Basic Electrical Units


  • Define basic electrical concepts
  • Examine Ohm's Law
  • Review symbols and units


Centrifugal Pump
(eg: lamp)

Voltage is the difference in electrical potential between two points.

This difference in electrical potential is what causes electrons to flow from one point to the other.

In hydraulic terms, circuit voltage would be the pump that gives electricity the push it needs to move electrons through a circuit.

Voltage is measured in Volts (V).


DC Current
AC Current

Current is the flow of electrons in a circuit.

Current that flows in one direction is called direct current (DC) and current that periodically reverses direction and constantly changes in magnitude is called alternating current (AC).

We measure current in Amperes, which is often shortened to just amps. The SI unit for current is A, but the common symbol I is still heavily used around the world.

But Why "I"?

Current is measured in Amperes, after André-Marie Ampère's contributions to electrical theory. Ampère referred to current as l'intensité (intensity), which is why I is frequently used to stand in for current in common electrical equations, including Ohm's Law.


The opposition to current is called resistance, and is measured in Ohms.

Ohms are represented with the omega symbol ().

Any component in an electrical circuit that opposes the flow of current is called a resistor or load.


Ohm's Law

Voltage (V) = Current (I) x Resistance (R)

Ohm's Law is a formula which describes the relationship between Voltage, Current and Resistance.

The formula can be rearranged based on which variable we are trying to derive.

Current = Voltage / Resistance

Voltage = Current x Resistance

Resistance = Voltage / Current

Adjust the Volts and Ohms sliders to see what happens to the Amps reading.


We know that work is being done in and by a component when we have a voltage change or voltage drop across that component.

For example, as a current passes through a light bulb, electrical energy is converted into light and heat.

Work is not only a loss of electrical energy though!

A generator which is providing electrical energy to a circuit is also doing work.

By using an external energy supply (e.g. mechanical energy from a turbine) a positive voltage change is generated.


Power is the rate of transfer/conversion of electrical energy per a given unit of time and is measured in watts.

Put another way, power is the rate at which work is being done.

Electric power can be calculated using the formula P = V × I.


P = Power

V = Voltage

I = Current


VoltageVVolt (V)
CurrentIAmpere (A)
ResistanceROhm (Ω)
PowerPWatt (W)
Work/EnergyEJoules (J)
TimetSecond (s)


The opposition to electrical current is called:

Power is measured in:

We hope you enjoyed Basic Electrical Units

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