Electric current and potential difference guide for KS3 physics students

Electric charge, current and circuits

Electric charge

Some particles carry an . In electric wires these particles are . We get an electric current when these charged particles move from place to place.

Electric current

An is a flow of charge, and in a wire this will be a flow of electrons. We need two things for an electric current to flow:

something to transfer energy to the electrons, such as a battery or power pack
a complete path for the electrons to flow through (an )

The simplest complete circuit is a piece of wire from one end of a battery to the other. An electric current can flow in the wire from one end of the battery to the other, but nothing useful happens. The wire just gets very hot and the battery loses stored internal energy – it ‘goes flat’ and stops working.

To do something useful with the electric current, you need to put an electrical into the circuit, that can use the current in a useful way.

Three images: 1 - bulb unlit, wires have a gap in them, labelled "incomplete circuit". 2 - bulb unlit no battery in the circuit diagram, labelled "no battery". 3 - Bulb lit, circuit includes a bulb and wires all the way around - Labelled "complete circuit" — Figure caption,
The lamp will only light up if there is a complete closed circuit with a battery.

You usually add a to the circuit. This lets you break the circuit and stop the electric current when you want to.

Game - series and parallel circuits

Play an Atomic Labs experiment exploring different arrangements of series and parallel circuits.

You can also play the full game

Circuit symbols

We use to draw diagrams of electrical circuits, with straight lines to show the wires. The diagram shows some common circuit symbols.

Circuit symbols

1. a horizontal line with two circles in the middle - between the circles the line is at a 45 degree angle - Labelled 'switch'. 2. A horizontal line, in the middle a vertical line stops the horizontal line, a break and then a shorter vertical line then starts the continuation of the horizontal line, labelled 'cell'. 3. Same as 2, but there are two sets of the vertical lines joined by a horizontal dashed line. 4. A horizontal line, in the middle a circle with a x cross through it, labelled 'lamp'. 5. A horizontal line, in the middle a circle with a V written in it, labelled 'voltmeter'. 6. A horizontal line, in the middle a circle with an A written in it,, labelled 'ammeter'. 7. A horizontal line, in the middle an empty rectangle, labelled 'resistor'. 8. A horizontal line, in the middle a rectangle with an arrow at 45 degrees through it, labelled ' variable resistor'. 9. A horizontal line, in the middle a circle with an M written in it, labelled 'motor'. — Figure caption,
Some common circuit symbols

Cells and batteries

The symbol for a is made by joining two more symbols for a together.

Two diagrams: 1 - A straight horizontal line goes to a short vertical line, then a small gap and another, shorter vertical line (this indicates the positive and negative side of the cell) from the shorter vertical line, the horizontal line continues - This is a cell. Diagram 2 - Starts as with the first diagram, but after the shorter vertical line, there is a dashed horizontal line that leads to a longer vertical line, with a gap and then a shorter vertical line. From the 2nd shorter vertical line, the solid horizontal line continues - Labelled a battery. — Figure caption,
Make sure you know the difference between these two symbols, a cell is one big and little vertical line (to indicate positive and negative sides) and a battery is more than one cell joined with a dotted line

Think of what we usually call a single battery, like the type you put in a torch. In physics, each of these is actually called a . It is only when you have two or more of these cells connected together that you call it a battery.

Do not confuse electrical cells with the cells in living organisms.

Circuit diagrams

The idea of a is to use circuit symbols instead of drawing each component in the circuit.

Always draw wires as straight lines - use a ruler. Do not be tempted to make them wiggly because the whole point is to make it easier to see what is connected to what.

Here you can see how the symbols for a cell and a look in a circuit diagram:

Two images: 1 - A picture of a bulb in the bottom line of a rectangle, with a battery with plus and minus at the top. Image 2: a diagram of lines in a rectangle, the bottom line has a circle with a cross in the middle of it (a lamp) and the top line has a break with two vertical lines, one longer than the other (a cell). — Figure caption,
You could make the circuit by laying out the cell and lamp on the table as shown in the circuit diagram, then joining them using wires

Measuring current

You can measure current and potential difference in circuits. They are different things and so are measured in different ways.

