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Apply Current and Charge

Worksheet Overview

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.

 

Image of metallic bonding

 

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, 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

Don’t forget your units!

Let's try some questions on this! 

Define the term 'current'. 

 

[1]

Write the equation involving charge, current and time. 

 

[1]

Match the units to their symbols. 

Column A

Column B

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

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

 

[3]

Using the idea of attraction, explain how to create a current in a conductor. 

 

[4]

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

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

A charge of 10 C creates a current of 2.5 A.

 

Calculate the amount of time the charge was flowing for. 

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]

  • Question 1

Define the term 'current'. 

 

[1]

CORRECT ANSWER
EDDIE SAYS
This is one of those definitions that you need to remember - it will be helpful for understanding the 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. If you weren't too sure, it might help to write it out several times until you can remember it.
  • Question 2

Write the equation involving charge, current and time. 

 

[1]

CORRECT ANSWER
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 be given 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 it would be better to have learned it!!
  • Question 3

Match the units to their symbols. 

CORRECT ANSWER

Column A

Column B

Current (I)
Amperes (A)
Charge (Q)
Coulombs (C)
Time (t)
Seconds (s)
EDDIE SAYS
As always, these symbols need to be learned so that you can recognise them in an equation. If you can do that, then solving equations becomes simple.
  • Question 4

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

 

[3]

CORRECT ANSWER
EDDIE SAYS
This was a challenging question and you had to work hard to get all three of those marks. You need to be able to describe how the structure of a material will determine 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, the 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 need to talk about how the electrons are charged - this allows you to create a current (the movement of charges).
  • Question 5

Using the idea of attraction, explain how to create a current in a conductor. 

 

[4]

CORRECT ANSWER
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 talk not only about electrons being charged particles, but also how they are charged (negatively or positively?) You also need to mention the movement of charged particles rather than the movement of electrons. Ideally, you should also refer to potential difference rather than voltage, but the mark is given here for either term. So, moving electrons need to be attracted to something and repelled from something else. This means you need a big positive or negative charge near the electrons in order to get them to move (the potential difference). Then mention how the electrons will move (away from the like charge and towards the unlike charge). And finally, refer to how to make a current and link it all back together. What an awesome question! How did you cope with it?
  • Question 6

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

CORRECT ANSWER
288 C
288
EDDIE SAYS
Let's take it slowly, one step at a time. Convert the time into seconds by multiplying by 60 4 x 60 = 240 s Put this into the equation: Q = ? I = 1.2 A t = 240 s Q = It Q = 1.2 x 240 Q = 288 C
  • Question 7

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

CORRECT ANSWER
EDDIE SAYS
How did you find this one? To work out the answer, you don't have the time in seconds, 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 It's simple if you can remember the equation!
  • Question 8

A charge of 10 C creates a current of 2.5 A.

 

Calculate the amount of time the charge was flowing for. 

CORRECT ANSWER
EDDIE SAYS
A nice simple rearrangement to ease you into some more complex questions now. Let's work our way through it. Q = 10 C I = 2.5 A t = ? Rearrange the equation: Q = It t = Q ÷ I Put the numbers in: t = 10 ÷ 2.5 t = 4 s
  • Question 9

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.

CORRECT ANSWER
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 I = 33.33 A or 33.3 if you round it to 1 decimal place.
  • Question 10

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]

CORRECT ANSWER
EDDIE SAYS
Okay, if you got this question right, 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, 4 ÷ ( 9.6 x 10-19) will give us how many electrons there are in 4 coulombs. 4 ÷ ( 9.6 x 10-19) = 4.17 x 1018 Phew - what a question! Well done for even tackling it, even if you didn't get it right. Hopefully, you'll find the easier equations a doddle to solve after this one!
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