Electric Circuits · Grade 10
What is actually being measured in volts?
Energy per unit of charge. One volt means one joule of energy for every coulomb that passes. So a nine-volt battery gives each coulomb nine times the energy a one-volt battery does, and the voltage across a component tells you how much energy that component takes from every coulomb going through it.
Learning objectives
- Define voltage as energy per unit of charge and use it to find the energy delivered to a given amount of charge
- Explain why a voltmeter is connected in parallel with the component being measured
- Show that the voltages across components connected in series add up to the voltage of the source
- Read the voltage across each part of a simple circuit from a labelled diagram
Not a push, but energy per coulomb
Voltage is often described as 'pushing' the current, which is a useful picture that stops working quickly. The precise definition is simpler: voltage is energy divided by charge.
So the energy delivered is voltage times charge. A coulomb passing through a 9 V battery receives 9 joules; if 5 coulombs pass, 45 joules have been delivered.
It is also why a 9 V battery is far less dangerous than a wall socket at 230 V: each coulomb there receives about twenty-five times the energy.
Voltage is always between two points
There is no 'voltage at a point'. You always ask for the voltage between two places, exactly as there is no 'height' without saying above what.
That is why a voltmeter is connected in parallel with a component — at two points, one on each side of it. It asks how much energy each coulomb lost travelling from one side to the other.
So that it does not alter the circuit, a voltmeter's resistance is very high: it sits in parallel and must not open a convenient path that steals current from the component being measured.
In series, the voltages divide
A coulomb leaving the battery passes through every component in series and returns. The energy it received from the battery is divided among them, and at the end of the loop it is back exactly where it started.
So the voltages across components in series add up to the source voltage. If a 12 V battery feeds two resistors in series and the first has 8 V across it, the second must have 4 V.
This is an excellent check: voltages that do not add up to the source mean an arithmetic slip or a component someone forgot.
Worked examples
How much energy do 3 coulombs receive passing through a 12 V battery?
- Energy equals voltage times charge
- 12 times 3
Answer: 36 joules
A 9 V battery, three resistors in series. Two of them have 3 V and 4 V. What is across the third?
- Series voltages add up to the source voltage
- 3 plus 4 is 7
- 9 minus 7
Answer: 2 volts
Why does a voltmeter have high resistance rather than low?
- It is connected in parallel with a component
- Low resistance would draw current through it
- The current through the component would change
Answer: So the measurement does not alter what it is measuring
Common mistakes
- Talking about 'the voltage at a point'
- Voltage is a difference between two points, like a difference in height. A question about voltage that does not say between what and what is not a complete question.
- Connecting a voltmeter in series
- Its high resistance would block nearly all the current and the circuit would stop working. A voltmeter is always in parallel, an ammeter always in series.
- Confusing voltage with current
- Voltage is energy per coulomb; current is how many coulombs pass per second. They have different units and do not substitute for each other.
What to remember
- Voltage equals energy over charge. A volt is a joule per coulomb.
- Voltage is always between two points.
- A voltmeter goes in parallel and has high resistance.
- In series the voltages add up to the source.