Make sense of an electric circuit
A power source establishes a potential difference across a resistor. Charge flows in a closed conducting path, and electrical energy is transferred to the resistor. For an ohmic component at constant physical conditions, current is proportional to voltage: double the applied voltage and current doubles, provided resistance does not change.
Learning goals
- Use V = IR to connect potential difference, current and resistance with correct units.
- Explain why an I–V graph is a straight line for an ideal ohmic resistor.
- Calculate dissipated electrical power and energy.
- Recognise why heated lamp filaments, diodes and many real devices may not obey a constant-resistance linear model.
Voltage, current and resistance
Voltage V is energy transferred per unit charge (1 volt = 1 joule/coulomb). Current I is charge flow rate (1 ampere = 1 coulomb/second). Resistance R is measured in ohms (Ω); for an ohmic resistor, the ratio V/I stays constant when temperature and other conditions are held fixed. The direction of conventional current is from the positive terminal through the external circuit toward the negative terminal.
When plotting V on the vertical axis and I on the horizontal axis, the slope is R. If I is plotted vertically and V horizontally, the slope is 1/R. Always name the axes before interpreting a gradient.
Electrical power and heating
The energy transferred per second is electrical power. If a resistor is the only load and ideal wires and a DC source are assumed, P = VI. Substituting Ohm’s law gives two other equivalent expressions. Energy transferred over a time interval Δt is E = PΔt for constant power.
A higher power rating does not imply that a component always consumes that much power; actual power depends on the applied voltage and operating conditions. A real resistor must be chosen with a safe power rating, but this is a software model and should not be used as wiring or mains-electrical safety advice.
Worked example: 6 V across 12 Ω
Choose V = 6 V and R = 12 Ω. Current I = 6/12 = 0.50 A, and power P = VI = 3.0 W. Keep R = 12 Ω and double V to 12 V: current rises to 1.00 A, power rises to 12 W. The current doubles, but power quadruples because P = V²/R. Restore V = 6 V and halve resistance to 6 Ω: current becomes 1.00 A and power becomes 6 W.
Guided virtual investigation
- Open the Ohm’s Law Circuit and select V = 6 V and R = 12 Ω.
- Predict current before reading the virtual meter. Check 0.50 A.
- Change only voltage, keeping R fixed; record pairs (V, I) and sketch the I–V graph.
- Restore the original voltage, then halve R. Compare current and power.
- Try two different voltage values at fixed resistance and calculate V/I in each case.
- Explain why the model uses a constant resistor temperature even though an actual resistor may warm when power increases.
Why Ohm’s law is not universal
For a conductor that heats substantially, resistance may vary with temperature. A filament lamp often shows a curved current–voltage relationship because its filament becomes hotter as current increases. A semiconductor diode is nonlinear and directional. Thus the statement “V = IR” is useful as a definition of resistance at a particular operating point, but the stronger condition “R stays constant as V changes” holds only for an ohmic device under stated conditions.
Common misconceptions
- “More voltage always means more current in the same proportion.” Not if resistance changes with voltage, temperature or device state.
- “A resistor consumes current.” In a simple series circuit, the same steady current enters and leaves the resistor; energy is dissipated, not charge.
- “Power equals resistance times voltage.” The correct formulas are P = VI, I²R or V²/R, under the corresponding definitions.
- “A battery forces its rated voltage across every device in every circuit.” Real sources have internal resistance and operating limits omitted here.
Exam practice with answers
- A 6 Ω resistor across 12 V: I = 12/6 = 2 A.
- A resistor carrying 2 A with 5 Ω resistance: V = IR = 10 V.
- If fixed V is doubled across a fixed R, I doubles and P quadruples.
- A 4 W resistor dissipates energy for 30 s at constant power: E = 120 J.
- From a V-versus-I graph passing through (I = 0.5 A, V = 5 V), slope R = V/I = 10 Ω.
Classroom and NCERT connection
For Class 10 electricity, focus on understanding the experimental condition “constant temperature,” using a sensible circuit diagram, naming graph axes and showing units in every calculation. The virtual lab is excellent for testing mathematical ratios but should be paired with discussion of measured deviations in physical resistors.
Extend the learning
After verifying the basic single-resistor model, progress to series and parallel networks, equivalent resistance and power distribution. Do not assume this single-resistor lab models the full safety, heating or AC behaviour of household appliances.
Frequently asked questions
Does V = IR mean resistance is always constant?
No. Constant resistance is the additional condition for an ohmic device under fixed physical conditions.
If voltage doubles at fixed resistance, what happens to power?
Power becomes four times larger because P = V²/R.
Is current used up in a resistor?
No. Charge is conserved; the resistor converts electrical energy into heat or other forms.
How do you get resistance from a V–I plot?
When V is vertical and I horizontal, the slope ΔV/ΔI is resistance.
Further reading and references
References are for further independent learning; explanations and examples on this page are written for NANDAX.