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### Course: Biology library>Unit 7

Lesson 2: Laws of thermodynamics

# First Law of Thermodynamics introduction

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. For example, kinetic energy may be converted into thermal energy, or potential energy may be converted into kinetic energy. Energy is never "lost"—it is transferred or converted in some way.

## Want to join the conversation?

• Why can't energy be created or destroyed? Doesn't the sun create light energy or when we are coasting on a bicycle don't we create kinetic energy?
• The sun doesn't create energy. It simply chemically changes hydrogen atoms into helium atoms through a process of Nuclear Fission. The byproduct of this reaction is a massive volume of light and heat energy.

Bicycles don't create kinetic energy. We give the bike kinetic energy by pedaling the bike.
• energy can be destroyed or created.does it mean that "heat energy" can be transformed into a more useful type of energy that has the ability to do work? thus decreasing the level of entropy in the universe after all energy can be transformed from one form to another
• It depends on what you mean by 'heat energy'. If you mean there is a temperature difference between things, then yes, you can use that energy and convert it to a different form (this happens for example in a power station). However, when we talk about thermodynamics, 'heat' often describes energy lost to surroundings by increasing the random motions of molecules. Although energy can be converted from one form to another, it cannot be converted back and forth any way you want. For things to go forward, you need to have an increase of entropy in the universe (this is the second law of thermodynamics), and there is no way you could collect back all the heat energy that has been dissipated into lots of molecules. If you think about a fridge, you can decrease energy of molecules inside it, but at the cost of increasing entropy by increasing heat outside it.
• At , if the light heats up the glass bulb, then how is it intact? Shouldn't the glass overheat and explode?
• The light doesn't actually heat up anything too much, and since the glass is transparent, barely any heating will occur. The light will simply pass through the glass. If you were talking about the heat from the filament, it would disperse slowly through the collisions of molecules and get more and more dffused with each collision, so it wouldn't heat up the glass much. Even if it did, the heat would quickly disperse out through the glass, without heating it much, as Sal mentioned at .
• If energy can not be created or destroyed, what energy was there before the Big Bang?Energy has to be created,but only once.Because the energy has to be there for the big bang to happen, right?
• Well if you agree that there was absolutely nothing before the Big Bang, we may well say that none of the laws of conservation were applicable (in this context the law of conservation of energy which says energy can neither be created nor be destroyed), hence we may say that matter could be formed and that's exactly where the matter that exploded during Big Bang came from.
• Once you start running there will be air friction towards you then why don't we feel hot while we run?
• I mean, I don’t know about you, but I feel hot after running for a while. However, the hotness isn’t really due to air friction to a noticeable degree, but rather by the heat released by your body expending chemical energy to move. Air fraction, or drag, does cause an increase in temperature to an object in motion, but you have to be going fast, like jet plane fast, for the effect to be noticeable. This is why returning spacecraft reentering earth’s atmosphere have heat shields to protect from the air friction burning craft up.

As mentioned in another reply, your body also sweats too to try to release that excess heat and help prevent your body from overheating. So there are heating and cooling effects at play when running.

Hope that helps.
• At , Sal said the 1st Law of Thermodyamics is that energy cannot be created or destroyed, it can only be converted. Isn't that the Law of Conservation of Energy? Are these two the same? What is going on here?
• Yes they are essentially saying the same thing. Thermodynamics was developed looking at heat energy where as conservation of energy applies to all types of energy.
• What kind of energy, does our body generate on a normal basis?
Like, when we walk or jump.
• We are not really 'generating' energy, we are using energy (chemical energy stored in food) and converting it into another form. As Davin commented, we convert a lot of energy to heat. But since you ask about walking or jumping, if you are moving, you are converting energy to kinetic energy. And if you jump or climb up a hill, you need to use a source of energy to gain gravitational potential energy. Does this answer your question?
And in case you are wondering where the energy from the food comes from, it all (indirectly), comes from the light energy from the sun. Plants capture this energy and use it to drive chemical reactions that make sugars from CO2 and water, producing sugars and oxygen. Normally that chemical reaction would go the other way, but it is the light energy that makes allows the plant to produce sugars. Then by 'burning' the sugars, we can release energy again, this time not as light, but in a way the lets our body do chemical reactions, and somewhere down the line this for example makes our muscles move. But we can't use all the energy in a productive way, some is always lost as heat.
• what is the difference between Radiant energy & Light energy??
• Radiant energy refers primarily to energy that radiates from the source, which is commonly in the form of light energy or thermal energy, while light energy is specifically electromagnetic.
• So the First Law of Thermodynamics is the Law of Conservation of Energy?
• Essentially the First law of Thermodynamics and the Law of Energy Conservation imply the same thing but the First law of Thermodynamics is focused on thermodynamics and the transfer of heat energy where as Law of Energy Conservation is more generic in its scope.
• is thermal energy and kinetic energy the same thing ?
• According to what I've read thermal energy is a somewhat ill-defined property for which usage varies.

You are correct in thinking that a significant fraction of what is usually meant by thermal energy is due to kinetic energy at the atomic level. However, kinetic energy is typically used to describe the energy associated with "organized" motion through space, while thermal energy is "disorganized" motion and also includes vibration and rotation.

An example might be if you kicked a stone out of the edge of a fire. It has kinetic energy imparted by your kick and thermal energy imparted by the fire. The thermal energy from being next to the fire is (mostly {I think}) expressed by enhanced vibration of the minerals within the rock. My understanding is that, while this thermal energy is kinetic (due to movement), it is qualitatively different from the kick imparted kinetic energy.

The following links have more details:
https://en.wikipedia.org/wiki/Thermal_energy