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Understand Current and Charge

Worksheet Overview

QUESTION 1 of 10

You know at lunch, when you’re finally set free from the classroom, you tend to move around in groups with people you like and avoid people you don’t like, right? Well, electrons are like that as well, only they don’t like other electrons. If you get two electrons and try to put them close together, they will try to move apart from each other, which causes them to move. This movement of electrons is what we will be looking at in this activity.

 

Electrons have a negative charge. Things that have a negative charge will repel each other, pushing each other away and moving in the process. We can see this in the picture below. We have forced a load of electrons onto this poor kid’s hair and they are all trying to get away from each other -  they are literally picking up the hair in order to get away from each other.

 

Child with static hair

 

We can use this to our advantage because not only do electrons hate each other, but they love positive charges. They are attracted to them and want to get to them as quickly as possible. This means that if we make a negative charge on one side of a wire and a positive charge on the other, then the electrons in the wire will move from one side to the other.

 

 

Why is this so important? It is all because of the definition of current. An electric current is a flow of electric charge, carried by negatively charged electrons in the atoms of metals. 

 

Now for some maths! We can work out the current by using the following equation:

 

I = Q ÷ t

current = charge divided by time

 

I = current measured in amperes (A)

Q = charge measured in coulombs (C)

t = time measured in seconds (s)

 

You will have to remember this equation!

 

Let's go through an example of how to use it:

 

Let's imagine that a charge of 3 C passes through a wire every second. What is the current in the wire?

 

Step 1   Find and highlight all of the numbers.

A charge of 3 C passes through a wire every second. What is the current in the wire?

 

Step 2   Write out the numbers next to their symbols

I = ?

Q = 3 C

t = 1 s

 

Step 3   Put them into the equation I = Q ÷ t

I = 3 ÷ 1

 

Step 4  Put this into your calculator and work out the answer and don’t forget the units!

I = 3 A

 

Now, let's have a go at some questions.

What is an electric current?

A flow of protons

A flow of neutrons

A flow of charge

What provides charge in an electric current?

Electrons

Protons

Neutrons

What is the unit of electric current?

Ammeters

Amperes

Volts

What is the unit for electrical charge?

Ampere

Coloumb

Volts

What is needed to make a current flow? 

A negative charge on one end of the wire

A positive charge on one end of the wire

Both a positive and a negative charge on opposite ends of the wire

What is the current if you have a charge of 10 C that passes through a wire in 5 seconds?

A negative charge on one end of the wire

A positive charge on one end of the wire

Both a positive and a negative charge on opposite ends of the wire

What is the current if it takes 4 seconds for a charge of 0.5 C to pass through a wire?

A negative charge on one end of the wire

A positive charge on one end of the wire

Both a positive and a negative charge on opposite ends of the wire

A man gets struck by lightning. A total charge of 40 C passes through him in 0.2 seconds. 

 

If a current of 0.2 A will kill a human, was there enough current in this lightning strike to kill this man?

Yes

No

What is the current if a charge of 0.1 C passes through your finger in 0.1 s?

A Van-De-Graff generator is capable of delivering up to 40,000 volts in one strike with a charge of 0.01 C. The strike lasts a very short time, only 0.3 s.

 

What is the current in this strike?

  • Question 1

What is an electric current?

CORRECT ANSWER
A flow of charge
EDDIE SAYS
How did you do with this first question? Even if you weren't sure of the answer, you could have a good guess that it is something to do with charge, based on the title of this activity! An electric current is a flow of electric charge.
  • Question 2

What provides charge in an electric current?

CORRECT ANSWER
Electrons
EDDIE SAYS
This one really followed on from the previous question, didn't it?Did you get this one right? Electric current is carried by negatively charged electrons in the atoms of metals.
  • Question 3

What is the unit of electric current?

CORRECT ANSWER
Amperes
EDDIE SAYS
The unit used for electric current is amperes. This can sometimes be shortened to just amps (or A) but amperes is the full title. It's named after Mr Ampere who first defined it!
  • Question 4

What is the unit for electrical charge?

CORRECT ANSWER
Coloumb
EDDIE SAYS
The unit for charge is the coloumb, with the symbol C. It is named after the person who first defined it, this time it was Mr Coloumb, not Mr Ampere! Any guesses for what Mr Volta defined?!
  • Question 5

What is needed to make a current flow? 

CORRECT ANSWER
Both a positive and a negative charge on opposite ends of the wire
EDDIE SAYS
This was a bit trickier but don't worry if you weren't sure of the answer. You can always go back and read the Introduction again to get things clearer in your mind. Current can only flow if there is both a positive and a negative charge on opposite ends of the wire. We call this a potential difference - look at our activity on voltage and resistance for more information on this.
  • Question 6

What is the current if you have a charge of 10 C that passes through a wire in 5 seconds?

CORRECT ANSWER
EDDIE SAYS
Right then, you need to bring in that lovely equation that we were looking at. Can you remember it? I = Q ÷ t We don't know the current (I) but we know the rest: Q (charge) = 10 C t (time) = 5 seconds So, I = 10 ÷ 5 I = 2 A It's easy when you know how, isn't it?!
  • Question 7

What is the current if it takes 4 seconds for a charge of 0.5 C to pass through a wire?

CORRECT ANSWER
EDDIE SAYS
Okay, you know what to do now. Remember the equation: I = Q ÷ t Q = 0.5 C t = 4 s I = 0.5 ÷ 4 I = 0.125 A
  • Question 8

A man gets struck by lightning. A total charge of 40 C passes through him in 0.2 seconds. 

 

If a current of 0.2 A will kill a human, was there enough current in this lightning strike to kill this man?

CORRECT ANSWER
Yes
EDDIE SAYS
A lovely juicy maths question to end with! Don't let all those words muddle you up, you've got to start with that same old equation to find out how much current passes through the man. I = Q ÷ t Q = 40 C t = 0.2 s I = 40 ÷ 0.2 I = 200 A 200 is bigger than 0.2, so yes, there is enough current to kill him - in fact 1,000 times more current! This has been quite a brain-stretching set of questions so well done for working your way through them all. Just remember that all-important equation and you've got this sorted!
  • Question 9

What is the current if a charge of 0.1 C passes through your finger in 0.1 s?

CORRECT ANSWER
1
1 A
1A
1 amp
EDDIE SAYS
Ouch, that sounds painful - but actually it really isn't a very big charge! So, what's that lovely equation? Oh yes it's I = Q ÷ t Q = 0.1 C t = 0.1 s so I = 0.1 ÷ 0.1 I = 1 A
  • Question 10

A Van-De-Graff generator is capable of delivering up to 40,000 volts in one strike with a charge of 0.01 C. The strike lasts a very short time, only 0.3 s.

 

What is the current in this strike?

CORRECT ANSWER
EDDIE SAYS
Did you work your way through this one okay? It's really just the same as the previous questions so don't let all those extra words confuse you! So, what's the equation we need? That's right: I = Q ÷ t Q = 0.01 C t = 0.3 s I = 0.01 ÷ 0.3 I = 0.03 A
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