# Analyse Current and Charge

In this worksheet, students will gain an understanding of current and charge and use this to solve complex problems.

Key stage:  KS 4

Difficulty level:

### QUESTION 1 of 10

It’s dark outside, big black clouds are blotting out the sun. You sit in your chair staring out of the window as the rain begins to fall. Then you see it – a bright flash lighting up the whole sky. You sit and count the seconds 1, 2, 3… Then comes the distinctive roll of thunder echoing across the landscape. Nature's best showcase of electricity - one billion volts blasting down to the ground with 30,000 amperes of current. Enough to burn a hole in anything that it meets. But what is it? What is electricity and why do we have lightning strikes?  In this activity, we will be looking at what a current is and how to make one, as well as some calculations to allow us to work out current.

To start with, we need to look at the atom, specifically the atoms of conductors (materials that allow electricity to move through them). Electricity needs electrons -  no electrons means no electricity. Where do you find electrons? In atoms. Some atoms have as few as 1 electron and some have as many as 130 - it all depends on the type of atom that it is. But electrons are trapped inside the atom, they cannot be used to make electricity. For these electrons to be used, we need to separate them from their atoms. Luckily, this happens in metals where the outer electrons form a ‘sea’ of electrons that are free to move about. You can take a look at our activity on metallic bonding for more information on this.

When you have some electrons that can move around, you then need to actually start moving them. This is where the idea of charge comes in and it is quite important here. Electrons are negatively charged and they are normally attracted to the positively charged nucleus of the atom (where the protons and neutrons live). This is because opposite charges attract each other. When we have removed electrons from their atoms, all we need to do is to place them near a positive charge and the electrons will flow towards that charge. The opposite is true as well - if you place a negative charge near to them, then they will move away from that charge.

When these electrons are flowing, you have a current. The definition of current is ‘the number of electrons passing a point per second’ so the more electrons there are, the more current there is. Let’s think about lightning again. The electrons must have been free to move in the clouds – this happens because when ice particles collide with other ice particles, it frees up some electrons. There must have been either an opposite charge or a similar charge near them – both in this case - the ground is the opposite charge, pulling the electrons down and the cloud is the similar charge, pushing the electrons away. This then gives us enough energy to have a whole bunch of electrons moving from the cloud to the ground. That is your lightning strike.

Now, how many electrons are in a lightning strike? Well, we can work out the amount of charge by using the equation below, which is fondly known as the QUIT equation.

Q = It

Q = charge (measured in coulombs (C))
I = current (measured in amperes or amps (A))
t = time (measured in seconds (s))

Let’s use this equation in an example:

Question:   A wire has a charge of 4 C moving though it in 5 s. Calculate the current in the wire.

Step 1   Highlight all of the numbers in the equation:

A wire has a charge of 4 C moving though it in 5 s. Calculate the current in the wire.

Step 2   Write out the numbers next to their symbols:

Q = 4 C
I = ?
t = 5 s

Step 3   Rearrange the equation:

Q = It

I = Q ÷ t

Step 4   Put the numbers into the equation:

I = 4 ÷ 5

Step 5   Do the maths:

I = 0.8 A

Let's try some questions on this.!

Define the term 'current'.

[1]

Match the units to their symbols.

## Column B

Current (I)
Coulombs (C)
Charge (Q)
Amperes (A)
Time (t)
Seconds (s)

Describe why a metallic conductor is able to carry a current and an insulator is not.

[3]

Explain how to create a current in a conductor.

[4]

Write the equation involving charge, current and time.

[1]

A charge of 4 kC is allowed to pass through a wire in the charging process for a fusion reactor. The reactor takes 120 s to fully charge.

Calculate the current needed to supply this charge.

A flash of lightning has a current of 20 A for 0.2 seconds.

Calculate the number of electrons that are in the lightning strike.

Charge on an electron 9.6 x 10-19

[2]

A current of 30 MA is needed to power the national rail links out of London.

Calculate the charge needed by the railway system.

A charge of 540 C is needed by a PC over a period of 3 minutes.

Calculate the current needed by the PC.

A device is split into two machines. In order to correctly operate, the first machine needs a current of 3 A and a minimum of 2.1 C. The second machine needs a current of 1.1 A and a minimum of 4.1 C.

Calculate the amount of time that these machines could run for.

• Question 1

Define the term 'current'.

[1]

EDDIE SAYS
This is one of those definitions that you need to remember - it will help you to understand and use the all-important equation for charge and current. Current is the amount of charge (or the number of electrons) passing a point every second - there is your definition. See if you can learn it off by heart.
• Question 2

Match the units to their symbols.

## Column B

Current (I)
Amperes (A)
Charge (Q)
Coulombs (C)
Time (t)
Seconds (s)
EDDIE SAYS
As always, these symbols need to be learned to be able to spot them in a question. Once you can recognise them, you will find it easy to add them into equations.
• Question 3

Describe why a metallic conductor is able to carry a current and an insulator is not.

