Ohm's Law Investigation
In 1827, Georg Ohm measured 13 different wire lengths and found that doubling voltage exactly doubled current, but only for some materials.
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Q1 · Ohm's Law states that doubling the voltage doubles the current, but a student asks "does that always work for every material?" Before reading, predict: do you think all conductors obey this rule, or might some behave differently?
Q2 · In an investigation, how would you test whether a component obeys Ohm's Law? What would you measure, what would you vary, and what result would you expect to see if it does obey the law?
Set up a simple circuit with a battery, a resistor, and a voltmeter and ammeter. Connect a 1.5 V battery, the ammeter reads, say, 0.15 A. Swap in a 3 V battery, the reading doubles to 0.30 A. Swap in a 6 V battery, it doubles again to 0.60 A. Every time you double the voltage, the current doubles, in a perfectly constant ratio. That ratio is resistance, and the relationship is Ohm's Law: V = I × R, where V is voltage in volts, I is current in amps, and R is resistance in ohms. If you know any two of these quantities, you can calculate the third.
For many materials, called ohmic conductorsthis relationship is linear. Double the voltage across a resistor and the current doubles. Triple the voltage and the current triples. This proportionality makes circuits predictable and designable. However, not all materials are ohmic. Diodes, LEDs and transistors have non-linear behaviour.
A 12 V car battery connected to a 4 Ω starter motor draws I = V/R = 12/4 = 3 A. If the battery voltage drops to 10 V during cranking, the current falls to 2.5 A and the motor turns more slowly.
What to write in your book
- Ohm's Law: V = I × R
- For ohmic conductors, current is directly proportional to voltage
- Not all materials are ohmic, resistance can change with temperature
Drag the sliders to explore Ohm's Law. Watch the current update live.
🧮 Ohm's Law Calculator
Enter any two values to calculate the third. Use the magic triangle above if you get stuck.
Ohm's Law lets you predict circuit behaviour before you build it. Engineers use it to select appropriate wire thicknesses, calculate power dissipation, and design safety systems. The law is not universal, it fails for superconductors (zero resistance), semiconductors, and components like light bulbs whose resistance changes with temperature.
The power dissipated in a resistor can also be calculated from Ohm's Law. Combining P = V × I with V = I × R gives P = I²R and P = V²/R. These forms are essential for checking whether a component will overheat.
A phone charger labelled 5 V, 2 A delivers P = 5 × 2 = 10 W. The internal resistance of the charger circuit must be low enough that most of this power reaches the phone rather than being wasted as heat in the charger itself.
Students often think Ohm's Law applies to everything electrical. It does not. A light bulb filament has much higher resistance when hot than when cold, so its current is not proportional to voltage. Batteries have internal resistance that changes with charge level. Real circuits are more complex than ideal resistors.
What to write in your book
- Always state Ohm's Law before substituting numbers
- Power in a resistor: P = I²R = V²/R
- Check whether the material is approximately ohmic
Complete this Ohm's Law calculation.
📊 V vs I Graph, Ohmic vs Non-Ohmic
Click buttons to see how different conductors behave. Ohmic conductors show a straight line; non-ohmic conductors curve.
Ohmic conductors are materials where resistance stays constant regardless of voltage or current. Copper wire, carbon resistors, and nichrome heating elements are approximately ohmic at constant temperature. This predictability makes them essential for circuit design.
Non-ohmic materials have uses too. Diodes allow current in one direction only. Light-dependent resistors change resistance with light level. Thermistors change resistance with temperature. These non-linear behaviours are the basis of sensors and electronic control systems.
A dimmer switch uses a semiconductor device called a triac to chop the AC waveform. The bulb receives less average voltage and dims. This is non-ohmic behaviour that could not be achieved with a simple resistor.
What to write in your book
- Ohmic conductors have constant resistance at constant temperature
- Copper wire and carbon resistors are approximately ohmic
- Diodes, LEDs and light bulb filaments are non-ohmic
The assumption that all conductors are ohmic is one of the most common errors in introductory physics. While Ohm's Law is an excellent approximation for metals at constant temperature, it breaks down for many important materials and devices.
