Science 7–10 · Year 9
Ohm's law: current against voltage for two fixed resistors
Physical sciences — Energy, content group Electrical energy (NSW Stage 5 focus area; ACARA places electric circuits in Year 6, AC9S6U03, and has no Year 9 or Year 10 content description on circuits, so only Year 9 inquiry codes are cited)
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The idea
For a metal resistor at steady temperature the current is proportional to the voltage across it, and the constant ratio V / I is its resistance, so a larger resistance lets less current through at the same voltage.
What you need
- variable DC power supply 0 to 12 V, 1
- 10 ohm 5 W resistor on a mounting board, 1
- 22 ohm 5 W resistor on a mounting board, 1
- digital multimeters as ammeter (0 to 2 A) and voltmeter (0 to 20 V), 2
- switch, 1
- 4 mm leads, 8
How to do it
- Connect the supply, switch, ammeter and 10 ohm resistor in series, with the voltmeter across the resistor.
- Set the supply to 1.0 V, close the switch, read current and voltage, open the switch. Repeat at 2.0, 3.0, 4.0, 5.0 and 6.0 V.
- Reverse the supply connections and repeat at 2.0 and 4.0 V to check the negative side.
- Plot current (A) against voltage (V); draw the line of best fit and calculate its gradient. Resistance = 1 / gradient.
- Replace the 10 ohm resistor with the 22 ohm resistor and repeat from 1.0 V to 6.0 V.
- Plot the second resistor's line on the same axes and compare the two gradients and the two resistances.
What you should see
10 ohm resistor: a straight line through the origin; at 3.0 V the current is 0.30 A and at 6.0 V 0.60 A, gradient 0.10 A/V, so R = 10.0 ohm within the resistor's stated tolerance, and the reversed readings lie on the same line. 22 ohm resistor: also a straight line through the origin but less steep, 0.27 A at 6.0 V (gradient 0.045 A/V). The learner knows it worked when both lines are straight and the larger resistance gives the smaller current at every voltage.
What changes
- What you change
- voltage across the component (V)
- What you measure
- current through it (A)
- What you keep the same
- same resistor for each line
- switch closed only while reading, so the resistor stays cool
- meters on the same ranges
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Voltage flows through a component.
- A larger resistance means a larger current.
- Current is used up in a resistor, so less leaves it than enters it.
Safety card
Hazards
- resistors become hot
Controls
- switch closed only to take a reading
- do not exceed 6 V (3.6 W in the 10 ohm resistor, within its 5 W rating)
- do not touch the resistor body
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-EGY-01SC5-WS-04SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9I03AC9S9I04AC9S9I05
Sources
The pages the author read to write this activity.