Use Ohm’s law for approximate value: given a source of 18 V and a current of 220 mA, estimate the resistance of the rheostat (round to the nearest practical value).

Difficulty: Easy

Correct Answer: 82 Ω

Explanation:


Introduction / Context:
Ohm’s law (V = I * R) is the cornerstone of basic circuit calculations. When voltage and current are known, resistance follows directly. This is common when setting rheostats or selecting series resistors to achieve a target current.


Given Data / Assumptions:

  • Voltage V = 18 V.
  • Current I = 220 mA = 0.22 A.
  • We assume a simple DC circuit where the rheostat dominates the resistance.


Concept / Approach:

Solve the rearranged Ohm’s law: R = V / I. Convert milliampere to ampere to avoid unit errors. Round to a standard-ohmic value if needed for options.


Step-by-Step Solution:

Convert current: 220 mA = 0.22 A.Apply Ohm’s law: R = V / I = 18 / 0.22.Compute: 18 / 0.22 ≈ 81.818… Ω.Nearest listed practical value: ≈ 82 Ω.


Verification / Alternative check:

Back-calculate current: I = V / R = 18 / 82 ≈ 0.2195 A = 219.5 mA, which is consistent with the given 220 mA.


Why Other Options Are Wrong:

8.2 kΩ and 820 Ω would yield currents far smaller than 220 mA. 8.2 Ω would give I ≈ 2.2 A, much larger than specified.


Common Pitfalls:

Forgetting to convert mA to A; misplacing decimals; using R = I / V instead of R = V / I.


Final Answer:

82 Ω

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