Even a simple torch bulb is non-ohmic. When cold, its filament has low resistance and draws a large inrush current. As it heats to over 2,000°C, its resistance increases by a factor of ten or more. The steady-state current is much lower than the initial surge. This is why bulbs often burn out when switched on, the initial current spike stresses the filament.
A 60 W, 240 V bulb has a hot resistance of R = V²/P = 240²/60 = 960 Ω. But when cold, the same filament might measure only 100 Ω. The initial switch-on current is nearly 2.4 A, ten times the normal operating current of 0.25 A.
What to write in your book
- Not all conductors are ohmic, resistance often changes with temperature
- A cold light bulb filament has much lower resistance than a hot one
- Real circuits require more complex analysis than ideal resistors
At the start of this lesson you read that Georg Ohm published his famous law in 1827 and was ridiculed for it being too simple, yet nearly 200 years later, V = IR sits at the centre of every circuit investigation on Earth. You were challenged to test it yourself and find out whether it holds for every material.
Now that you've completed the investigation, does Ohm's Law hold up for all materials? What did your evidence show, and why did Ohm's critics turn out to be wrong?
Before you begin, estimate:
A 12 V car battery is connected to a resistor. If the current is 3 A, what is the resistance? And if you replace the resistor with one of double the resistance but keep the same battery, what is the new current? Use V = IR. Record your estimates, then verify with the calculator.
Model answers (click to reveal)
📖 Model Answers
▼MCQ Answers
1. BI = V/R; double V with constant R → double I.
2. CR = V/I = 9/0.5 = 18 Ω.
3. ALight bulb filaments heat up, changing resistance. Non-ohmic.
4. BAmmeters must be in series to measure current through the component.
5. CGradient = ΔV/ΔI = R for ohmic conductors.
SAQ 1, Ohm's Law Calculation (3 marks)
Model answer: Using Ohm's Law: R = V / I = 2.0 V / 0.4 A = 5 Ω. When the voltage increases to 6.0 V, the new current is I = V / R = 6.0 V / 5 Ω = 1.2 A. The current has tripled because the voltage has tripled while resistance stayed constant. This demonstrates the direct proportionality in Ohm's Law: for an ohmic conductor, current is directly proportional to voltage when resistance is unchanged.
SAQ 2, Investigation Design (4 marks)
Model answer: To test whether a metal resistor is ohmic, I would set up a circuit containing a variable DC power supply, the test resistor, an ammeter connected in series, and a voltmeter connected in parallel across the resistor.
The independent variable is the voltage across the resistor, which I change using the variable power supply in steps (e.g., 1 V, 2 V, 3 V, 4 V, 5 V). The dependent variable is the current through the resistor, measured by the ammeter. Controlled variables include the same resistor (to ensure consistent resistance), the same equipment, and allowing the resistor to cool between measurements (to control temperature).
I would record my results in a table with columns for Voltage (V) and Current (A). Then I would plot a graph of Voltage (y-axis) versus Current (x-axis). If the resistor is ohmic, the graph will be a straight line passing through the origin, and the gradient will equal the resistance. If the graph curves, the resistor is non-ohmic.
SAQ 3, Light Bulb Non-Ohmic Behaviour (5 marks)
Model answer: A light bulb does not obey Ohm's Law because its filament heats up dramatically as current increases. The tungsten filament in an incandescent bulb reaches temperatures of 2,500°C. In metals, resistance increases with temperature because the vibrating metal ions scatter electrons more effectively. At low current (cold filament), resistance is low. At high current (hot filament), resistance is much higher. This means the V-I relationship is not proportional, the graph curves upward with a decreasing gradient.
This non-ohmic behaviour has important consequences for efficiency. A 60 W incandescent bulb transforms only about 5% of its electrical energy into visible light; the remaining 95% becomes heat. When the bulb first turns on, the cold filament has low resistance, causing a brief inrush current up to 10× the normal operating current. This is why bulbs often burn out at the moment of switching on, the sudden current surge stresses the filament. Modern LED bulbs avoid this problem entirely: they are non-ohmic in a different way (diode behaviour) but operate at much lower temperatures, achieving 80–90% efficiency in converting electricity to light.
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