PhysicsChapter 11: Electricity
Ohm's Law
Explanation level
Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided its temperature and other physical conditions remain constant.
Mathematical form
- V
- potential difference across the conductor (V)
- I
- current through the conductor (A)
- R
- resistance of the conductor (Ω)
For an ohmic conductor under constant conditions, the ratio VI remains constant. This constant is its resistance R.
Resistance and the ohm
Resistance is the opposition a component offers to electric current. Its SI unit is the ohm, symbol Ω.
A component has resistance 1 Ω when a potential difference of 1 V produces a current of 1 A through it.
V-I characteristic of an ohmic conductor
Plot potential difference V on the vertical axis and current I on the horizontal axis. An ohmic conductor gives a straight line through the origin while its temperature is constant.
If I is plotted vertically against V horizontally, the slope is 1R. Always inspect the labelled axes before interpreting a graph.
Experimental verification
- Connect the test resistor, ammeter, rheostat, cell and switch in series.
- Connect a voltmeter in parallel across the test resistor.
- Begin with the rheostat set to limit current.
- Close the switch briefly and record corresponding V and I readings.
- Change the rheostat and take several readings while keeping the conductor's temperature nearly constant.
- Plot V against I. A straight line through the origin verifies Ohm's law, and its slope gives R.
The ammeter has very low resistance and is placed in series. The voltmeter has very high resistance and is placed in parallel.
Ohmic and non-ohmic behavior
| Ohmic conductor | Non-ohmic device |
|---|---|
| V is directly proportional to I under fixed conditions | V and I are not directly proportional |
| V-I graph is a straight line through the origin | Characteristic is curved or direction-dependent |
| Resistance remains constant over the tested range | Resistance changes with voltage, current, temperature or direction |
| Example: metallic resistor kept at constant temperature | Examples: filament lamp, diode and thermistor |
A filament lamp is non-ohmic over a wide range because its filament heats as current rises, increasing its resistance. A diode is non-ohmic because its current also depends strongly on voltage direction.
Conditions and limitations
- Temperature must remain constant.
- The material and dimensions of the conductor must remain unchanged.
- The physical state of the conductor must not change.
- The law does not describe every device; it is an empirical relation obeyed by ohmic materials over suitable ranges.
Microscopic link
An applied potential difference creates an electric field in the conductor. The field produces electron drift and hence current. Collisions with the lattice oppose this drift and appear macroscopically as resistance.
Worked example
A device connected across 12 V draws 200 mA. Find its resistance.
Exam-ready method
- State the proportionality and the constant-condition clause.
- Write V=IR and use proper SI units.
- For graphs, name both axes before finding the slope.
- In the experiment, place the ammeter in series and voltmeter in parallel.
- Explain how heating can cause departure from a straight line.
- Distinguish ohmic conductors from non-ohmic devices using their V-I characteristics.
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Exam preparation
Exam Focus
Important points
- Statement with conditions
- V=IR and rearrangements
- Definition and unit of resistance
- V-I graph slope
- Experimental verification
- Ohmic versus non-ohmic behavior
Common mistakes
- Omitting the constant-temperature condition.
- Reading slope as R without checking graph axes.
- Using milliamperes as though they were amperes.
- Connecting an ammeter in parallel or a voltmeter in series.
- Assuming every electrical component obeys Ohm's law.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Ohm's Law
Sticky Notes — Quick Revision
Key Point 1
- At constant temperature and fixed physical conditions, current is directly proportional to potential difference.
Key Point 2
- Ohm's law is V=IR.
Key Point 3
- For a V-versus-I graph, the slope equals resistance.
Key Point 4
- In verification, the ammeter is in series and the voltmeter is in parallel.
Ready to test yourself?
You've completed the learning section.
- 8 concept questions
- Easy → Medium → Hard
- Explanation after each answer
Exam Practice
Definition
Q1. State Ohm's law.
Experiment
Q2. Describe an experiment to verify Ohm's law.
Graph
Q3. How is resistance found from a V-I graph?
Reasoning
Q4. Why does a filament lamp not obey Ohm's law over a wide range?
Numerical
Q5. A device draws 200 mA from a 12 V source. Find its resistance.
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