Use a digital voltmeter with a suitable parallel connection
Interpret signed digital voltage displays and range limits while distinguishing resolution, loading and source conditions.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Connect a voltmeter in parallel.
- Interpret volt prefixes and chosen polarity.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is 420 mV in volts?
What you will learn
- Describe digital p.d. measurement with appropriate range and terminals.
- Interpret sign, prefixes and the limits of display resolution.
Record the two-terminal reference
Connect the digital voltmeter's voltage terminals across the two points in the isolated low-voltage measurement. A display of +4.20 V means the first stated terminal is at higher potential than the second under the meter's reference. Reversing the leads can give −4.20 V for the same physical p.d. with the reference reversed.
Choose a range above the expected magnitude. If the meter displays an overload indication, do not record it as zero p.d.; follow its instructions and select a larger suitable range. A 420 mV display is 0.420 V, not 420 V, so retain the displayed unit and prefix with the digits.
Know what the meter can establish
A stated 0.01 V display step gives voltage resolution. It does not alone prove an accuracy of exactly 0.01 V. A high-resistance voltmeter draws relatively little current, but its effect can still matter in a sensitive circuit; an ideal meter approximation must be stated when used.
An open-circuit source reading can approximate e.m.f. when the meter's current is negligible. A loaded real source's terminal reading need not equal its e.m.f. because of internal energy transfers. Record whether the source was loaded rather than treating every voltage display as the same source quantity.
Sources for this lesson
Pause and explain
Reversing voltage leads changes +4.20 V to −4.20 V. What changed?
Worked example
A digital voltmeter gives +4.20 V. The same two points are measured with reversed leads. Interpret the expected sign change in the ideal reading.
Show the worked solution
- The physical pair of points is unchanged.
- The measurement polarity is reversed.
- The magnitude stays 4.20 V and the sign reverses.
Answer −4.20 V on the reversed reference.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Interpret a +4.20 V reading and its −4.20 V value after reversing the leads across the same points.
Give me a hint
Distinguish the physical magnitude from the chosen reference.
Compare my reasoning
- The p.d. magnitude is 4.20 V.
- Reversed leads swap the reference points.
- The sign reverses while the ideal magnitude remains.
Same 4.20 V magnitude; opposite measurement reference.
Look for these in your work
- State the magnitude.
- Explain the sign convention.
Convert 420 mV to V and identify what a 0.01 V display step establishes.
Give me a hint
Convert the prefix, then distinguish resolution from accuracy.
Compare my reasoning
- 420 mV = 0.420 V.
- The step is 0.01 V resolution.
- Accuracy is not established by that step alone.
0.420 V; resolution 0.01 V, with accuracy not determined from the step alone.
Look for these in your work
- Convert the displayed unit.
- Separate resolution and accuracy.
A loaded cell's terminal display is called its exact e.m.f. without any source assumptions. Assess the interpretation.
Give me a hint
Identify the terminal account and possible source-internal transfers.
Compare my reasoning
- The meter measures terminal p.d.
- A real loaded source can transfer energy internally.
- Equality with e.m.f. requires an appropriate ideal or negligible-current condition.
The display is terminal p.d.; exact equality with e.m.f. is not established.
Look for these in your work
- Name the measured quantity.
- State the missing source condition.
Common mistakes
- Read voltage terminals and prefixes.
- Overload is not zero p.d.
- Resolution and loaded terminal p.d. have separate limitations.
What can you explain now?
Does a 0.01 V display step alone prove an accuracy of 0.01 V?
Compare with the explanation
No.
The step specifies resolution; accuracy requires the supplied meter specification.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Interpret digital p.d. with range, polarity, resolution and explicit source-loading conditions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.