In the space below, sketch a circuit diagram that she could use to determine this resistance. Show a battery, a variable resistor, a voltmeter and an ammeter in the circuit.
At switch-on, the potential difference (p.d.) across the resistor is $4.2\,\text{V}$ and the current through the resistor is $1.2\,\text{A}$. Calculate the resistance of the resistor.
Soon after the circuit is switched on, the resistor becomes very hot and produces smoke. Explain why it overheats. Include a calculation in your answer.
The other two resistors are also marked $3.9\,\Omega$, $3\,\text{W}$. The first resistor is now taken out, and the student connects these other two resistors in series to the p.d. of $4.2\,\text{V}$. State and explain whether these resistors overheat.