Appliance Ratings and Bulb Circuits
For a resistive appliance at rating, R_rated = V_rated squared/P_rated and I_rated = P_rated/V_rated. Actual power follows the actual circuit voltage or current; incandescent cold resistance can differ greatly.
Why this shows up in the exam
Bulbs in series and parallel · Choosing a series dropping resistor · Comparing illumination qualitatively
Learn the idea
A rated voltage and power specify an appliance's hot operating resistance, not its power at every connection. A 100 W, 220 V bulb draws 100 W only at 220 V. In series or at another voltage, first infer its operating resistance under the usual constant-resistance exam model.
🧠 Memory hook: Ratings define the designed operating point.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- R_rated = V_rated²/P_rated — effective resistance at rated operation
- I_rated = P_rated/V_rated — current drawn at the rated operating point
- P_actual = V_actual²/R_model — actual power under the stated constant-R model
How to approach it
- 1Convert every rating to R or I
- 2Solve the actual connection
- 3Compute actual power and compare
Common slip-ups that cost marks
- •Treating wattage as resistance
- •Keeping rated power after changing voltage
- •Ignoring hot-versus-cold filament resistance
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A cell of emf 6 V and internal resistance 1 ohm is connected to a 2 ohm resistor. Find the circuit current.
More from Current Electricity
Resistivity, Conductivity, and Material Properties
Learn how resistivity and conductivity depend on material, length, area, and how resistance changes with dimensions and temperature.
Series and Parallel Combinations of Resistors
Master the calculation of equivalent resistance, voltage division, and current distribution in series and parallel resistor networks.
Electrical Energy, Power, and Heating Effect
Calculate electrical energy, power dissipation, and understand the heating effect of current in various circuit configurations.
EMF, Internal Resistance, and Terminal Voltage
Understand the concepts of EMF, internal resistance of cells, and how terminal voltage is affected in real circuits.
Drift Velocity and Microscopic View of Current
Explore the microscopic origin of current, including drift velocity, relaxation time, current density, and the effect of electric field in conductors.
Kirchhoff's Laws and Circuit Analysis
Apply Kirchhoff's current and voltage laws to analyze complex circuits and solve for unknowns.