E.M.F & Modelling Resistance

Revise E.M.F & Modelling Resistance for Physics 2 (IAL) WPH12 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Physics · Unit 4

E.M.F & Modelling Resistance

Big idea: Every real battery wastes a bit of its own energy pushing charge through itself — that "waste" is internal resistance, and it's why the voltage you actually get is always a little less than the voltage the battery promises.

Summary — What This Chapter Covers

E.M.F. (ε) is the total energy a battery gives to each coulomb of charge — measured with a voltmeter when no current flows.
Internal resistance (r) is resistance inside the battery itself, which "eats" some of that energy as heat.
Lost volts (Ir) is the energy wasted inside the battery; terminal p.d. (V) is what's left over for the circuit.
The master equation ties it all together: ε = I(R + r) = V + Ir
Core Practical 8 uses a V–I graph to find ε (y-intercept) and r (negative gradient) experimentally.
Resistance rises with temperature in metals (more ion vibration = more electron collisions).
Resistance falls with temperature in thermistors (more free charge carriers released).
Resistance falls with light intensity in LDRs (more light = more freed electrons = easier flow).

1. Electromotive Force (E.M.F.)

Picture a battery as a tiny pump. It doesn't create charge — charge is already sitting in the wires — but it gives that charge a "push" of energy, like a pump lifting water uphill so it has potential energy to flow back down through your circuit and do useful work (light a bulb, turn a motor, whatever).

E.M.F. (electromotive force) is a measure of exactly how much energy the battery gives to each coulomb of charge that passes through it. It's a slightly misleading name — it isn't actually a force at all (no newtons involved), it's an energy-per-charge quantity, just like potential difference. The "force" in the name is a historical leftover from when scientists didn't fully understand what was going on.

Definition

ε = E / Q

ε = e.m.f. (volts, V) | E = energy transformed from chemical → electrical (joules, J) | Q = charge (coulombs, C)

The trick to measuring it: e.m.f. is defined as the potential difference across the battery's terminals when no current is flowing — i.e. an "open circuit" with just a voltmeter connected. Why no current? Because the moment current flows, some energy gets lost inside the battery itself (more on that in Section 2), and what you'd measure would be slightly less than the true e.m.f. With zero current, there's no internal energy loss to worry about, so the voltmeter reads the full, honest e.m.f.

┌──────( V )──────┐ │ │ │ ┤├────────┘ └────────┘ High-resistance voltmeter connected straight across the cell terminals, with NO other circuit connected. Reading = e.m.f. (ε)

Analogy

Think of e.m.f. as the total "budget" of energy the battery hands out per coulomb. Some of that budget gets spent immediately just running the battery's own internal machinery (internal resistance) — the rest is what's actually available to power your circuit.

Practice Question

A cell has an e.m.f. of 6.0 V. Explain, in terms of energy, what this value actually means.