Rated voltage and terminal voltage of a generator

The rated voltage of a load

Every electrical load (light bulb, motor, etc.) is designed to operate correctly at a specific voltage, known as the rated voltage. This is usually indicated on the appliance or its packaging.

For example, a lamp marked ‘6 V’ operates normally when the voltage across its terminals is approximately 6 V.

What happens if the voltage is not correct?

Situation	Consequence
Applied voltage < Rated voltage	The lamp glows dimly (under-voltage)
Applied voltage = Rated voltage	Normal operation
Applied voltage > Rated voltage	Risk of damage (over-voltage)

Voltage across the terminals of a power source

A power source (battery, power supply) imposes a voltage across its terminals when connected to a circuit. This voltage may be slightly lower than its rated value when the power source begins to wear out, or when the circuit draws a high current.

Practical example

We have a 4.5 V battery and a lamp marked ‘3.5 V’. If the bulb is connected directly to the battery without any other component, the voltage across it may exceed its rated voltage: the bulb will shine very brightly and then blow. Another component (such as a resistor) would need to be added to limit the voltage across it.

A common pitfall to avoid

Do not confuse the rated voltage (the reference value specified by the manufacturer) with the voltage actually measured across a dipole in a given circuit: the latter depends on the entire circuit, not just the component itself.