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1.3.6Development of Practical Skills in Physics

Circuits

5 min read 8 parts Lesson 23 of 29 1 quiz Virtual lab

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Lesson summary

1.3.6 Circuits — OCR A Level Physics A notes: Construct and use practical circuits with ammeters, voltmeters, variable resistors and potential dividers, and take safe, accurate electrical measurements.A practical circuit is built from a power supply (cell, battery or low-voltage …

  • Variable resistor (rheostat) in series
  • I–V characteristic
  • internal resistance
  • resistivity
  • Worked example 1
  • Worked example 2

Section 1 of 8Circuit components and symbols

A practical circuit is built from a power supply (cell, battery or low-voltage supply), connecting leads, a switch, the component under test and meters. The standard symbols are used in diagrams: a cell, a resistor (rectangle), a variable resistor (rectangle with an arrow), a filament lamp, a diode, a thermistor, a light-dependent resistor, and the meters shown as a circle containing A or V.

Build the main series loop first, from the positive terminal through each component back to the negative terminal. Add the voltmeter last, in parallel across the component being measured. Keep the switch open until the circuit has been checked, and close it only while taking a reading, since prolonged current heats components and changes their resistance.

Section 2 of 8Ammeters and voltmeters

Ammeter
An ammeter is connected in series with the component, so the same current flows through it. An ideal ammeter has zero resistance, so it does not alter the current. Real ammeters have a small resistance.
Voltmeter
A voltmeter is connected in parallel with the component, so that it measures the potential difference between the two ends. An ideal voltmeter has infinite resistance and draws no current. Real voltmeters have large resistance (digital: 10 MΩ typical).

Observe the polarity

  • the meter terminal marked + (red) goes towards the positive side of the supply.
  • Select a sensible range, starting with the highest range if the size is unknown and switching down for greater resolution.
  • Check for zero error before connecting, especially on analogue meters.

Section 3 of 8Varying the current: rheostat and potential divider

There are two ways to change the voltage or current across a component.

1Variable resistor (rheostat) in series
Variable resistor (rheostat) in series this changes the total resistance and therefore the current. The lowest voltage across the component is not zero, because the component always receives part of the supply voltage.
2Potential divider
Potential divider the two fixed ends of the resistor are connected across the supply and the component is connected between the slider and one end.
  • The voltage across the component can be adjusted smoothly from zero up to the full supply voltage.
  • A potential divider is preferred when a wide range of values starting from zero is needed, for example when plotting the I–V characteristic of a filament lamp or a diode.

In a simple divider of two resistors R1 and R2 in series across Vin, the output across R2 is Vout = Vin R2 / (R1 + R2), provided the load draws very little current.

Section 4 of 8Typical experiments

In an I–V characteristic experiment, the voltage across a component is increased in steps and the current recorded.

Typical experiments
Readings should also be taken with the supply reversed for a diode.
The plot for a fixed metal wire at constant temperature is a straight line through the origin (Ohm's law); the filament lamp gives a curve whose gradient decreases because the resistance rises with temperature.
Resistance at a point is R = V / I, not the gradient.

In an internal resistance experiment, a variable resistor changes the current from a cell and the terminal voltage V is plotted against I. The equation V = E − Ir shows that the intercept is the emf E and the gradient is −r. See the later topic on electrical circuits for the theory.

For resistivity of a wire, a length l of wire is connected between crocodile clips and R = V / I recorded at several lengths. Plotting R against l gives a straight line through the origin with gradient ρ / A.

Section 5 of 8Safety and accuracy

Use low-voltage supplies, keep water away from circuits, and switch off before changing connections.

1
Currents through thin wires should be kept low and the circuit switched on only briefly, to avoid heating the wire, which would change its resistivity and give a false trend.
2
Use a protective resistor in series with a diode or LED so the current does not exceed the rated value.
3
Check the meter ranges after setting up: if a reading is off the scale, switch off and select a larger range.
4
Quote results with the correct number of significant figures and percentage uncertainty (see 1.3.1 and 1.3.3).

Section 6 of 8Worked examples

  1. A potential divider has R1 = 4.0 kΩ and R2 = 2.0 kΩ across a 12 V supply. Find the voltage across R2 when unloaded.

  2. A filament lamp has I = 0.20 A when the voltage is 2.0 V and I = 0.50 A when the voltage is 6.0 V. Find the resistance in each case and comment.

  3. An internal-resistance experiment gives a straight line V = 1.52 V at I = 0, and V = 1.24 V at I = 0.40 A. Find E and r.

Section 7 of 8Key terms

TermMeaning
AmmeterMeter connected in series that measures current.
VoltmeterMeter connected in parallel that measures potential difference.
RheostatVariable resistor used in series to change current.
Potential dividerArrangement giving an adjustable fraction of the supply voltage.
I–V characteristicGraph of current against potential difference for a component.
Terminal pdPotential difference across the terminals of a cell when a current flows.

Section 8 of 8Exam technique

1
Connecting the ammeter in parallel or the voltmeter in series with the component.
2
Using a rheostat in series when a potential divider is needed to reach zero voltage.
3
Leaving the current on between readings, so a wire or filament heats up.
4
Taking the gradient of an I–V curve as the resistance at a point; use R = V / I.
5
Reversing the meter terminals so that the needle deflects the wrong way.
Try it in the virtual labCircular motion: centripetal force \n