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Understanding Current, Potential Difference and Resistance

Worksheet Overview

QUESTION 1 of 10

Hint! You should have a secure understanding of current before you try this activity. If not, we recommend that you go back and try out the Understanding Current and Charge activity. 

 

Did you know, when you use an electrical device, it gets hot? You did know because you’ve done the activity on energy transfers on EdPlace? It’s amazing isn’t it – the person that wrote it is so cool. If you haven’t done that worksheet (why haven’t you?) you might have noticed this effect when using your phone. Especially when you are on Wi-Fi, you’ll find it might get hot to the touch. This is because of an electrical effect called resistance. We will be looking at resistance, current and potential difference and how all three of these concepts are related in this worksheet. 

 

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

 

1 – resistance. This is how much the electrons are being slowed down. If you remember, in order to make current you have to move electrons but there is stuff in that 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 a difference in charge between two places. If you remember, 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 here I want to talk about the 2 different ways 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 through a corridor, should be easy right? Now imagine there are 100 of you trying to walk through a corridor – well, that’s a lot more difficult isn’t it. This means there is more resistance if there is more current. 

 

Hopefully, now, you can see that all three of these ideas are linked, and they are linked by the most famous of all of the electrical equations:

 

INSERT IMAGE OF EQUATION HERE

 

Where:
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 talk a look at that equation in action:


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

 

Step 1 – highlight all of the number 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 = 3 x 10


Step 4 – do the maths and write your answer:
V = 30 V

Don’t forget your units!

Let’s try some questions on that! 

Match the unit to the value

Column A

Column B

Resistence
Ω
Current
A
Potential Difference
V

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

Describe the function of the potential difference in a circuit. (2 marks)

Write an equation using the following terms. Current, Potential Difference and Resistance. 

I want to reduce the current in the circuit. Which one of the following things could I do? Tick all of the boxes that apply. 

Increase resistance

Increase potential difference

Decrease resistance

Decrease potential difference

A circuit has a quoted 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 in his circuit. He needs to reduce the voltage of the whole circuit to 200 V. He decided to take out 3 components reducing his resistance to 50 Ω and increasing his current to 4 A. Calculate if his circuit will now work. 

Yes

No

Brooke has just bought a car that runs on electric motors. Each motor has a battery and in 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. 

Describe how to work out the following to someone who has never done a maths question before. (3 marks)

 

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

  • Question 1

Match the unit to the value

CORRECT ANSWER

Column A

Column B

Resistence
Ω
Current
A
Potential Difference
V
EDDIE SAYS
If you are able to spot these in the 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 values - 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? 

CORRECT ANSWER
A resistor
Resistor
EDDIE SAYS
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 traveling. Slow electrons means less charge passing a point per second - this means less current.
  • Question 3

Describe the function of the potential difference in a circuit. (2 marks)

CORRECT ANSWER
EDDIE SAYS
This question is worth 2 marks, so you need to make 2 points in your answer. The obvious way would be to talk about the positive and then talk about 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 an equation using the following terms. Current, Potential Difference and Resistance. 

CORRECT ANSWER
EDDIE SAYS
This is a common exam question, they want to know that you know the equation. This equation is definitely going to come up in your exam - I have not seen an exam where this did not come up. This means that you just need to remember the equation - try writing it out 100 times all over a piece of paper until you can't feel your hand anymore. It always worked for me!
  • Question 5

I want to reduce the current in the circuit. Which one of the following things could I do? Tick all of the boxes that apply. 

CORRECT ANSWER
Increase resistance
Decrease potential difference
EDDIE SAYS
There are 2 correct answers to this question. The question itself relies on you understanding what current resistance and potential difference do. Let\'s have a recap and see if we can answer the question. Resistance - slows down the electrons (current) by having more stuff for them to smash into. Current - How many electrons are moving through the wires every second. Potential difference - How much energy (or how fast) each electron has. So if we want to reduce the current, we want 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 thought the wire. Get it? Awesome, we knew you would!
  • Question 6

A circuit has a quoted resistance of 6 Ω. The batteries are providing a current of 1.5 A. Calculate the voltage in the circuit. 

CORRECT ANSWER
9
EDDIE SAYS
This is one of those find the numbers and put them into the equation jobs we like to do when there are maths questions in science. Let's have a go together! 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 = 1.5 x 6 Step 3 - do the calculation and write down your answer. V = 9 V Awesome, done!
  • 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. 

CORRECT ANSWER
300
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
  • 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 in his circuit. He needs to reduce the voltage of the whole circuit to 200 V. He decided to take out 3 components reducing his resistance to 50 Ω and increasing his current to 4 A. Calculate if his circuit will now work. 

CORRECT ANSWER
Yes
EDDIE SAYS
There are a lot of words in this question, but the only thing you really need to pay attention to here is the final sentence. You can see that there are numbers all over this question, so you can 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 not 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 200 V.
  • Question 9

Brooke has just bought a car that runs on electric motors. Each motor has a battery and in 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. 

CORRECT ANSWER
350000
350,000
EDDIE SAYS
Did you spot the kΩ? It means that it was really 35,000 Ω, not 35! You need to factor this into your calculations, 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 Done! Easy!
  • Question 10

Describe how to work out the following to someone who has never done a maths question before. (3 marks)

 

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

CORRECT ANSWER
EDDIE SAYS
So, we have done some of the questions, now we want to know if you can explain this to someone else. This is simply a case of doing what you would have done every time with a simple set of instructions with each question. Have a look at the criteria to see if you have answered the questions correctly. You should have got an answer of 1.5 V.
Try it ---- OR ----

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