The potential difference between the terminals of an electrical heating device is 40 V, when it draws a current of 2 A from the source. Which one of the following is the correct value of current drawn by the heating device if the potential is increased to 140 V?
- A.7 A
- B.5 A
- C.4 A
- D.2 A
Correct answer
Explanation
The correct answer is A, 7 A. By Ohm's law the heater's resistance is 40 V divided by 2 A, which is 20 ohms, so at 140 V it draws 140 divided by 20, or 7 A.
Ohm's law states that, at constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = IR, where R is the resistance. Since the resistance of the heating element stays the same, the current rises in the same ratio as the voltage. The voltage goes up 3.5 times, from 40 V to 140 V, so the current also goes up 3.5 times, from 2 A to 7 A. The power it uses would rise from 80 W to 980 W.
B is wrong because 5 A would mean a resistance of 28 ohms, not 20. C is wrong because 4 A would fit only if the voltage had doubled to 80 V. D is wrong because the current cannot stay at 2 A when the voltage rises across the same resistance.
Exam tip: V = IR; with fixed resistance, current changes in the same ratio as voltage, and power P = VI.
Practice Questions
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