Basics on Current and Charge

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Current (as you know from the definitions section) is the rate of flow of charge flowing per second in a conductor. It has the symbol I and is measured in amps (A).

Remember if you forget this definition you can figure it out from the basic current equation on the datasheet, current = charge/time (I=Q/t) i.e. current is the rate of charge / time in seconds.

So we know that current, I is measured in amps (A) it's also important to remember charge, Q is measured in coulombs (C).

This basically means a current of 1 amp is 1 coulomb of charge passing through a point every second.

If a question says for example, the current in the circuit is 150A and the time is 3s (seconds) the charge would be (Q=It) Q=150x3 = 450C.

Remember to put units when asked in the exam. Also don't be fooled if the question gives you a value in, for example, milliamps or milliseconds. If it does you need to make sure you put this correctly in the calculator.

If that didn't make sense look at this question:
"What is the current flowing in the circuit if the charge is 10C over a period of 200ms?"

We know we can easily work out current by using I=Q/t but since the question is giving us the time in ms (milliseconds) and the charge in standard C (coulombs) they won't calculate properly to give you the right current value. You need to use standard form in your calculator. The time is 200ms (which is 200 x10^-3 milliseconds) not 200 seconds!

In your calculator type (using I=Q/t) I= 10 / 200x10^-3) and you should get an answer of 5 (5A).

A lot of the time you can just use the equations on the data sheet to answer questions. For example, if a wordy question says "Here is a graph of charge against time, what does the gradient represent?" Immediately some people start to think it's one of those important rules you needed to memorize. However when you re-read the question, you already know gradient = change in Y/change in X. The question tells us that it's a graph of charge against time, so if we link this to an equation (charge/time) we know this is equal to current. The gradient must be the current!

Conservation of charge means that it is not possible to create charge, or destroy it. It is possible to cancel out charge by adding an equal and opposite charge.
(e.g. a +ve charge of 1C could be canceled out by a -ve charge of 1C).

Conventional current is the flow of current from the positive to the negative terminals of the cells (batteries). Although we now know electrons flow in the opposite direction to the flow of current.

You can use an ammeter to measure the current.

We already know from GCSE that good conductors are materials in which current will flow very well. Insulators are the opposite. At A-level you need to be aware that conductors make it easy for current (electrons) to flow easily since the outer layer of atoms are free to move from atom to atom. This is usually true for most metals. If one end of a piece of metal is made positive the negative electrons at the opposite end will be attracted towards it and because they are free, they can easily move towards it.

Oppositely insulators make it difficult for charged particles to move freely across. This is because the materials used in insulators are made of atoms which hold onto their electrons strongly preventing the easy free movement of the electrons.