$A$ cell of constant $e.m.f.$ is connected first to an external resistance $R_1$ and then to an external resistance $R_2$. If the power consumed in both cases is the same,what is the internal resistance of the cell?

An external resistance $R$ is connected to a battery of $e.m.f.$ $V$ and internal resistance $r$. The joule heat produced in resistor $R$ is maximum when $R$ is equal to

The actual value of resistance $R$,shown in the figure is $30\,\Omega$. This is measured in an experiment as shown using the standard formula $R_{measured} = \frac{V}{I}$,where $V$ and $I$ are the readings of the voltmeter and ammeter,respectively. If the measured value of $R$ is $5\%$ less than the actual value,then the internal resistance of the voltmeter is ................. $\Omega$.

$A$ cell whose e.m.f. is $2\, V$ and internal resistance is $0.1\,\Omega$ is connected with a resistance of $3.9\,\Omega$. The voltage across the cell terminal will be ................ $V$.

$P$ and $Q$ are two identical wires of resistance $2000\, \Omega$ each, and $M$ is the midpoint of $PQ$. A voltmeter of resistance $1000\, \Omega$ is connected between $P$ and $M$. When a potential difference of $150\, V$ is applied across $PQ$, the reading of the voltmeter will be ............. $V$.

The unit of $e.m.f.$ is