Resistance

Five quantities do almost all the work in Topic 4. Charge \((Q)\) is the property that is carried. Current \((I)\) is the flow of that charge. E.m.f. \((E)\) is the electrical work a source does on each unit of charge sent round a complete circuit. Potential difference \((V)\) is the work each unit of charge does passing through a component. Resistance \((R)\) is the p.d. across a component per unit current in it. Power and energy then follow. Magnetism adds fields and forces, and a changing magnetic field adds induced e.m.f.

Charge is the property carried, and it is positive or negative. Current is the flow of that charge, measured in amperes with an ammeter in series. E.m.f. is the electrical work a source does moving unit charge round a complete circuit; potential difference is the work unit charge does passing through a component; both are measured in volts, read with a voltmeter in parallel. Resistance is the p.d. across a component per unit current in it, \(R = V/I\), measured in ohms. Power is the rate of energy transfer, \(P = IV\), and the energy transferred is \(E = IVt\).