Alight bulb transforms electric energy first into the thermal energy of the heated filament and then into light and heat. The rate at which these energy changes occur is determined by the way the bulb is constructed. The 100-watt bulb has a lower resistance filament—the thin wire in the bulb that gets hot. Assuming that both bulbs are connected to a 120-volt outlet, there will be more current through the 100-watt bulb than the 25-watt bulb. Lower resistance is created by making the filament out of thicker wire. The lower resistance means higher current, which in turn means higher power and more light and heat output.

A circuit is a circular path through which charge can flow. So, the first requirement is a complete, unbroken conducting path. Second, there must be a source of potential difference—most often a battery. The battery provides the voltage that will produce the current in the circuit. With only a battery and wire connecting the two ends of the battery the resistance in the circuit will be almost zero, and the amount of current will be very high. This situation is called a short circuit. The wire will become hot enough to burn you. So, for a useful circuit there must be a third element—a device with resistance. This may be a resistor, lamp, motor, etc.

In terms of energy, the energy input to the circuit is the chemical energy stored in the battery. When the circuit is complete, the chemical energy becomes electrical energy in the wires. That energy is then converted to thermal energy in the resistor or lamp, or kinetic energy in the motor. The hot resistor or lamp then radiates heat and light into the environment.

In our households the battery is replaced by the electric generating station operated by the utility company. It may use the chemical energy in fossil fuels such as coal, oil, or natural gas to boil water (i.e. produce thermal energy). The steam from the boiling water then turns the generator, converting the energy to rotational kinetic energy. The generator then converts this energy of rotation into the electrical energy that is transmitted to the home. A nuclear power plant uses the nuclear energy in the nucleus of the uranium atoms to heat the water and produce steam. From that point on the nuclear and fossil fuel power plants are essentially the same.