NUST NET Physics · Physics · Electricity. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Current is the rate of flow of charge (I = Q/t). The resistance of a wire depends on its material, length and thickness: a longer or thinner wire resists more. For metals resistance rises with temperature; for semiconductors it falls, because heating frees more charge carriers.
A real cell has internal resistance. Its EMF is the energy given to each coulomb, but some is lost inside the cell, so the terminal potential difference is less than the EMF whenever current flows.
Harder networks are solved with Kirchhoff's rules, which are conservation of charge and of energy. The Wheatstone bridge and the potentiometer are null methods: at balance no current flows through the galvanometer.
Leaving out the internal resistance. The current is E/(R + r), not E/R, and the p.d. across the terminals is E − Ir, which is less than the EMF.
Answer: 6.0 V
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