Why is the heat a board gives off a useful diagnostic signal?

Select one answer.

• Electrical power that is not delivered as useful output leaves as heat, so where a board is hot maps where its power is going — and a fault, dissipating abnormal power, changes that map at the faulty part.
• Heat is never a useful signal in any situation, because the temperature of a component has no relationship at all to how much power it happens to be dissipating at the time, so a part running hot is exactly as likely to be perfectly healthy as a part running completely cool.
• The heat is useful only because a warm board is more pleasant to work on, which is a comfort benefit that has nothing to do with locating any fault or reading where power is being spent.
• Heat matters solely because a board must be kept below room temperature to work, so the only thing a temperature reading tells you is whether the room's air conditioning is currently switched on.