Ohm's Law

• Ohm's law: V = IR at constant temperature.
• V–I graph for ohmic conductor — straight line through origin.
• Resistance — opposition to current; unit ohm (Ω).
• Factors affecting resistance (intro to next topic).
• Electric current I = Q/t; potential difference and voltmeter/ammeter use.

1. Ohm's law — V = IR; V in volts, I in amperes, R in ohms.
2. Ohmic conductor — R constant; V–I graph linear through origin (e.g. metal wire at fixed T).
3. Non-ohmic — filament lamp, diode — R changes with current/temperature.
4. Current — rate of flow of charge: I = Q/t; unit ampere (A).
5. Potential difference — work per unit charge; voltmeter across component.
6. Ammeter — in series; very low resistance.
7. Voltmeter — in parallel; very high resistance.
8. Rearrangements — I = V/R; R = V/I.
9. Limitation — valid only at constant temperature for many conductors.
10. NCERT experiment — measure V and I; plot graph; slope = R.

Find current through 10 Ω resistor connected to 5 V battery

Given: V = 5 V, R = 10 Ω
Step 1 — Ohm's law: I = V/R
Step 2 — I = 5/10 = 0.5 A
Step 3 — Check units: V/Ω = A ✓
Step 4 — If R doubles to 20 Ω, I halves to 0.25 A (V constant).
Conclusion: current 0.5 A through the resistor.

• Saying V ∝ I without constant temperature condition.
• Connecting ammeter in parallel (damages meter).
• Misconception: thick wire always has higher R (depends on length, area, material).
• Confusing Q = It with Ohm's law.
• Using mA values without converting to A in V = IR.

• State Ohm's law with units.
• Why is ammeter connected in series?
• R = 20 Ω, I = 0.3 A. Find V.
• What is shape of V–I graph for ohmic resistor?

Open the Practice tab for graded questions on Ohm's Law.