Define what is meant by the ohm.
A wire of uniform thickness has a resistance of $3.2\,\Omega$. It is $2.5\,\text{m}$ long and made from a metal with resistivity $460\,\text{n}\Omega\,\text{m}$. Calculate the wire’s cross-sectional area.
In Fig. 7.1, a cell that has electromotive force (e.m.f.) $E$ and internal resistance $r$ is joined to a variable resistor of resistance $R$. The current in the circuit is $I$.
State, in energy terms, why the potential difference across the variable resistor is smaller than the cell’s e.m.f.
State an expression linking $E$ with $I$, $R$ and $r$.
The resistance $R$ of the variable resistor is adjusted so that it becomes equal to $r$. Determine an expression, in terms of only $E$ and $r$, for the power $P$ dissipated in the variable resistor.