State Kirchhoff’s second law.
An electric heater that contains two heating wires X and Y is attached to a power supply with electromotive force (e.m.f.) $9.0\,\text{V}$ and negligible internal resistance, as shown in Fig. 6.1. Wire X has a resistance of $2.4\,\Omega$ and wire Y has a resistance of $1.2\,\Omega$. A voltmeter is placed in parallel with the wires. A variable resistor is used to control the power dissipated in wires X and Y. The variable resistor is adjusted so that the voltmeter reads $6.0\,\text{V$
Calculate the resistance of the variable resistor.
Calculate the power dissipated in wire X.
The cross-sectional area of wire X is three times that of wire Y. Assume that both wires are made of the same metal, so their resistivity and number density of free electrons are the same. Determine the ratio $$\frac{\text{length of wire X}}{\text{length of wire Y}}.$$
Determine the ratio $$\frac{\text{average drift velocity of free electrons in wire X}}{\text{average drift velocity of free electrons in wire Y}}.$$