Push more current through the internal resistance and watch the terminal voltage sag below the EMF — the volts you actually get out are never quite the volts the cell promises.
Every circuit is a story of charge being pushed by a cell and slowed down by resistance. The same handful of equations describes a single resistor, a whole network, and the cell driving it.
Current is the rate of flow of charge past a point. A bigger current means more charge passing per second.
Ohm's law relates the potential difference across a component to the current through it, for a component held at constant temperature.
A wire's resistance depends on what it's made of and its dimensions — long, thin wires resist more than short, fat ones.
Power is the rate at which electrical energy is transferred. Because V = IR, the same power can be written three equivalent ways.
Combining resistors changes the total resistance differently depending on how they're arranged. In series, the same current flows through both, so their resistances simply add. In parallel, both share the same voltage but split the current, so the combined resistance is always less than either one alone.
Real cells aren't perfect — they have their own internal resistance, which eats into the voltage they can deliver. The EMF is the total energy the cell gives per unit charge; some of that is always "lost" driving charge through the cell itself.
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