Course › Electricity
Ohm's law and power How voltage, current and resistance are tied together, and how to work out power.
What you will be able to do Find voltage, current or resistance when you know the other two Work out power from voltage and current Name the unit of power If you know two of voltage , current and resistance , how do you find the third?
Pick any two, and Ohm's law hands you the third. That one rule is behind most of the
arithmetic on the Foundation exam, so it is worth learning properly.
The rule
Voltage (E E E volts ) equals current (I I I amps ) times resistance (R R R ohms ):
E = I × R E = I \times R E = I × R
If you know any two of the three, you can calculate the third. Cover the thing you want in the
triangle below, and what is left tells you what to do.
Ohm's law triangle: E on top, I and R below E I R Ohm's law triangle: E on top, I and R below Three worked examples
Finding the voltage
A current of 2 A 2\ \text{A} 2 A 50 Ω 50\ \Omega 50 Ω resistor . What voltage is applied?
Known: I = 2 A I = 2\ \text{A} I = 2 A R = 50 Ω R = 50\ \Omega R = 50 Ω E E E
Cover E E E I I I R R R E = I × R E = I \times R E = I × R
E = 2 × 50 = 100 V E = 2 \times 50 = 100\ \text{V} E = 2 × 50 = 100 V Check: 100 / 50 = 2 A 100 / 50 = 2\ \text{A} 100/50 = 2 A current we started with.
Finding the current
A 200 V 200\ \text{V} 200 V 100 Ω 100\ \Omega 100 Ω resistor . How much current flows?
Known: E = 200 V E = 200\ \text{V} E = 200 V R = 100 Ω R = 100\ \Omega R = 100 Ω I I I
Cover I I I E E E R R R I = E / R I = E / R I = E / R
I = 200 / 100 = 2 A I = 200 / 100 = 2\ \text{A} I = 200/100 = 2 A Check: 2 × 100 = 200 V 2 \times 100 = 200\ \text{V} 2 × 100 = 200 V
Milliamps
The rule only works in volts , amps and ohms 500 mA = 0.5 A 500\ \text{mA} = 0.5\ \text{A} 500 mA = 0.5 A
Power
Power is the rate of energy consumption : how fast energy is being used. Its unit is the
watt Watt .
Beyond the RSK text
The tutorial names power and its unit but gives no formula. The one you will meet everywhere is
P = E × I P = E \times I P = E × I watts equal volts times amps .
You may be expected to remember these formulas; the tutorial does not say whether the exam
gives them.
You now know
E = I × R E = I \times R E = I × R I = E / R I = E / R I = E / R R = E / I R = E / I R = E / I Always work in volts , amps and ohms . Convert milliamps first.
Power is the rate of energy consumption, measured in watts , and P = E × I P = E \times I P = E × I
Source: RSK Foundation tutorial, KARE 04 §5; KARE 04 §6
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