There are a metric ton of different relays. Then, within those categories, there are a metric ton of different options that you can choose from. If you'd like to learn more, GlobalSpec has some nifty learning materials on figuring out what kind you'd like / need for a process, and you can even sort by a plethora of ranges.
If you need a specific relay per an application, we might be able to help you out a little more, but til then, you might have to do a little digging around or try to narrow your question a little bit more
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The first law of thermodynamics is you do NOT talk about thermodynamics.