26.7: Electrical Power
Electric power is the product of current and voltage, represented in units of joules per second, or watts. For example, cars often have one or more auxiliary power outlets with which you can charge a cell phone or other electronic devices. These outlets may be rated at 20 amps and 12 volts, so that the circuit can deliver a maximum power of 240 watts. Consider a 25 Watt bulb and a 60 Watt bulb. The conversion of electrical energy produces heat and light, while the kinetic energy lost by the electrons in collisions is converted into the internal energy of the conductor and radiation. Therefore, the 60 Watt bulb glows brighter and warmer than the 25 Watt bulb.
A fuse is a device that protects a circuit from a sudden surge in electric power. It is essentially a short piece of wire between two contacts. During a fluctuation in electricity or power, the kinetic energy of the charge carriers—current—running through a conductor is converted into thermal energy in the conductor. The piece of wire in the fuse is under tension and has a low melting point, designed to heat up and break at the rated current. Fuses act quickly, but there is a small time delay while the wire heats up and breaks. Once the fuse is destroyed, it must be replaced as it protects the rest of the circuit from high power. Similarly, circuit breakers are also rated for a maximum current and remain open to protect the circuit, but can be reset.