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ikihi
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So the formula for power is Power = work / time. There's two types of energy transfers: heat and work. How do you calculate the power of something using heat energy in BTU/hr when BTU isn't work?
DaveE said:BTU is work (or energy). 1 BTU = 1055.06 joules = 1055.06 Watt-sec.
Work is mechanical energy moved in a direction and heat is the energy of random particles moving around? So you are saying that heat energy may be used in this equation, even though it says work?Dale said:That is just a question of semantics. The calculation is fine, only the terminology is problematic.
yes. Energy is a more fundamental concept than a particular scenario. Energy is often converted from one form to another. For example, consider a Carnot engine that converts heat energy into mechanical energy.ikihi said:Work is mechanical energy moved in a direction and heat is the energy of random particles moving around? So you are saying that heat energy may be used in this equation, even though it says work?
I wouldn’t go that far. Work is a credit or debit, energy is the account balance.DaveE said:Work IS energy, energy IS work.
DaveE said:yes. Energy is a more fundamental concept than a particular scenario. Energy is often converted from one form to another. For example, consider a Carnot engine that converts heat energy into mechanical energy.
If we didn't know how to convert chemical potential energy (like gasoline) into heat energy (like combustion in cylinders), and then into mechanical energy (like spinning tires), you couldn't drive your car to the grocery store.
Work IS energy, energy IS work. Two different names for the same thing. Which, BTW, I always found annoying; just pick one. I choose "energy".
From what I read: The Second Law of Thermodynamics allows work to be transformed fully into heat, but forbids heat to be totally converted into work. This is confusing because they are not totally interchangeable it seems.Dale said:I wouldn’t go that far. Work is a credit or debit, energy is the account balance.
OK. That is how it's used. OTOH it's all the same stuff (money, in this case). You are adding process details to the definition of what it is.Dale said:I wouldn’t go that far. Work is a credit or debit, energy is the account balance.
Not always. You can store energy in the form of heat. The translation of equations into human language is fraught with additional complexities. Often you need to know the context for complete understanding.ikihi said:"Heat" is a form of energy in transit
Wouldn't "heat" imply a temperature difference? And when there is a temperature difference that suggests that thermal energy will start flowing from hot to cool areas? In other words: heat is an energy transfer of thermal energy. I was using "heat" in a similar manner to "work", in that I was referring to the transfer process.DaveE said:Not always. You can store energy in the form of heat. The translation of equations into human language is fraught with additional complexities. Often you need to know the context for complete understanding.
Yes. Work is a process in that sense.DaveE said:Then they have additional process names
Probably because none of those things are conserved. Many conserved quantities have something similar either denoting a transfer or a rate of transfer (or both). For energy it is work/power. For momentum it is impulse/force. For angular momentum it is torque. For charge it is current.DaveE said:The physics world isn't very consistent with this approach. Things like position, pressure, temperature, or time aren't treated that way.
Those all have different units. We aren't discussing work/power. We are discussing work/energy.Dale said:For energy it is work/power. For momentum it is impulse/force. For angular momentum it is torque. For charge it is current.
That is just historical. Originally heat and mechanical work were seen as completely different things.ikihi said:I do not understand why they don't say that heat transfer and work are both just called "energy transfer".
Yes, that makes sense to me. Again, there is a difference between what people say and what makes sense in basic definitions. You are much more likely to hear about heat energy storage than temperature energy storage, for example.ikihi said:Wouldn't "heat" imply a temperature difference? And when there is a temperature difference that suggests that thermal energy will start flowing from hot to cool areas? In other words: heat is an energy transfer of thermal energy. I was using "heat" in a similar manner to "work", in that referring to the transfer process.
OK, a perfectly reasonable definition of the result of a process of changing energy (a discrete chunk of energy). Is there an equivalent word for a cumulative change in angular momentum or charge, if not how do physicists get by without them?ergospherical said:@DaveE in this case it's not just semantics, it's an important distinction. The energy ##E = E(p,V)## of a system is a state function, but heat ##Q## and work ##W## are not state functions (they are processes). Similarly the one-form ##dE## is exact whilst the one-forms ##dW## and ##dQ## are not.
I agree that power is more useful than work, IMO.DaveE said:I think we could get by with "power" as a better name for the process of changing energy.
I think it is a bit unrealistic to think that we shouldn’t make and use a word unless we also make and use a word for every other analogous process. Words are made historically and organically, there is no overarching rationaleDaveE said:Is there an equivalent word for a cumulative change in angular momentum or charge, if not how do physicists get by without them?
Touché. You are 100% correct about this. Still it proves that physics doesn't need "work", people just like it.Dale said:I think it is a bit unrealistic to think that we shouldn’t make and use a word unless we also make and use a word for every other analogous process. Words are made historically and organically, there is no overarching rationale
this is a bad idea because the energy ##E## of the system can also change if heat is added to it i.e. ##\Delta E = W + Q##DaveE said:Or perhaps change the symbol for work from ##W## to ##\Delta E##.
cmon, this is a bit sillyDaveE said:Still it proves that physics doesn't need "work", people just like it.
Of course that could be fixed by calling heat “thermal work”. If I could wave a wand that is what I would do, but the history is too strong.ergospherical said:this is a bad idea because the energy ##E## of the system can also change if heat is added to it i.e. ##\Delta E = W + Q##
I was confused about how a unit of energy such as BTU or joules can fit into the power equation where work is. I was confused whether thermal energy transfer is work. Some websites have suggested that it is not. These websites list the term "heat" as similar but different to work: which was where my confusion began. But others have stated that thermal energy transfer it is in fact work. So can I conclude that thermal heat transfer from a temperature difference using radiation, conduction, and convection, is in fact work?DaveE said:OK, a perfectly reasonable definition of the result of a process of changing energy (a discrete chunk of energy). Is there an equivalent word for a cumulative change in angular momentum or charge, if not how do physicists get by without them?
I guess I don't have a problem with the semantics of defining work as a process instead of a thing (or really the result of a process). But I think processes usually require a more detailed description, which makes a unique word for the amount unnecessary. I think we could get by with "power" as a better name for the process of changing energy. Or perhaps change the symbol for work from ##W## to ##\Delta E##.
As I said, I know y'all don't agree with me. Maybe because that's what's written in the intro textbooks and we can't get rid of it. I think it's unnecessary and inconsistent, but then, I didn't live in a world without "work" in the physics books. Maybe it's crucial and makes things clearer; except for the OP who appeared confused about work and energy.
I personally would like that, but unfortunately that is not how those words are currently defined and used by the scientific community. The official usage is that work is a transfer of energy other than heat.ikihi said:But others have stated that thermal energy transfer it is in fact work. So can I conclude that thermal heat transfer from a temperature difference using radiation, conduction, and convection, is in fact work?
The power equation using heat instead of work is P = Q/t, where P is power in watts, Q is heat in joules, and t is time in seconds.
The traditional power equation, P = W/t, uses work in joules instead of heat. Work is the force applied to an object multiplied by the distance it moves, while heat is the energy transferred due to a temperature difference.
No, the power equation using heat instead of work is specifically for situations where heat is the only form of energy being transferred, such as in thermal power plants or heating systems.
The power equation using heat instead of work is useful in understanding and optimizing thermal processes, such as in designing more efficient heating systems or calculating the power output of a thermal power plant.
Yes, the power equation using heat instead of work assumes that all the heat transferred is being converted into work, and does not take into account any losses or inefficiencies in the system. It also assumes that the heat transfer is constant over time.