(i) How many bulbs of resistance 8 Ω each should be connected in parallel combination to draw a current of 2 A from a battery of 4 V ?
(ii) Name the device used for measuring electric current. How is it connected in a circuit ?
(iii) State Joule's law of heating.
Generated by claude-sonnet-4-6 · 2026-06-15 06:41 · grounding rag
Model Answer
(i) Let $n$ bulbs, each of resistance 8 Ω, be connected in parallel.
Equivalent resistance: $R_p = \dfrac{8}{n}$ Ω
Using Ohm's law: $I = \dfrac{V}{R_p}$
$$2 = \frac{4}{8/n} = \frac{4n}{8} = \frac{n}{2}$$
$$n = 4$$
4 bulbs should be connected in parallel.
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(ii) An ammeter is used to measure electric current. It is always connected in series in the circuit, so that the same current flows through it.
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(iii) Joule's Law of Heating: The heat produced in a resistor is
$$H = I^2 R t$$
It states that the heat produced in a conductor is directly proportional to (a) the square of the current ($I^2$), (b) the resistance ($R$) of the conductor, and (c) the time ($t$) for which current flows.
Source: Chapter 11 – Electricity, Sections 11.6.2 and 11.7
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Explanation
- Part (i): Use $R_p = R/n$ for $n$ identical resistors in parallel, then apply $V = IR_p$. Always show the formula and substitution.
- Part (ii): Many students confuse ammeter (series) with voltmeter (parallel). Examiner expects both the name and the connection method.
- Part (iii): State all three proportionalities clearly — examiners look for $I^2$, $R$, and $t$ explicitly. Writing just the formula without words may lose a mark.