# Understand Current and Charge

In this worksheet, students will revise electrical current in terms of electron flow. Students will also be introduced to equations to calculate current and charge.

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: There is a current of 3 A in a wire for 10 s. Calculate the charge in the wire.

Step 1   Highlight all of the numbers in the equation:

There is a current of 3 A in a wire for 10 s. Calculate the charge in the wire.

Step 2   Write out the numbers next to their symbols:

Q = ?
I = 3 A
t = 5 s

Step 3   Put the numbers into the equation:

Q = 3 x 5

Q  = 15 C

Let's try some questions on this!

Why are conductors able to allow an electrical current to pass through them?

Electrons are able to move

Electrons are fixed into place

Electrons are delocalised from their atoms

Electrons can move between atoms

What is the definition of current?

How many electrons there are

How metallic an object is

How much space there is for the electrons to move

How many electrons are passing a point per second

Write the equation involving charge, current and time.

[1]

A circuit has a current of 2 A and is left on for 10 s.

Calculate the charge that flows through the circuit.

A formula e car has a current of up to 12 A, and a single lap can last about 45 seconds.

Calculate the charge used by a formula e car over a lap.

Match the values to their units.

## Column B

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

What happens to two charges that are both negative when you put them close together?

They attract

They repel

Calculate the charge when a current of  3A moves for 2 minutes.

They attract

They repel

Calculate the charge when a current of 1.2 A flows for 4 minutes.

What happens when you put a like charge next to an unlike charge?

They attract

They repel

• Question 1

Why are conductors able to allow an electrical current to pass through them?

Electrons are delocalised from their atoms
EDDIE SAYS
A hard one to start with! Another word for when electrons are not fixed in their atoms is delocalized - this means they are not local to the atom anymore. Because of this, they are able to move around and this means that the material can conduct electricity. This happens in all metals and some other materials.
• Question 2

What is the definition of current?

How many electrons are passing a point per second
EDDIE SAYS
This is one of those definitions that you need to remember - and it will also help you when using the equation for charge and current. A current is the amount of charge (or the number of electrons) passing a point every second - there is your definition. Well done if you already knew this - if you're still a bit shaky on it, then try writing it out several times to help you to remember it.
• Question 3

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. You will generally be given all three of the things you need to include in the question, you just have to put them together in what you think is the right order. If in doubt, put anything involving all three! But it is better if you learn it!!
• Question 4

A circuit has a current of 2 A and is left on for 10 s.

Calculate the charge that flows through the circuit.

EDDIE SAYS
So, how did you do with this maths question? If you can remember the equation, it is easy - but did you remember it?! Let's work through it one step at a time. Highlight the numbers and then write them out: Q = ? I = 2 A t = 10 s Put them into the equation Q = It Q = 2 x 10 Do the maths, write your answer, and don't forget the unit! 20 C Do you see how simple it is once you've got that equation sorted out?
• Question 5

A formula e car has a current of up to 12 A, and a single lap can last about 45 seconds.

Calculate the charge used by a formula e car over a lap.

EDDIE SAYS
Did you remember that coulombs has a capital C when it's used as a single letter? Little things like this are important, believe it or not. It is possible to lose marks if you forget things like this. So, if we follow our procedure, you should have done the following! Q = ? I = 12 A t = 45 s Q = 12 x 45 Q = 540 C Remember the units!
• Question 6

Match the values to their units.

## Column B

Current (I)
Amperes (A)
Charge (Q)
Coulombs (C)
Time (t)
Seconds (s)
EDDIE SAYS
As always, these values need to be learned to be able to spot them in a question. Once you become confident with them, it will easy for you to put them into an equation and then you'll be awesome!
• Question 7

What happens to two charges that are both negative when you put them close together?

They repel
EDDIE SAYS
Like charges repel each other - this means that electrons (with a negative charge) will be attracted to the nucleus of an atom (with a positive charge) and repelled from each other. To get these electrons moving, all you need to do is put a big negative charge near them and they will move away from it, or a big positive charge and they will move towards it!
• Question 8

Calculate the charge when a current of  3A moves for 2 minutes.

EDDIE SAYS
This time, you have one extra thing to do before you can work out the answer. You don't have the time in seconds, but in minutes. So, you need to convert from minutes into seconds first of all. This is not too hard - you just multiply by 60 (the number of seconds in a minute) 2 x 60 = 120 s Then put the numbers into the equation: Q = It Q = 3 x 120 Q = 360 C
• Question 9

Calculate the charge when a current of 1.2 A flows for 4 minutes.

288 C
288
EDDIE SAYS
Did you convert the time into seconds? Let's do it all together. Convert the time into seconds by multiplying by 60 4 x 60 = 240 s Put this into the equation: Q = It Q = 1.2 x 240 Q = 288 C
• Question 10

What happens when you put a like charge next to an unlike charge?

They attract
EDDIE SAYS
Like charges repel and unlike charges attract. If you want to move a charge from one place to another, then all you need to do is to place an opposite charge near it and it will be attracted towards it! Just like magic, or magnets... How did you get on with this activity? Hopefully, you're feeling more confident with using this all-important equation now.