[8-Ball]

Not quite sure what you're referring to. If it's why the cpu is at the temperature it is, then I'll try and explain in a little more detail.

A few points.

1. Heat flows DOWN a temperature gradient.
2. The amount of thermal energy that will flow down a unit thermal gradient (ie 1 degree cooler per metre) for a block of unit cross section (1mx1m) is the coefficient of thermal conductivity.

So, for a given heat exchange process, if we want to transfer MORE thermal energy, then we need to INCREASE the thermal gradient.

If we have a fixed coolant temp and a fixed heat load, the thermal gradient must be increased until the thermal energy which will flow down the gradient matches the heat dissipation of the source. The heat source will then stay at a constant temperature, as the thermal energy it is producing is ALL being removed. This doesn't mean it will cool down, as this would reduce the thermal gradient.

The whole heat exchange from cpu to air can be considered as a series of individual heat exchanges, each with an associated thermal resistance (reciprocal of the efficiency), where a greater resistance requires a greater thermal gradient (driving force) in order to transfer the same amount of thermal energy.

1. CPU > Waterblock base - (TIM C/W)
2. Waterblock base > waterblock fins - (block C/W)
3. Waterblock fins > water - (convective heat transfer coefficient)
4. Water > radiator wall - (convective heat transfer coefficient)
5. Radiator wall > radiator fins - (radiator C/W)
6. Radiator fins > air (convective heat transfer coefficient)

For a given heat load, each of these steps will have an associated temperature difference required to transfer that exact amount of heat. These can simply be added up to find the total temperature difference between the air and the cpu, where the air temp is the controlling variable.

Factors such as flow rate, fluid viscosity, thermal diffusivity of fluid and the geometry of heat exchanger surfaces will affect the resistances/efficiencies, thus affecting the temperature difference associated with each step.

As for the difference between idle and load.

If the thermal energy load is resuced, ie, cpu at idle, then the temperature difference required at each stage to transfer that amount of thermal energy will be reduced. Thus the overall temperature difference will be reduced, and since the ambient is still at the same temperature, the cpu will cool down.

Obviously, there will be a delay due to the time it takes for the water and the copper to drop in temperature, which will happen because the heat transferred into them from the cpu will drop, yet they are still at a temperature to transfer the initial load to the next medium, so more thermal energy will be transferred out than that which is coming in, so it will cool down.

That's why larger copper blocks and systems with large volumes of water will not react so quickly to heat loads.

I hope all of that makes sense.

8-ball