Voltage and Current

Voltage and Current are electrical quantities that describe the potential (voltage) and the flow (current).

Voltage is often shown as E or V, and the unit is volts. It is measured with a voltmeter. In Europe, "U"^[1] is used.

Current is shown as I, the unit is Amperes (or amps). It is measured with an ammeter.

Ohm's Law describes the relationship between voltage and current in a circuit. The resistance of a circuit is determined by dividing the voltage by the current (E/I = R). Resistance is measured in ohms, the symbol is the Greek symbol Omega (Ω), and is measured with an ohmmeter. Ohmmeters are never used on energized circuits!

Ohm's Law describes how things will happen in a circuit, or is used to determine the quantities you should measure. This is useful for troubleshooting.

Voltage and Current

Now The Details

The picture shows a series circuit and a parallel circuit. This is how voltage and current are related in such circuits:

Series Circuit

The electrical current flows from the power supply into bulb A, out of bulb A and into bulb B, out of bulb B and returns to the power supply.

The current flows single file through both light bulbs, and the light bulbs have the same resistance (we are assuming the bulbs are identical). Ohm's law tells us that I = V / R, or V = I · R, and since the current I and the resistance R are the same for both bulbs, the voltage V on each bulb is also the same.

Since the supply voltage is 12 V, that means there is a voltage of 6 V on each bulb.

Parallel Circuit

The electrical current flows from the power supply to the junction that leads to 2 identical light bulbs. The bulbs are identical, and they are both connected to the 12 V of the power supply. The missing piece of the puzzle is the current flowing through each bulb. Ohm's law tells us that I = V / R. Since the voltage V and the resistance R are the same for each bulb, the current I flowing through each bulb is also the same.

The picture labels the current flowing into the parallel circuit with Y, so the current flowing through each bulb is Y/2.

General Rules

Voltage

In a circuit with two devices in series, the total voltage on the circuit is divided up between the devices. If the devices have the same resistance, there is half of the total voltage on each device.

Current

In a circuit with two devices in parallel, the total current flowing through the circuit is divided up between the devices. If the devices have the same resistance, there is half of the total current flowing through each device.

Voltage and Current

Contents

Voltage and Current

Now The Details

General Rules

See Also