Answer
Electrons often are the primary carriers of both heat and current.
Work Step by Step
Mobile electrons can carry both heat and current, so good thermal conductors are often good electrical conductors too.
This poses problems when trying to design safe appliances that heat things, such as irons or heaters. The casing is often metallic to transmit heat easily, but an electrical fault inside can electrically energize the case and shock the user.
There are materials that do not follow that rule – diamond is a notable exception. It is an electrical insulator, but its rigid lattice transmits heat very well.