[3]

EDDIE SAYS
You need to be able to describe how the structure of a material determines its properties, and this is where chemistry and physics start to collide. The ability to conduct is a physical property, but it is the chemical structure that allows the material to be able to conduct. Basically, electrons are able to move in metals because they are not in a fixed position. If you try the same with an insulator, nothing will happen because the electrons are in a fixed position. You also needed to mention how the electrons are charged - this allows you to create a current ( the movement of charges). Well done if you managed to get all three marks in this tricky question.
• Question 4

Explain how to create a current in a conductor.

[4]

EDDIE SAYS
This is getting a little more complex, but we are aiming for the higher grades now. You need to start thinking about the language you are using and if it is the right language in each part. You need to mention not only about the charge of electrons but how they are charged (negative or positive?). You also need to refer to the movement of charged particles, rather than the movement of electrons. So, moving electrons need to be attracted to something and repelled from something else. This means you need both a positive and a negative charge near the electrons in order to get them to move (the potential difference). Then, you need to mention how the electrons will move (away from the like charge and towards the unlike charge). And finally, you need to mention how to make a current and link it all back together. What an awesome question!
• Question 5

Write the equation involving charge, current and time.

[1]

EDDIE SAYS
You have to remember this equation. This type of question comes up a lot in exam papers to test your knowledge of important equations. They will give you all three of the things you need to include in the equation, you just have to put them together in what you think is the right order. If in doubt, put anything involving all three! But ideally, learn it so you know the equation!
• Question 6

A charge of 4 kC is allowed to pass through a wire in the charging process for a fusion reactor. The reactor takes 120 s to fully charge.

Calculate the current needed to supply this charge.

EDDIE SAYS
A simple question, but there is a kC in there - did you spot it? It is a kilo-coulomb or 1,000 coulombs. Let's have a go together. Q = 4,000 C (converted into C from kC by multiplying by 1,000) I = ? t = 120 s Rearrange the equation: Q = It I = Q ÷ t Put the numbers in: I = 4000 ÷ 120 1 = 33.33 A or 33.3 to 1 decimal place.
• Question 7

A flash of lightning has a current of 20 A for 0.2 seconds.

Calculate the number of electrons that are in the lightning strike.

Charge on an electron 9.6 x 10-19

[2]

EDDIE SAYS
If you got this question, well done! This is very much an extension question, so don't worry if you didn't quite get it. Let's look at how to tackle it. Step one is simple - work out the charge (hopefully you got this step!) Q = ? I = 20 A t = 0.2 s Q = It Q = 20 x 0.2 Q = 4 C Now comes the difficult part - how many electrons are there in one coulomb? One electron has a charge of 9.6 x 10-19, so if we do 1 ÷ (9.6 x 10-19) we will get how many electrons there are in one coulomb. So, we need to work out 4 ÷ (9.6 x 10-19) to find out how many there are in 4. 4 ÷ (9.6 x 10-19) = 4.17 x 1018 Phew - what a question! You've reached the end of this activity and hopefully, you feel a bit more confident with this topic now. Don't worry if you found some of these questions challenging - you will not be the only one!
• Question 8

A current of 30 MA is needed to power the national rail links out of London.

Calculate the charge needed by the railway system.

EDDIE SAYS
This question looks as if it hasn't given you enough information to find the answer - but that is never the case in these types of question - you just need to look out for the hidden information. In this case, the hidden information is in the definition of current - the amount of charge passing a point per second. The per second is the important part there! That means that this measurement was taken over one second, so we can assume that the time was one second as well. The only thing that you need to look at now is the charge - it is in MA (mega amperes), so we need to convert it. There are one million amperes in a mega ampere so: 30 MA = 30,000,000 A That's a lot of charge!
• Question 9

A charge of 540 C is needed by a PC over a period of 3 minutes.

Calculate the current needed by the PC.

3 A
3
EDDIE SAYS
Did you see that the time was given in minutes and not seconds? This means you need to convert the time first of all. That's easy to do - all you need is to multiply the minutes by 60. 3 x 60 = 180. Q = 540 C I = ? T = 180 Q = It I = Q ÷ t I = 540 ÷ 180 1 = 3 A Simple!
• Question 10

A device is split into two machines. In order to correctly operate, the first machine needs a current of 3 A and a minimum of 2.1 C. The second machine needs a current of 1.1 A and a minimum of 4.1 C.

Calculate the amount of time that these machines could run for.

EDDIE SAYS
This looks tricky, doesn't it, but you just have to take it one step at a time. Firstly, you need to add these together to get the total, and then you need to do the Q = It equation to find out the length of time it could stay on for! Let's have a go. Q1 = 2.1 C Q2 = 4.1 C I1 = 3 A I2 = 1.1 A Qtotal = 2.1 + 4.1 = 6.2 Itotal = 3 + 1.1 = 4.1 Q = It t = Q ÷ I t = 6.2 ÷ 4.1 t = 1.5 s Well done!