Final answer:
When electrical devices are in use, resistive heating caused by electrical energy converting into thermal energy makes them warm. This is due to electron collisions within the device's components, which increases the temperature over time.
Step-by-step explanation:
Electrical devices can feel warm to the touch when they have been on for a long time due to a process called resistive heating. The warmth you feel is caused by the electrical energy being converted into thermal energy, primarily as electrons move through a conductor and collide with particles, losing kinetic energy, which results in resistance. Over time, this transfer of energy causes the temperature of the components within the device to increase.
In cases of a short circuit, the undesired and rapid conversion of electrical energy to thermal energy can cause materials to melt or ignite due to the low resistance path that results in a high power dissipation. However, even in normal operation, devices can warm up. This is perfectly demonstrated when you touch a cellphone charger while charging a cell phone; it gets warm because of the resistors included in its circuits.