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Circuit Analysis: Ohm's law and Kirchhoff's laws · Ohm's Law and Electrical Power

How voltage, current and resistance relate, what sets a conductor's resistance, and how electrical power turns into heat and energy use.

Circuit Analysis · 28 questions

Try 3 questions

1. A 12 V source drives current through a 4 Ω resistor. What current flows?

  1. 0.33 A
  2. 3 A
  3. 48 A
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Answer: 3 A

Ohm's law gives I = V / R = 12 V / 4 Ω = 3 A; multiplying or inverting the ratio are the usual slips.

2. If the voltage across a fixed resistor doubles, what happens to the current through it?

  1. It halves
  2. It doubles
  3. It stays the same
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Answer: It doubles

With resistance constant, current is directly proportional to voltage, so twice the voltage pushes twice the current.

3. Why doesn't an incandescent bulb's filament follow Ohm's law from cold to full brightness?

  1. Current can pass through it only one way
  2. Its resistance rises as it heats up
  3. Its resistance falls as it heats up
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Answer: Its resistance rises as it heats up

A tungsten filament's resistance grows roughly tenfold from room temperature to operating heat, so current isn't proportional to voltage across that range.

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