4.1.5P4. Electricity and Magnetism
Voltage — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 18 of 19 Unit quiz
Section 1 of 6What voltage is
Source of voltage
- Source of voltage A cell, a battery (two or more cells) or a generator provides the voltage in a circuit.
- One terminal of the source is positive and the other negative, and this difference is what pushes current round the circuit.
Potential difference
- Potential difference Voltage is also called potential difference (p.d.) and is measured in volts (V).
- It is the energy transferred per unit charge between two points, so it always describes a difference between two places in the circuit.
Cause of current
- Cause of current A larger voltage across the same circuit makes a larger current flow.
Section 2 of 6Voltage in series and parallel
| Circuit | Rule for voltage | Example |
|---|---|---|
| Series | The supply voltage is shared between the components, and their voltages add up to the supply voltage | Two lamps across a 6 V battery might have 2 V and 4 V |
| Series with identical components | The voltage is shared equally | Two identical lamps on a 6 V battery get 3 V each |
| Parallel | Each branch has the same voltage as the supply | Three lamps on separate branches of a 6 V battery each have 6 V |
Section 3 of 6Measuring voltage with a voltmeter
1
Build the complete circuit first and check that it works, using only the ammeter and components.
2
Pick up the voltmeter and connect it last, with one lead on each side of the component being measured.
3
This puts the voltmeter in parallel with the component, because it measures the difference between two points.
4
Connect the red terminal towards the positive side of the supply and the black terminal towards the negative side.
5
Read the value from a position directly in front of the scale if the meter is analogue.
Section 4 of 6Analogue and digital meters
Voltmeters
Analogue voltmeter
- Analogue voltmeter Has a needle moving over a scale, with typical ranges of 0 to 1 V or 0 to 5 V.
- It is liable to parallax error, so read it from directly in front with your eye perpendicular to the scale.
Digital voltmeter
- Digital voltmeter Shows the value as digits, which are easy to read and more precise, and it can measure tiny voltages in mV.
- The last digit may flicker between values, so a decision must be made about which to record.
Zero error
- Zero error Check that the reading is zero before the experiment;
- if not, subtract the zero value from every measurement.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Voltage | The potential difference between two points, measured in volts. |
| Potential difference | Another name for voltage; the difference in electrical potential between two points. |
| Voltmeter | An instrument connected in parallel that measures the voltage across a component. |
| Parallax error | A reading error caused by viewing a scale from an angle. |
| Zero error | A fault where a meter gives a non-zero reading when it should read zero. |
Section 6 of 6Exam technique
1
Connecting a voltmeter in series with the component. It must go in parallel across it.
2
Connecting the voltmeter before the rest of the circuit is built, which makes faults hard to find.
3
Saying voltage flows through a component. Current flows; voltage is the difference across the component.
4
Ignoring zero errora meter that starts at 0.1 V adds 0.1 V to all readings unless corrected.
5
Believing that the voltage is used up in a series circuit. Energy is transferred, and the voltages add up to the supply voltage.
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