Current and voltage

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Early experiments with electricity

He carried out many experiments with electricity, including this one.

Benjamin Franklin was a scientist in the 18th century.

Don’t try this at home!

Franklin proved the link between lightning and electricity using a kite attached to a metal key.


An electrical circuit is like a central heating system: There is a pump that pushes water around the system. The water everywhere starts to move at the same time. There are pipes that carry the water. The water flows through the pipes.

What is an electrical circuit?


What is an electrical circuit?


Scientists don’t draw circuits as they look in real life. This is because it takes too long and each person would draw something slightly different.

Drawing electrical circuits

Instead, they use circuit symbols to represent each component in the circuit. They are drawn exactly the same way each time!

Wires are drawn as straight lines. They don’t have to represent the real lengths of the wires.


Match the circuit symbols


Components in an electrical circuit


What is a series circuit?

This is a simple series circuit.

In a simple series circuit, everything is connected in one loop across the terminals of the battery. There are no points where the current can splitor join (these are called junctions).

This circuit has two lamps connected in series. Circuits are always drawn using straight lines.


Examples of series circuits

Here are some other simple series circuits:

1. Two resistors connected in series.

2. A variable resistor (or rheostat) and a bulb connected in series.


The unit of measurement for current is the amp, which has the symbol A.

Measuring current

Current is measured using a device called an ammeter.

When measuring the current through a component, the ammeter is always connected in series (in the same loop) with that component.

In a circuit diagram, an ammeter is shown by an ‘A’ in a circle.


Experiment 1: current in a series circuit


Current in a series circuit: summary


Voltage is measured using a device called a voltmeter. In a circuit diagram, a voltmeter is given the symbol V.

Measuring voltage

When measuring the voltage across a component, the voltmeter is always connected in parallel with (or across) the component.

The voltage supplied by the battery is shared between all the components in a series circuit.

This is still a series circuit, as the voltmeter does not affect the circuit.


Experiment 2: voltage in a series circuit


Voltage in a series circuit: summary


Current or voltage?


Experiment 3: cells in a series circuit


Cells in a series circuit: summary


Series circuits – key ideas

1. In a series circuit the current is the same in all parts of the circuit. Series circuits are found in torches and strings of Christmas lights.

3. The current depends on the voltage in any circuit.

2. The supply voltage is shared between the components in a series circuit. (The sum of the voltage across each component is the same as the total supply voltage.)


Make your own series circuit


Series circuits: summary


A parallel circuit is one that contains a point (a junction) where the current can split (point A) or join (point B).

What is a parallel circuit?

This means that there is more than one path around the circuit.

A B


Modelling parallel circuits


1. Set up the circuit as shown.

Measuring current in a parallel circuit

2. Place an ammeter, in turn, at positions 1, 2, 3 and 4 and record the readings in a table.

Current (A)

A1

A2

A3

A4

Ammeter


In a parallel circuit, the current that leaves the cell is the same as the current that returns to the cell.

Current in a parallel circuit

This is because the current does not get used up by the circuit, just the energy that the current is carrying.

The ammeter readings for A1 and A4 should be the same.


The current splits up at the first junction and then joins together at the second junction.

Current in a parallel circuit

The following is always true for this type of parallel circuit:

If the bulbs are identical, then the current will split evenly. If the bulbs are not identical, then the current will not split evenly.

A1 = (A2 + A3) = A4


Connect up this circuit and measure, in turn, the voltage at V1, V2 and V3. Record your results in the table.

Measuring voltage in a parallel circuit

What do you notice about the results?

How can you explain this?

Voltage (V)

V1

V2

V3

Voltmeter


Make your own parallel circuit


Cells in a parallel circuit: summary


Series and parallel circuits: summary


Glossary


Anagrams


Multiple-choice quiz


True or false?

Current and voltage

Technology

Transcript of Current and voltage