Measuring current and potential difference

In a nutshell

Current $(I)$ is the flow of electrical charge, has the units of amps and is measured using an ammeter. Potential difference $(V)$ is the difference in electric potential between two different points in an electrical circuit, is measured in volts and is measured using a voltmeter.

Current

What is current?

Current is the flow of electrical charge, which in the case of wires and electrical circuits will be electrons. When electrons are flowing through the circuit, there is an electrical current. If there is no flow of electrons, there is no current.

Current is measured using a device called an ammeter. To understand how an ammeter works, think of it counting how many electrons are passing a certain point every single second. The more electrons which are passing that particular point, the more charge is flowing and hence a larger current.

Current has the symbol " $I$ " and is measured in a unit called amperes, which is shortened to amps ( $A$ ). Named after the scientist who discovered it.

Measuring current

An ammeter is counting how many electrons are going past a particular point in the circuit every second. So the ammeter needs to be inserted into the circuit where the electrons can pass through it to be counted. Therefore, the ammeter needs to be in series with the circuit.

Factors affecting current

Having more electrons in the circuit means more electrons will pass that point every second. The amount of electrons available depends on several factors, but is heavily influenced by the type of material being used in the circuit. The more available electrons in the circuit, the higher the current.

Pushing the electrons around the circuit quicker will also increase the current. This is because whilst the amount of electrons in a circuit is fixed for a particular type of material, if the electrons are pushed around the circuit quicker, there will be more passing a point in a second.

Note: Current is never used up in a circuit.


Measuring current in different circuits. Notice that the ammeter is always placed in series, which means that the current has to flow through it.

Potential difference

What is potential difference?

Potential difference is sometimes referred to as voltage. As the name suggests it is the difference in electrical potential between two points in a circuit. The power supply supplies the electrical charge with electrical energy called potential. This is the energy which gets used up around the circuit in the different components.

Potential difference is measured using a device called a voltmeter. To understand how a voltmeter works, think of it as measuring the difference in potential before and after an electrical component. The more energy used across a component, the greater the potential difference.

Potential difference has the symbol " $V$ " and is measured in a unit called voltage, shortened to volts ( $V$ ). Again, named after the scientist who discovered it.

Measuring potential difference

A voltmeter is measuring the difference in electrical potential between two different points. In order to do this, the voltmeter needs to measure the electric potential before and then again after.

Therefore, if measuring the potential difference across a component, the voltmeter goes across the component, which is in parallel.

The only thing which affects voltage around a circuit is the power supply or battery which is providing the energy to the circuit. The greater the voltage of the power source, the greater the voltage around the circuit. The voltage provides the "push" which moves the electrons around the circuit.


Measuring potential difference in different circuits. Note that the voltmeter is always placed in parallel across the component to be measured.