Answer to Question #237121 in Electric Circuits for Taiwo

Question

A cell of emf 3v an internal resistance 0.5ohms passes current through an external resistor of 7o.calculate the potential drop across the cell

Answer 1

Explanations & Calculations

• The potential drop across the cell is exactly the same as that drops across the external resistance if the connecting wires & other stuff do not contain resistance in them.
• Therefore, just by applying $\small v=ir$ to that external one, the needed quantity can be evaluated.
• Before that, the current flowing through the circuit should be calculated & which can be done by applying Kirchhoff's rule to the circuit. Therefore,

$\qquad\qquad \begin{aligned} \small i &=\small \frac{E}{R+r}=\frac{3V}{0.5\Omega+7\Omega}=0.4\,A \end{aligned}$

• Then simply,

$\qquad\qquad \begin{aligned} \small V&=\small iR\\ \small \bold{V _{drop}}&=\small \frac{ER}{(R+r)}=0.4\,A\times7\Omega=\bold{2.8V} \end{aligned}$

• This is also obtained by the relationship $\small V_{drop} = E\pm ir_{internal}$, + during the cell being charged & - during the normal service.