Final answer:
The useful output energy from a fan is the kinetic energy of air, from a table lamp is light (although it also produces heat), and from a heater is thermal energy. All electric appliances eventually contribute to heat in a room. A heat pump can transfer more heat than the electrical work it uses by utilizing ambient air, making it more efficient.
Step-by-step explanation:
The useful output energy of different appliances can be measured by how these appliances convert electrical energy into other forms for their intended purpose. For a fan, the useful output is the kinetic energy of the moving air, although it also produces some heat energy due to its motor's inefficiency. A table lamp, especially one with an incandescent light bulb, primarily produces light as its useful output, but a significant portion of its energy is also emitted as heat. Lastly, the main output of a heater is the thermal energy or heat it transfers to the surrounding environment, warming up the space; this is its intended useful energy.
Nevertheless, it's important to note that all of these devices ultimately increase the amount of heat transfer in a room. This is because, over time, most forms of useful work done by electric appliances are converted to heat energy due to energy dissipation and inefficiencies.
For example, a heat pump can transfer more heat energy than the electrical work it consumes, due to the extraction of heat from the ambient air. This is a more efficient heat transfer than using a simple electric room heater.