(a) How will you infer with the help of an experiment that the same current flows through every part of a circuit containing three resistors in series connected to a battery?
(b) Consider the given circuit and find the current flowing in the circuit and potential difference across the 15 Ω resistor when the circuit is closed.
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 Resistance of the resistor, $R_2=10\Omega$

 Resistance of the resistor, $R_3=15\Omega$

Potential difference, $V=30V$

To find: Current flowing in the circuit, $I$.

Solution:

In the given figure, all the resistors are connected in series. 

We know that in series combination total resistance is given as-

$R_S=R_1+R_2+R_3$

Substituting the given values we get-

$R_S=5+10+15$

$R_S=30\Omega$

Thus, the total resistance of the circuit is 30 Ohm.

From Ohm's law, we know that-

$I=\frac {V}{R}$

Putting the value of $V$ and $R$, we get-

$I=\frac {30}{30}$

$I=1A$

Thus, the current flowing in the circuit is 1 Ampere.

Now, to find the potential difference across $15\Omega$ resister we use Ohm's law-

$V=I\times R$

Putting the value of $I=1A$ and $R=15\Omega$, we get-

$V=1\times 15$

$V=15V$

Thus, the potential difference across $15\Omega$ resister is 15 Volt.