 15 Ω. Calculate the voltage.

[3]

• Question 1

Match the terms to their units.

## Column B

Resistance
Ω
Current
A
Potential Difference
V
EDDIE SAYS
If you are able to recognise these in a question, then you can find out where to put them in the answer. It is quite common that you will be given the equation in the question and you just have to find out where to put the terms - by knowing the units, you'll be more likely to get the stuff in the correct place.
• Question 2

What could you add to a circuit to reduce the current?

A resistor
Resistor
EDDIE SAYS
Remember how important it is to read the question carefully. To reduce the current in a circuit, you could take away some potential difference, but this is not what the question is asking. In this question, you need to add something to the circuit, not take it away - so what can you add to lower the current? Resistance is the thing that slows the electrons down, and the more resistance you have, the slower the electrons will be travelling. Slow electrons means less charge passing a point every second - this means less current. So you need to add a resistor.
• Question 3

Describe the function of the potential difference in a circuit.

[2]

EDDIE SAYS
This question is worth two marks, so you need to make two points in your answer. The obvious way would be to talk about the positive and the negative charges, and this would get you full marks, but in a mark scheme, the answer is always laid out in increasing difficulty of scientific knowledge. In this case, one mark for getting both the positive and the negative charge correct and one mark for saying how they affect the electrons.
• Question 4

Write the equation for current, potential difference and resistance.

[1]

EDDIE SAYS
This is a common exam question - they want to test your knowledge of important equations. This equation is definitely going to come up in your exam - it always does! This means that you must remember it - try writing it out a hundred times all over a piece of paper!
• Question 5

Which of the following things could be done to reduce the current in a circuit?

Increase resistance
Decrease potential difference
EDDIE SAYS
There are two correct answers to this question. Did you get them both? The question itself relies on you understanding what current, resistance and potential difference do. Let's have a recap: Resistance - slows down the electrons (current) by having more stuff for them to smash into. Current - is how many electrons are moving through the wires every second. Potential difference - is how much energy each electron has, or how fast it is. So if we want to reduce the current, we need to slow down the electrons. If we increase the resistance, there will be more stuff for the electrons to smash into, slowing them down. If we slow the electrons down by reducing the potential difference, then we will have fewer electrons going through the wire. Get it? Awesome, we knew you would!
• Question 6

A circuit has a resistance of 6 Ω. The batteries are providing a current of 1.5 A.

Calculate the voltage in the circuit.

9
9 V
EDDIE SAYS
This is a straightforward maths question! You simply have to remember the equation and add in the numbers. Step 1 Find the numbers and write them down: V = ? I = 1.5 A R = 6 Ω Step 2 Put them into the equation: V = IR V = 1.5 x 6 Step 3 Do the calculation and write down your answer: V = 9 V Don't forget to leave one space before adding the unit!
• Question 7

Sam has a circuit that runs with a current of 3 A and a resistance of 100 Ω.

Calculate the voltage supplied by the power supply.

EDDIE SAYS
Another find the numbers and put them into the equation. Let's do it together again. V = ? I = 3 A R = 100 Ω V = 3 x 100 V = 300 V Don't forget the unit!
• Question 8

Sam finds that his circuit doesn't work correctly because he doesn't have enough potential difference to power all of the components. He needs to reduce the voltage of the whole circuit to 200 V. He decides to take out three components, reducing his resistance to 50 Ω and increasing his current to 4 A.

Will his circuit work now?

Yes
EDDIE SAYS
There are a lot of words in this question, but don't be distracted by them! You can see that there are numbers all over the place, so you need to pick out the numbers and calculate something with them - that will get you some marks. Then you can go back and read the rest of the question and see what it is asking you to do. The question is essentially asking you to do the calculation and work out if the potential difference is now 200 or higher. We can do that easily. V = ? I = 4 A R = 50 Ω V = 4 x 50 V = 200 V So the answer is yes because the voltage is exactly 200 V which is what Sam was aiming for.
• Question 9

Brooke has just bought a car that runs on electric motors. Each motor has a battery and is independently run. The motors need a minimum current of 10 A to run and have an operating resistance of 35 kΩ.

Calculate the voltage needed by each motor.

EDDIE SAYS
Oops - did this one trip you up? What went wrong, you ask? Well, did you spot the kΩ? The k means 1,000, so this means that it was really 35,000 Ω, not 35! You need to factor this into your equation - apart from that, it's a normal calculation. Let's try it together. V = ? I = 10 A R = 35,000 Ω (converted from 35 kΩ) V = 10 x 35,000 V = 350,000 V Did you remember the unit?
• Question 10

Describe how to work out the following to someone who has never done a maths equation question before!

James has a circuit with a current of 0.1 A and a resistance of 15 Ω. Calculate the voltage.

[3]

EDDIE SAYS
How did you do with this one? You've had some practice at answering these sort of questions now, so can you explain how to do it to someone else. This is a really excellent way to help yourself to revise a topic and to check your own understanding. This is simply a case of doing what you would have done every time with a simple set of instructions for each question. Have a look at the criteria to see if you have answered the question correctly. You should have got an answer of 1.5 V. Well done - another activity safely completed - brilliant work!