A filament lamp is connected to a 12 V d.c. supply. When the lamp is operating normally, a current of 0.40 A flows through it. What is the resistance of the lamp at this current?

Answer: D. Step 1: Recall the relationship between resistance, p.d. and current Resistance is defined by: R = V/I Step 2: Substitute the values The potential difference across the lamp is V = 12 V and the current through it is I = 0.40 A: R = 12 V/0.…

Electric Circuits and Resistance: Question 6

Syllabus 4.2.4

Multiple choice Core 1 mark

A filament lamp is connected to a $12\text{ V}$ d.c. supply. When the lamp is operating normally, a current of $0.40\text{ A}$ flows through it. What is the resistance of the lamp at this current?

Show worked solution Hide worked solution

Step 1: Recall the relationship between resistance, p.d. and current

Resistance is defined by:

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

Step 2: Substitute the values

The potential difference across the lamp is $V = 12\text{ V}$ and the current through it is $I = 0.40\text{ A}$ :

$R = \frac{12\text{ V}}{0.40\text{ A}}$

Step 3: Work out the resistance

$R = 30\,\Omega$

Why the other options are wrong

Option	How it arises	Error
$0.033\,\Omega$	$0.40 \div 12$	Formula inverted (current ÷ p.d.)
$4.8\,\Omega$	$12 \times 0.40$	Multiplied instead of dividing
$11.6\,\Omega$	$12 - 0.40$	Subtracted instead of dividing

Final answers

Resistance $= \boxed{30\,\Omega}$ , option D