EMF, Terminal Voltage, and Internal Resistance
For discharge current leaving the positive terminal, V_terminal = E - Ir. For charging current entering the positive terminal, V_terminal = E + Ir, with signs always checked from the chosen traversal.
Why this shows up in the exam
Battery terminal readings · Charging versus discharging tests · Including source resistance in circuits
Learn the idea
A real cell's terminal voltage differs from its emf because current produces an internal voltage drop. EMF is energy supplied per unit charge inside the source. When the cell delivers current, some energy is dissipated internally; during charging, the external source must overcome the emf plus the internal drop.
🧠 Memory hook: Delivering subtracts Ir; charging adds Ir.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- V_terminal = E - I r — terminal voltage of a cell delivering current
- V_terminal = E + I r — terminal voltage of a cell being charged
- I = E/(R+r) — single discharging cell and external resistor R
How to approach it
- 1Mark current through the cell
- 2Decide delivering or charging
- 3Apply a consistent loop sign convention
Common slip-ups that cost marks
- •Calling terminal voltage always equal to emf
- •Using the discharge sign while charging
- •Leaving internal resistance outside the loop
🌟 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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