# What is internal energy

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Thread starter 1 month ago
#1
what is internal energy in engineering terms

ive found several definations. some say it includes the kinetic energy and potential energy of the system and some say that it excludes them.
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1 month ago
#2
(Original post by mlm1234)
what is internal energy in engineering terms

ive found several definations. some say it includes the kinetic energy and potential energy of the system and some say that it excludes them.
Generally, you exclude the change in KE and PE as it’s negligible, however, technically, they are,part of the internal energy
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Thread starter 1 month ago
#3
so a defination of would
The sum of the energy located with the thermodynamic system due to its motion, excluding the potential and kinetic energy due to minimial effect of it on the system.

am i right the the internal energy keeps account of the total energy and that the sum of the input and out put of energy equals the internal energy. so that the overal energy change would be zero.

(Original post by EmberPlayer)
Generally, you exclude the change in KE and PE as it’s negligible, however, technically, they are,part of the internal energy
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1 month ago
#4
(Original post by mlm1234)
so a defination of would
The sum of the energy located with the thermodynamic system due to its motion, excluding the potential and kinetic energy due to minimial effect of it on the system.

am i right the the internal energy keeps account of the total energy and that the sum of the input and out put of energy equals the internal energy. so that the overal energy change would be zero.
I may have confused you a bit, internal energy is the mechanical energy (KE + PE) possessed by the particles in the systems. When you look at the first law of thermodynamics, it technically is the change in the total energy of the system ( which includes the internal energy of the system, the KE of system and PE of the system), however, it’s generally assumed the change of KE/PE of the system (not the particles in the system) is negligible compared to the change in internal energy, hence it isn’t included within the total change of energy of the system. So basically engineers usually (not always) assume that the change of energy of a system is only due to the change in internal energy of the system.
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1 month ago
#5
To give an example: let’s say you had a balloon (the ballon along with the air particles inside it consist of the system). You do 3 actions to the ballon - you heat it, you move the entire balloon around, and you place the balloon at an elevated position.

Now to calculate the total change in energy of the system (the balloon):

1. the change in internal energy (the particles in the balloon move faster due to being heated, ie particles of air gain KE),
2. the change in KE of the entire system (the balloon is being moved around, hence the balloon is gaining KE)
3. The change in PE energy (the balloon is elevated, hence it gains gravitational potential energy)

So now if you calculate each individually using reasonable assumptions, eg. say the balloon is heated by 20K, it is being moved around at 100m/s and it has been raised by 10m. You’ll realise that the majority of the contribution to the total change in energy is due to heating the balloon (increasing the internal energy). Therefore, it’s generally a reasonable assumption to neglect the effects of the change of KE /PE of the system in regards to the change of total energy of the system.
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Thread starter 1 month ago
#6
(Original post by EmberPlayer)
To give an example: let’s say you had a balloon (the ballon along with the air particles inside it consist of the system). You do 3 actions to the ballon - you heat it, you move the entire balloon around, and you place the balloon at an elevated position.

Now to calculate the total change in energy of the system (the balloon):

1. the change in internal energy (the particles in the balloon move faster due to being heated, ie particles of air gain KE),
2. the change in KE of the entire system (the balloon is being moved around, hence the balloon is gaining KE)
3. The change in PE energy (the balloon is elevated, hence it gains gravitational potential energy)

So now if you calculate each individually using reasonable assumptions, eg. say the balloon is heated by 20K, it is being moved around at 100m/s and it has been raised by 10m. You’ll realise that the majority of the contribution to the total change in energy is due to heating the balloon (increasing the internal energy). Therefore, it’s generally a reasonable assumption to neglect the effects of the change of KE /PE of the system in regards to the change of total energy of the system.
See if i got this right

Intenral energy is the sum on energy that resides within the thermodynatic system. due to the minimial effect that KE and PE energy have on the system they are excluded. only heat can work can effect the internal energy of the system.

sorry just started mechanical engineering thermodynatics and trying to get head around it. so what would be simple defination of what it is then.
Last edited by mlm1234; 1 month ago
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Thread starter 1 month ago
#7
dsf
Last edited by mlm1234; 1 month ago
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1 month ago
#8
(Original post by mlm1234)
See if i got this right

Intenral energy is the sum on energy that resides within the thermodynatic system. due to the minimial effect that KE and PE energy have on the system they are excluded. only heat can work can effect the internal energy of the system.

sorry just started mechanical engineering thermodynatics and trying to get head around it. so what would be simple defination of what it is then.
Heat transferred into/out of the system and also work done on/by the system affect the change in internal energy (first law of thermodynamics). Yeah, thermo is pretty confusing at first. Don’t worry it’ll make more sense as you go
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Thread starter 1 month ago
#9
(Original post by EmberPlayer)
Heat transferred into/out of the system and also work done on/by the system affect the change in internal energy (first law of thermodynamics). Yeah, thermo is pretty confusing at first. Don’t worry it’ll make more sense as you go
thanks for that you are life saver.
it was the kinetic and potential that throw me.

was that defintion right
Think i may exclude the potential and kinetic bit when writing about it, saves confusing and only need broad understanding off it.
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1 month ago
#10
(Original post by mlm1234)
thanks for that you are life saver.
it was the kinetic and potential that throw me.

was that defintion right
Think i may exclude the potential and kinetic bit when writing about it, saves confusing and only need broad understanding off it.
I’d define internal energy as the sum of KE and PE energies of the particles that make up the system
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