Describe the use of voltmeters (analogue and digital) with different ranges

4.2.3 Electromotive Force and Potential Difference

Learning Objective

Describe the use of voltmeters (analogue and digital) with different ranges.

Key Concepts

• Electromotive force (EMF) – the energy supplied per unit charge by a source, measured in volts (V). It is the maximum potential difference the source can provide when no current flows.
• Potential difference (PD) – the work done per unit charge in moving between two points in a circuit, also measured in volts.
• Voltmeters – instruments used to measure the potential difference between two points. They must be connected in parallel with the component whose PD is to be measured.

Types of \cdot oltmeters

Using an Analogue \cdot oltmeter

1. Identify the required measurement range. Choose a range that is just above the expected PD to obtain the best resolution.
2. Rotate the range selector knob to the chosen range (e.g., 0–2 V, 0–20 V, 0–200 V).
3. Connect the voltmeter in parallel with the component:

   • Red lead to the point at higher potential.
   • Black lead to the point at lower potential.

4. Read the needle position. Estimate the value to the nearest scale division. If the needle is between two marks, add a fraction (e.g., 0.5 division).
5. Calculate the actual PD using the formula:

   \$V = \text{(reading)} \times \text{range factor}\$

   where the range factor converts the scale reading to volts.

Using a Digital \cdot oltmeter

1. Power the D \cdot M and set it to the voltage (V) mode. Many D \cdot Ms have a dedicated \cdot button.
2. Choose the range:

   • Manual range: Press the range button (e.g., 2 V, 20 V, 200 V) to select the appropriate scale.
   • Auto‑range: Press the auto‑range button; the D \cdot M will automatically select the smallest range that can accommodate the measured PD.

3. Connect the leads in parallel as described for the analogue meter.
4. Read the displayed numeric value. The D \cdot M typically shows the value to three significant figures and may indicate the least count (e.g., 0.01 V).
5. If the reading is “OL” (over‑load), the PD exceeds the selected range; switch to a higher range.

Precautions and Common Errors

• Always connect the voltmeter in parallel; connecting in series will give a false reading and may damage the instrument.
• Do not exceed the maximum voltage rating of the voltmeter (often 250 V for classroom meters).
• For analogue meters, avoid parallax error by viewing the needle straight on.
• Allow the D \cdot M to stabilise after changing ranges; the reading may drift for a few seconds.
• When measuring low voltages, use the lowest possible range to improve resolution.

Example: Measuring the EMF of a Cell

Suppose a student expects a cell to have an EMF of about 1.5 V.

1. Using an analogue voltmeter set to the 0–2 V range, the needle points between the 1.4 V and 1.5 V marks, about three‑quarters of the way to 1.5 V. Estimated reading ≈ 1.45 V.
2. Using a digital voltmeter on auto‑range, the display reads 1.498 V.
3. Both readings are close to the true EMF; the digital reading is more precise.

Summary

Voltmeters are essential for measuring potential difference. Analogue meters require manual range selection and careful needle reading, while digital meters offer auto‑ranging, higher internal resistance, and more precise numeric displays. Selecting the correct range—just above the expected voltage—optimises accuracy for both types of instruments.