# Why are we not able to use heat as a power source, only heat difference?

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When the topic of renewable energy comes up I cant help but think that since anything greater in temprature than absploute zero has some level of thermal energy, why are we not able to harness it? I get the law of entropy and heat and whatnot but even if its inefficient, why can we not use heat, only heat differences, to generate electricity. I get how RTGs and nuclear power works, but those both require some kind of thing that is cool in addition to what is hot. Why not just use the energy in the heat of whatever we have? If you believe in the concept of the universe having a “heat death” then you also get that its not that one part of the universe loses it heat and one part gains it, its that every atom in the universe loses heat. If thats the case then why cant we have uniform heat loss on earth?

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Heat death is where everything reaches the same temperature, so there is no difference to exploit to get useful energy. It isn’t about things getting uniformly cooler, but everything trending towards a middle point.

This is a bit of an unsatisfactory answer but we can’t just make use of the thermal energy of an object because the laws of thermodynamics say that in order to turn heat into work you need to also move some heat from a hot to a cold place.

Heat is a form of energy.

Another form of energy would be the potential energy in a gravity well. If you hold a basketball at arm’s length, that basketball has a potential energy based on the fact you’re holding it above ground. So what power can you harvest from holding that basketball at arm’s length? None.

Power is energy differential transferred over time. If you drop that basketball, you’re harvesting its potential energy into kinetic energy that will eventually be harvested into the impact when it strikes the ground. To gain that power, you must transform the energy in some fashion.

Heat is no different. Unless you’re moving from hot to cold or vice versa, you’re not moving the energy at all. You’re not producing work from it. It’s just a feature of the system like the basketball suspended above the ground.

I think the first sentence of the wikipedia definition might be helpfull here.

> In physics, energy is the quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it.

This means that energy itself is just a quantitative property which stays the same in a closed system. When you want to use/harness energy, you perform physical work. Work itself is the process of converting one form of energy to another.

This conversion/transfer requires a system to not be in perfect equillibrium but have some kind of difference in temperature/height/etc…

After all, performing work means changing something which implies that there is some form of structure to be changed. In your example, everything in the system is equal and has no potential to be changed