Basic Electronic Circuit Relations

TOPIC 2. Ohm's Law

The electrical relationship between voltage, current and resistance is called Ohm's law. Ohm's law is a simple mathematical formula that says the voltage in a circuit can be computed by multiplying the current flowing in the circuit times the resistance of the circuit. The resistance of a circuit is measured in ohms and is represented by the letter R. Ohm's Law is written as

V = I x R where
V = voltage (volts)
I = current (amps)
R = resistance (ohms)

The electrical impedance of a circuit is made up of both the electrical resistance and the electrical reactance of the elements in the circuit. Both the resistance and reactance impede the current flow through a circuit.

Resistance loads include incandescent light bulbs and electric heating elements. Reactive loads include electric motors and other devices where the magnetic field created around a coil of wire or in a capacitor is utilized as electrical energy.
Electricity is measured in units of power(video) called watts. It was named to honor James Watt, the inventor of the steam engine.

Electric Power (watts) = Current (amps) x Voltage (volts)

P (watts) = I (amps) x V (volts)

Using Ohm's law, we can also write

P (watts) = I (amps) x (I x R) (volts)

= I2 R (watts)

= E2 / R (watts)


One watt is a very small amount of power. It would require nearly 750 watts to equal one horsepower. A kilowatt represents 1,000 watts. A kilowatt-hour (kWh) is equal to the energy of 1,000 watts working for one hour. The amount of electricity a customer uses over a period of time is measured inwatthours(wh) or kilowatthours (kWh). kilowatthours are determined by multiplying the number of kW's required by the number of hours of use.

Electric Energy ( kwhrs) = Electric Power (watts) x Time (hrs)

For example, if you use a 40-watt light bulb 5 hours a day, you have used 200 watts of power, or .2 kilowatthours of electrical energy. To convert units, please click here.

Ohm's Law and Other Useful Formula
Resistive Loads

Go to TOPIC 3. Series and Parallel Circuits

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