Current is a measure of how much electric charge flows through a circuit. The more charge that flows, the bigger the current. Current is measured in . The symbol for ampere is A. For example, 20 A is a bigger current than 5 A. The word ampere is often abbreviated to ‘amp’, so people talk about how many amps flow.

Measuring current:

A device called an is used to measure current. Some types of ammeter have a pointer on a dial, but most have a digital display. To measure the current flowing through a component in a circuit, you must connect the ammeter in with it.

Two diagrams: 1 - a diagram of lines in a rectangle, the bottom line has a circle with a cross in the middle of it (a lamp) and the top line has a break with two vertical lines, one longer than the other (a cell), the left side of the rectangle has a circle with an A in it (an ammeter). 2. 1 - a diagram of lines in a rectangle, the bottom line has a circle with a cross in the middle of it (a lamp) and the top line has a break with two vertical lines, one longer than the other (a cell), the right side of the rectangle has a circle with an A in it (an ammeter). — Figure caption,
A circuit with an ammeter connected in two different places, both in series with the cell and lamp.

When two components are connected in series, you can follow the path through both components without lifting your finger or going back over the path you have already taken.

Measuring potential difference

is a measure of the difference in energy between two parts of a circuit. The bigger the difference in energy, the bigger the potential difference.

Potential difference is measured in . The symbol for volts is V.

For example, 230 V is a bigger potential difference than 12 V. Instead of referring to potential difference, people often talk about voltage, so you may hear or see ‘voltage’ instead of ‘potential difference’.

Measuring potential difference

Potential difference is measured using a device called a . Just like ammeters, some types have a pointer on a dial, but most have a digital display. However, unlike an ammeter, you must connect the voltmeter in to measure the potential difference across a component in a circuit.

A diagram of lines in a rectangle, the bottom line has a circle with a cross in the middle of it (a lamp) and the top line has a break with two vertical lines, one longer than the other (a cell). Along the bottom line is a second smaller rectangle, the bottom line of second rectangle has a circle with a V in it in the middle (a voltmeter) — Figure caption,
A circuit diagram showing a voltmeter in parallel with a lamp

You can measure the potential difference across a cell or battery. If the two or more cells point in the same direction, the more cells, the bigger the potential difference.

Two circuit diagrams. On the first, the voltmeter is across a cell and reads 1.5v, in the second diagram, the voltmeter is across a 3 cell battery and reads 4.5v. — Figure caption,
Each cell has a potential difference of 1.5 V, so three cells give 4.5 V.

Series circuits

In a television series, you get several episodes, one after the other. A series circuit is similar. You get several components one after the other.

If you follow the circuit diagram from one side of the cell to the other, you should pass through all the different components, one after the other, without any branches.

Two images: 1 - A picture of two bulbs in the bottom line of a rectangle, with a battery with plus and minus at the top. Image 2: a diagram of lines in a rectangle, the bottom line has two circles with a cross in the middle of it (a lamp) and the top line has a break with two vertical lines, one longer than the other (a cell). — Figure caption,
Two lamps in a series circuit

In a series circuit, if a lamp breaks or a component is disconnected, the circuit is broken and all the components stop working.

An image of two bulbs in a circuit - One of the bulbs is broken, the other is unlit. — Figure caption,
In a series circuit, if one lamp is removed or broken the other goes out.

Current in series circuits

An image of a circuit with two bulbs, ammeters are placed on the sides of the sides of the circuit and in between in bulbs. Each ammeter reads 0.5 A. — Figure caption,
All three ammeters read 0.5 A in this series circuit.

The current in a series circuit depends upon the number of cells. If you make the cells face in the same direction, the more cells you add, the greater the current.

