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Analysing Current and Charge

Worksheet Overview

QUESTION 1 of 10

It’s dark outside, big black clouds are blotting out of the sun. You sit in your chair staring out of the window as the rain slowly begins to fall, big drops of rain smashing against the window with a distinctive clunk. 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. Natures best showcase of electricity, one billion volts blasting down to the ground with 30 kilo 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 stakes. In this activity, we will be looking at what a current is, how to make one and 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 to work, no electrons mean no electricity. Where do you find electrons? On atoms. Some atoms have as few as 1 electron and some have 130, it all depends on the type of atom that it is. But these electrons are trapped in 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, the outer electrons form a ‘sea’ of electrons that are free to move wherever they want to. You can take a look at our activity on metallic bonding for more information on this.

 

 

When you have got some electrons that you can move around, you then need to actually start moving them. This is where our 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 the electron from the atoms, all we need to do to move them is to place them near a positive charge, the electrons will flow towards that charge. That opposite is true as well, if you place a negative charge near 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 our lightning again. The electrons must have been free to move in the clouds – this happens because when ice particles collide with other ice particles then it frees up some electrons. There must have been 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 moves 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 our handy equation Q = It, or are it is fondly known the QUIT equation. 

 

PICTURE OF EQUATION

 

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:

 

Q - 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 number 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 that! 

Define the term 'Current'. 

Match the unit to the value. 

Column A

Column B

Current (I)
Ampare (A)
Charge (Q)
Seconds (s)
Time (t)
Coulomb (C)

Describe why a metallic conductor is able to carry a current and an insulator is not (3 marks). 

Explain how to create a current in a conductor (4 marks)

Write an equation involving charge, current and time. 

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 in this time. 

A blot 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 9.6x10-19

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

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'. 

CORRECT ANSWER
EDDIE SAYS
This is one of those definitions that you need to remember - it helps to remember the equation (this will come up in some other questions). The current is the amount of change (or the number of electrons) passing the point every second - there is your definition. Get it? Got it? Good!
  • Question 2

Match the unit to the value. 

CORRECT ANSWER

Column A

Column B

Current (I)
Ampare (A)
Charge (Q)
Coulomb (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've found them in question then you can put them into the equation and then you'll be awesome! Everyone will be totes jell of your amazing science skills!
  • Question 3

Describe why a metallic conductor is able to carry a current and an insulator is not (3 marks). 

CORRECT ANSWER
EDDIE SAYS
You need to be able to describe how the structure of a material will give it it's properties - and this is where Chemistry and Physics start to collide. A physical property is the ability to conduct, but it is the chemical structure that allows the material to be able to conduct. Basically - the electrons are able to move in metal because they are not in a fixed position. If you try the same with an insulator, noting will happen because the electrons are in a fixed position. You also needed to talk about how the electrons are charged - this allows you to create a current (movement of charges).
  • Question 4

Explain how to create a current in a conductor (4 marks)

CORRECT ANSWER
EDDIE SAYS
This is not 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. We need to talk not only about electrons charge but HOW they are charged (negative or positive?). We need to talk about the movement of charged particles not the movement of electrons. So, moving electrons need to be attracted to something and repelled from something else. This means you need a big +ve an -ve charge near the electrons in order to get them to move (the potential difference). Then we talk about HOW the electrons will move (away from the like charge and towards the unlike charge). And finally, we talk about how we make a current and link it all back together. What an awesome question!
  • Question 5

Write an equation involving charge, current and time. 

CORRECT ANSWER
EDDIE SAYS
So, you have to remember this equation. This type of question comes up a lot in exam papers as a way of testing if you can remember the equations they want you to remember. They will give you all three of the things you need to include int eh question, you just have to put them together in what you think is the right order. If in doubt, put anything involving all three!
  • 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 in this time. 

CORRECT ANSWER
33.3
33.33
EDDIE SAYS
A simple question, but there is a kC in there - did you spot it? It is a kilo-Coulomb or 1000 Coulombs. Let's have a go together. Q = 4,000 C (converted into C from kC) I = ? t = 120 s Rearrange the equation: Q = It I = Q/t Number in: I = 4000/120 I = 33.3 A
  • Question 7

A blot 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 9.6x10-19

CORRECT ANSWER
EDDIE SAYS
Okay, 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 I did it. Step one is simple, work out the charge (hopefully you got this step!) Q = ? I = 20A t = 0.2 s Q = It Q = 20 x 0.2 Q = 4 C Now comes the difficult part - how many electrons in one coulomb? One electron has a charge of 9.6x10-19, so if we do 1/9.6x10-19 we will get how many electrons where are in one Coulomb. OR we could do 4/9.6x10-19 to find out how many are in 4, right? 4/9.6x10-19 = 4.17 x 1018
  • Question 8

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

CORRECT ANSWER
30000000
30,000,000
EDDIE SAYS
This question looks like it hasn't given you enough information to answer the question - but that is never the case in these types of question, you just need to look out for the hidden information. In this question, 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 1 second, so we can assume that the time was 1 second as well. Then, the only thing that you need to look at is the charge - it is MA (mega Amperes), so we need to convert it: 30 MA = 30,000,000 A
  • 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. 

CORRECT ANSWER
3
EDDIE SAYS
Did you see that the time was given in minutes and not seconds? So you need to convert the time first of all. That's easy to do, all you need to do is multiply the minutes by however many seconds there are in a minute. 3 x 60 = 180. Q = 540 C I = ? T = 180 Q = It I = Q/t I = 540/180 I = 3 A
  • 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. 

CORRECT ANSWER
1.5
EDDIE SAYS
Two steps, 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 QT = 2.1 + 4.1 = 6.2 IT = 3 + 1.1 = 4.1 Q = It t = Q/I t = 6.2/4.1 t = 1.5 s
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