Electromotive Force, Internal Resistance and the Potential Divider
A real source has resistance of its own, so the voltage across its terminals drops as it delivers more current and the missing part heats the source. Two resistors across a supply share that voltage in the ratio of their resistances, which is how a fixed supply is turned into whatever voltage a circuit needs.
What a learner can do afterwards
- Explains why terminal potential difference falls as current rises and gets the internal resistance from a graph of the two
- Calculates the output of a potential divider and predicts how it moves when one resistance changes
- Says where the energy lost inside a source goes and why a battery warms when it is heavily loaded
1 · Read
Electromotive force is not a force at all. It is energy per charge supplied by the source, measured in volts. A real source is an ideal emf plus one small series resistor, the internal resistance. With no current the terminals show the full emf, but as current rises part of the push is spent inside, so terminal voltage falls: V equals emf minus I r. The missing energy becomes heat, which is why hard-worked batteries warm up.
Plot terminal voltage against current and you get a straight line. Its intercept is the emf and the size of its slope is the internal resistance, which is the standard lab route to both numbers. An open circuit shows the full emf, while a short circuit draws huge current with huge internal drop and dangerous heating.
Two resistors in series split the supply in proportion to their values. With the output across R2, Vout equals Vin times R2 over (R1 plus R2). If the output resistor R2 grows, the output grows, and if its partner R1 grows, the output shrinks. Loading the output with a meter disturbs the split, so keep loads light.
Predict movement before calculating. Ask which resistance changed, and whether the output sits across it or across its partner.
Internal resistance drags terminals down with current, and dividers share voltage by resistance.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.