# Calculate Current, Potential Difference and Resistance

In this worksheet, students will learn what resistance and voltage are and how they interact. Key stage:  KS 4

Difficulty level:   ### QUESTION 1 of 10

We recommend that you take a look at our activity on current before you take a look at this one.

Resistance is futile – there’s a dated Star Trek reference for you all! But it is actually true – you cannot have any motion without something resisting it – physics, boom! The same is true of electrons, and that is what we are going to be looking at in this activity: how electrons are affected by resistance and how potential difference is in there helping out as well.

When electrons move, you can see them bashing into each other in the positive ions of the metal, right? This is slowing down the movement of the electrons and, therefore, the movement of the charge. If you can remember our definition of current, you might recall that it is the movement of charge per unit of time. This means that if the charge is slowed down by bashing into the positive ions in the metal, then the current is also going to be affected. The current will go down.

So, what is resistance then? It is just how many different things there are in the way of the electrons – or how much a material will slow down the electrons – or how hard it is for the electrons to get through a material. The unit of resistance is ohm (Ω). The higher the resistance, the lower the current.

So, resistance negatively affects the current. Is there a way that we can increase the current? Yes, yes there is! Remember from the activity on current that we said that there needs to be a positive and a negative charge to attract and repel the electrons? Well, that charge is called the potential difference (voltage, measured in volts (V)). If we make this higher, then the electrons will travel faster and if they travel faster, we will be able to fit more of them through the wire every second. This means that we will be able to increase the current using the potential difference.

Okay – so you should get the idea now, it is all about how the electrons move through the wire. We need to be able to predict this movement when we are working with circuits, and so we have come up with an equation to help us do this:

current = potential difference divided by resistance

This can be shown by the formula triangle below: V represents the potential difference  (measured in volts), I represents the current (measured in amps) and R represents the resistance (measured in ohms) The unit of resistance is ohm (Ω), current (ampere, A), voltage (volt, V)

Uses of resistance

A piece of wire has a resistance of 5 Ω and melts if the current through it exceeds 8 A. What is the maximum possible voltage that could cross the wire, without melting it?

To answer this, you need to rearrange the equation. For current, it would become: and for voltage, it would be V = I x R. So, in our question V = 8 x 5 = 40 V

Let's do some questions!

What can a resistor do?

It can change the electrical current in a circuit

It can split up current

It can create more current

Which of the options below are types of resistor?

Fixed

Split

Variable

Double

What do you divide voltage by to get resistance?

Resistor

Current

Power

Does increasing the resistance make the current go up or down?

Up

Down

If the voltage is 6 V and the resistance 3 Ohms, how much is the current?

2 A

2 V

2 Ohms

If the current is 6 A and the resistance is 5 Ohms, what is the voltage?

If the current is 3 A and the voltage is 12 V, what is the resistance?

4 Ω

36 Ω

0.25 Ω

What is the current when a voltage of 2 V passes through a resistor of 0.5 Ω?

A circuit has a potential difference of 6 V and a current of 5 A, what is the resistance of the circuit?

What is the voltage when a resistor of 8 Ω has a current of 2 A?

• Question 1

What can a resistor do?

It can change the electrical current in a circuit
EDDIE SAYS
That was a fairly hard one to start off with, wasn't it? Did you select the right answer? Resistors can change the electrical current in a circuit by putting stuff in the way of the electrons. These are normally positive metal ions.
• Question 2

Which of the options below are types of resistor?

Fixed
Variable
EDDIE SAYS
This was a tricky one to finish with - it's a question of you either know it or you don't! Hopefully, you'll remember these names for the next time you're asked this question! There are two types of resistor: fixed and variable. The variable resistor is known as a rheostat. Well done - you've completed this activity and worked your way through some challenging calculations. Remember that all these equations become easier the more you practise them.
• Question 3

What do you divide voltage by to get resistance?

Current
EDDIE SAYS
Did you remember your equation here? If you find it hard to recall complicated equations, try drawing the formula triangle (maybe even in different colours) and this could help you to visualise the calculation in an easier way. To work out the resistance (R), you need to divide the voltage (V) by the current (I).
• Question 4

Does increasing the resistance make the current go up or down?

Down
EDDIE SAYS
How did you do with this one? More resistance means more things to bash into, which means slower electrons and therefore less current. Simple really when you think about it!
• Question 5

If the voltage is 6 V and the resistance 3 Ohms, how much is the current?

2 A
EDDIE SAYS
Now here's a chance to use that formula in the Introduction! How did it go? Current = potential difference ÷ resistance. The potential difference = 6 V The resistance = 3 Ohms So, the current = 6 ÷ 3 = 2 A
• Question 6

If the current is 6 A and the resistance is 5 Ohms, what is the voltage?

30
30 V
EDDIE SAYS
Once again, it's that wonderful equation! Picture your formula triangle or remember the word equation: here you need V = I x R I = 6 and R = 5, so you do the calculation 6 x 5 = 30 V
• Question 7

If the current is 3 A and the voltage is 12 V, what is the resistance?

4 Ω
EDDIE SAYS
Did you remember the formula? Have a look back at the Introduction if you are not sure. Here you need: V ÷ I (potential difference divided by current) 12 ÷ 3 = 4 Ω
• Question 8

What is the current when a voltage of 2 V passes through a resistor of 0.5 Ω?

4
4 A
EDDIE SAYS
Here we go again! This time we need to work out the current (I) so we need: I = V ÷ R V = 2 V R = 0.5 Ω So I = 2 ÷ 0.5 I = 4 A
• Question 9

A circuit has a potential difference of 6 V and a current of 5 A, what is the resistance of the circuit?

EDDIE SAYS
Are you getting the hang of these yet? Use that equation or formula triangle to help you out. Here you need R = V ÷ I V = 6 V I = 5 A so R = 6 ÷ 5 = 1.2 Ω
• Question 10

What is the voltage when a resistor of 8 Ω has a current of 2 A?

16
16 V
EDDIE SAYS
Now what's that equation we need again? This time we need: V = I x R I = 2 A R = 8 Ω V = 2 x 8 Voltage = 16 V Terrific work!
---- OR ---- 