Processor

The microprocessor, or CPU, works with the operating system to control the computer. It essentially acts as the computer's brain. The CPU produces a lot of heat, so a desktop computer uses circulating air, a fan and a heat sink -- a system of plates, channels and radiator fins used to draw heat off of the processor -- to cool off. Since a laptop has far less room for each of these cooling methods, its CPU usually:

• Runs at a lower voltage and clock speed -- This reduces heat output and power consumption but slows the processor down. Most laptops also run at a higher voltage and clock speed when plugged in, and at lower settings when using the battery.

• Mounts to the motherboard without using pins -- Pins and sockets take up a lot of room in desktop PCs. Some motherboard processors mount directly to the motherboard without the use of a socket. Others use a Micro-FCBGA (Flip Chip Ball Grid Array), which uses balls instead of pins. These designs save space, but in some cases mean that the processor cannot be removed from the motherboard for replacement or upgrading.

• Has a sleep or slow-down mode -- The computer and the operating system work together to reduce the CPU speed when the computer is not in use or when the processor does not need to run as quickly. The Apple G4 processor also prioritizes data to minimize battery drain.

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An IBM ThinkPad’s processor

Some laptops use desktop CPUs that are set to run at lower clock speeds. Although this can improve performance, these laptops typically run much hotter and have a significantly reduced battery life.

A laptop heat sink and fan

Laptops usually have small fans, heat sinks, heat spreaders or heat pipes to help dissipate the heat from the CPU. Some higher end laptop models reduce heat even further with liquid coolant kept in channels alongside the heat pipe. Also, most laptop CPUs are near the edge of the unit. This allows the fan to move the heat directly to the outside instead of across other components.