A 12V battery supplies a current of 0.4A to a lamp. Calculate the resistance of the lamp.

Analyze the question: We are given the voltage (V = 12 V) and the current (I = 0.4 A). We need to find the resistance (R) of the lamp.

Concept: Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it (at constant temperature):

V = IR, so R = V / I

Solution:

R = V / I R = 12 / 0.4 R = 120 / 4 R = 30 Ω

Check the options: 30.00 Ω matches option A. The other options come from common mistakes:

• 4.80 comes from multiplying (12 × 0.4) instead of dividing.
• 0.03 and 0.30 come from misplacing the decimal point.

Tip: Remember the triangle V over I × R. Cover the quantity you want:

• Want R? V is on top, so R = V ÷ I.
• Want V? V = I × R.
• Want I? I = V ÷ R.

A quick sense check: when dividing by a number less than 1 (like 0.4), the answer must be bigger than the number you started with. So R must be greater than 12, which immediately eliminates B, C, D and E.