Electric Power in DC Circuits
Using the passive sign convention, positive VI means an element absorbs power and negative VI means it supplies power. For a resistor, absorbed power is nonnegative.
Why this shows up in the exam
Power in resistor networks · Battery output and internal loss · Comparing circuit configurations
Learn the idea
Electrical power is the instantaneous rate of energy transfer, P = VI. Power tells how fast a circuit element receives or supplies energy. For an ohmic resistor, Ohm law gives two alternative forms, but P = VI is the safest starting point.
🧠 Memory hook: Start with VI; substitute Ohm law only when the element is resistive.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- P = V I — general two-terminal electrical power with signed convention
- P = I² R — ohmic resistor when current is known
- P = V²/R — ohmic resistor when voltage is known
How to approach it
- 1Mark the voltage across the target element
- 2Find its own current
- 3Choose VI, I squared R, or V squared/R
Common slip-ups that cost marks
- •Applying V squared over R to a non-ohmic source
- •Using total current for every branch
- •Ignoring source-versus-absorber sign
🌟 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.
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