Ohm's law
V = I x R. Potential difference (V) equals current (A) times resistance (ohms). For a fixed resistor at steady temperature, current is proportional to voltage.
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Analogy
Pushing harder (voltage) on the same pipe gives a bigger flow (current).
Example
V = 12 V, R = 4 ohmsI = V / RI = 12 / 4I = 3 A
In practice
Electricians use it to choose the right resistor, or to predict the current in a circuit.
Practice
Flashcards
- State Ohm's law.V = I x R.
Interview questions
- A 230 V kettle draws a current of 8 A. Calculate its power and its resistance.
- Power = V x I = 230 x 8 = 1,840 W
- Resistance = V / I
- R = 230 / 8 = 28.75 ohms
Questions people ask
How do I use V = IR?
Cover the quantity you want in the V-I-R triangle. For current, divide voltage by resistance. For resistance, divide voltage by current. For voltage, multiply. A 12 V supply across a 6 ohm resistor gives a current of 12 / 6 = 2 A. Always write the answer with the correct unit.
What does a straight line through the origin on a V-I graph mean?
It shows the component obeys Ohm's law: the current is directly proportional to the voltage, so resistance is constant. A fixed resistor at steady temperature gives such a line. A bulb filament gives a curve, because its resistance rises as it gets hotter.