Investigating Science 11–12 · Year 11
Ohm’s law as a prediction: resistor against filament lamp
Module 4: Theories and Laws
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The idea
A law earns acceptance by predicting results before they are measured: Ohm’s law predicts the current in a resistor, and a filament lamp shows the conditions under which it stops applying.
What you need
- 100 Ω ± 1 % resistor rated 0.5 W; a 6 V filament lamp in a holder
- variable low-voltage DC supply 0 to 6 V (or four 1.5 V cells with a rheostat), switch
- digital ammeter (mA range) and voltmeter, connecting leads
How to do it
- Write the predicted current for the resistor at 1, 2, 3, 4, 5 and 6 V before connecting anything.
- Build the circuit with the ammeter in series and the voltmeter across the resistor; measure the current at each voltage, three times.
- Plot current against voltage and compare with the prediction line.
- Replace the resistor with the lamp and measure current at 0.5 V steps to 6 V.
- Calculate V/I at each step for the lamp and plot it against current.
- State the conditions under which Ohm’s law predicted the results and where it failed, and why.
What you should see
The resistor gives 10.0, 20.0, 30.0, 40.0, 50.0 and 60.0 mA at 1 to 6 V, within the 1 % tolerance plus the meters’ stated accuracy, a straight line through the origin with gradient 0.0100 A/V; it dissipates 0.36 W at 6 V, inside its rating. The lamp’s graph bends: V/I rises steadily with current because the filament’s resistance rises as it heats, so a single resistance value predicts too much current at high voltage. The law holds for a conductor at constant temperature.
What changes
- What you change
- applied voltage
- What you measure
- current
- What you keep the same
- same component and meters
- switch off between readings so the resistor stays cool
- three readings per voltage
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Current is used up as it goes round the circuit.
- Ohm’s law applies to every component.
- A law is a theory that has been tested many times.
Safety card
Hazards
- hot resistor or lamp
- short circuits
Controls
- keep voltage within component ratings
- switch off between readings
- check the circuit before switching on
Note
Low-voltage electrical work with no hazardous chemicals: record a RiskAssess risk assessment following the NSW Department of Education Science safety and compliance page; use only cells or a laboratory low-voltage supply, never mains.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-11INS11/12-5INS11/12-6INS11/12-7
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/education-and-training/nesa/curriculum/science/investigating-science-stage-6-2017
- education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/investigating-science/m4-theories-and-laws-investigating-science.docx
- instructional-resources.physics.uiowa.edu/5f1010-ohms-law
- phet.colorado.edu/en/simulations/circuit-construction-kit-dc
- phet.colorado.edu/en/simulations/ohms-law