3 diagrams: 1 - One bulb, one cell, ammeter reads 0.5 A. 2 - One bulb, two cells, ammeter reads 1.0 A. 3 - One bulb, 3 cells, ammeter reads 1.5 A. — Figure caption,
The current in this series circuit increases as more cells are added

Current is not used up

If you put more lamps into a series circuit, the lamps will be dimmer than before because less current will flow through them.You might think that the current gets less as it flows through one component after another, but it is not like this. The current is not used up by the components in a circuit. This means that the current is the same everywhere in a series circuit, even if it has lots of lamps or other components.

Parallel circuits

In a parallel circuit, different components are connected on different branches of the wire. If you follow the circuit diagram from one side of the cell to the other, you can only pass through all the different components if you follow all the branches.

Two images: 1 - A picture of a bulb in the bottom line of a rectangle, with a battery with plus and minus at the top, another rectangle below the first contains another bulb. Image 2: a diagram of lines in a rectangle, the bottom line has a circle with a cross in the middle of it (a lamp) and the top line has a break with two vertical lines, one longer than the other (a cell)., another rectangle joins below the first containing another circle with a cross through it (lamp). — Figure caption,
Two lamps connected in parallel

In a parallel circuit, if a lamp breaks or a component is disconnected from one parallel wire, the components on different branches keep working. Unlike a series circuit, the lamps stay bright if you add more lamps in parallel.

Two diagrams - 1 - A battery in a parallel circuit with a broken bulb, the bulb below the broken bulb is lit. 2 - A battery in a parallel circuit with a broken bulb, the bulb above the broken bulb is lit. — Figure caption,
In a parallel circuit, if one lamp is removed or broken the other stays on

When two components are connected in parallel, the current is shared between the components. The current is shared when it reaches the branches, then adds again where the branches meet.

An image of a parallel circuit - Ammeter at the top of the circuit reads '6A' - Ammeters on each of the 3 branches each read '2A'. — Figure caption,
Current flowing through three identical lamps in parallel

Resistance

The wires and the other components in a circuit reduces the flow of charge through them. This is called resistance.The unit of is the , and it has the symbol Ω (an uppercase Greek letter omega).

For example, a 2 Ω component has a greater resistance than a 1 Ω component, and will reduce the flow of charge through it more effectively.

Adding components

The resistance increases when you add more components in series. For example, the resistance of two lamps is greater than the resistance of one lamp, so less current will flow through them.

Two circuit diagrams. 1 - One cell, one bulb, ammeter reads 0,4A. 2 - One cell, two bulbs in series, ammeter reads 0.2A. — Figure caption,
The more lamps, the greater the resistance and the lower the current.

Calculating resistance

To find the resistance of a component, you need to measure:

the potential difference across it
the current flowing through it

The resistance is the ratio of potential difference to current. We use this equation to calculate resistance:

resistance = potential difference ÷ current

For example:
3 A flows through a 240 V lamp. What is the resistance of the lamp?

resistance = 240 ÷ 3 = 80 Ω

If you plot a graph of current against potential difference for a wire, you get a straight line.

Conductors and insulators of electricity

Different materials have different resistances:

an electrical has a low resistance
an electrical has a high resistance

You can easily find out which materials are conductors and which are insulators using a simple circuit.

You set up a series circuit with a cell, lamp and wires. Leave a gap in the circuit between two of the wires. Then connect the two wires using pieces of each material and see if the lamp lights up:

it will light up if the material is a conductor
it will not light up if the material is an insulator

The table lists some examples of conductors and insulators:

Conductors	Insulators
Metal elements	Most non-metal elements eg sulphur, oxygen
Graphite (a form of carbon; a non-metal element)	Diamond (a form of carbon; a non-metal element)
Mixtures of metals, eg brass, solder	Plastic
Salt solution	Wood
Liquid calcium chloride	Rock

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