Electronics Basics: Use Ohm's Law Carefully

Calculate with V = I R and state the model's limits.

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For an ideal ohmic resistor, V = I R. Keep resistance and temperature conditions fixed when predicting proportional changes. Use volts, amps and ohms; 1000 mA = 1 A and 1000 ohms = 1 kilohm. Not every component has constant resistance. Use diagrams and calculations only. Never use wall outlets, short batteries, open powered equipment or handle charged capacitors.

Worked example

With 5 V across 1000 ohms, I = 5/1000 = 0.005 A = 5 mA. Doubling resistance at the same voltage halves current. This is a calculation, not a wiring activity.

Five volts across 1000 ohms gives five milliamps. With the same five volts across 2000 ohms, current is two and a half milliamps. These are ideal ohmic-resistor calculations.

Would you believe it?

Conventional current uses the direction positive charge would move. In metal wires, negatively charged electrons drift in the opposite direction.

OpenStax: electrical current

Question 1 What current flows through 1000 ohms at 5 V? 5000 mA 0.5 mA 5 mA

Question 2 At the same 5 V, what current flows through 2000 ohms? 5 mA 2.5 mA 10 mA

Question 3 What voltage is across 2000 ohms at 0.003 A? 6 V 0.0000015 V 6000 V

Question 4 What resistance gives 0.01 A when 4 V is applied? 0.04 ohms 40 ohms 400 ohms

Question 5 Can one constant resistance model describe every LED over all voltages? Only if current is written in mA No Yes

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