What is the first law of thermodynamics? (article) | Khan Academy (2024)

Log in

• Azmi

  7 years agoPosted 7 years ago. Direct link to Azmi's post “I understand that Q isn't...”

  I understand that Q isn't the same thing as T, Q is heat that enters/exits the gas. ΔT might increase as Q enters, but ΔT also might stay constant or even decrease as Q enters the system, and so the opposite.

  But it's still confusing me that from this expression Q = m.c.ΔT it shows that Q is the function of ΔT (delta T) which is the change in temperature, and from this ΔT is proportional to Q.

  I would be grateful if anyone can help me with this confusion.

  Button navigates to signup page•Button navigates to signup page

  (11 votes)

  • mliu

    6 years agoPosted 6 years ago. Direct link to mliu's post “You say "ΔT also might st...”

    

    You say "ΔT also might stay constant or even decrease as Q enters the system, and so the opposite" - that is to do with the effect of W (work done on or by the gas in question), where ΔT is proportional to ΔU. In the expression Q = m.c.ΔT, W is not taken into account as it assumes a constant volume. You're just looking at raising the temperature of the substance with regard to the mass and specific heat capacity of the material, but not really considering the expansion of the gas pushing a piston or the piston pushing down on the gas (W). The Q = m.c.ΔT expression is only really appropriate for a constant volume.

    See: https://chemistry.stackexchange.com/questions/4722/when-is-it-okay-to-use-q-mc-delta-t-is-this-equation-only-for-calorimetry-que

    Comment on mliu's post “You say "ΔT also might st...”

    (21 votes)
• Muhammad Talha Ashraf

  8 years agoPosted 8 years ago. Direct link to Muhammad Talha Ashraf's post “why do we need second law...”

  why do we need second law of thermodynamics?Is energy really lost in second law of thermodynamics?

  Button navigates to signup page•Button navigates to signup page

  (10 votes)

  • APDahlen

    8 years agoPosted 8 years ago. Direct link to APDahlen's post “Hello Muhammad,This law...”

    

    Hello Muhammad,

    This law concerns energy conversion. Suppose we had a machine that could convert electrical energy to mechanical energy (motor). In an ideal world it could do so without any loss. In reality the efficiency is not 100% and so we loose some energy to waste heat. The 2nd law of thermodynamics states that you can NEVER make a machine that converts 100% of the energy.

    Likewise with a battery. When you charge the battery (electrical to chemical energy) there will be losses. When you discharge the battery (chemical to electrical) there will be losses.

    Don't let anyone tell you otherwise - there is no such thing as perpetual motion machines!

    Regards,

    APD

    Comment on APDahlen's post “Hello Muhammad,This law...”

    (14 votes)

    See Also

    5.13 The First Law of Thermodynamics – Douglas College Physics 1207First Law Of Thermodynamics - Equation, Statement, Examples2.4: The First Law of ThermodynamicsThe First Law of Thermodynamics
• nidhi bharadwaj

  8 years agoPosted 8 years ago. Direct link to nidhi bharadwaj's post “How does doing work on th...”

  How does doing work on the system increase the kinetic energy of the gas molecules

  Button navigates to signup page•Button navigates to signup page

  (4 votes)

  • Joleen Teo

    8 years agoPosted 8 years ago. Direct link to Joleen Teo's post “If the piston is pushed d...”

    

    If the piston is pushed down (form of work done), its base would collide against the gas molecules, causing them to move faster and hence their kinetic energy is increased.
    Hope this makes sense :)

    Button navigates to signup page

    (11 votes)
• Hafsa Kaja Moinudeen

  8 years agoPosted 8 years ago. Direct link to Hafsa Kaja Moinudeen's post “If the piston moves up wh...”

  If the piston moves up when the gas expands (due to heat) and if the volume of the gas does not decrease, what allows the piston to move down again?
  Thanks

  Button navigates to signup page•Button navigates to signup page

  (4 votes)

  • Teacher Mackenzie (UK)

    8 years agoPosted 8 years ago. Direct link to Teacher Mackenzie (UK)'s post “not sure of your exact qu...”

    not sure of your exact question

    but if the piston moves back down, it is usually because it is being pushed by a mechanical force to squash (compress) the gas (or to push the gas out of the cylinder as waste)

    The mechanical force comes from another part of the engine such as fly-wheel or other picston.

    OK?

    Comment on Teacher Mackenzie (UK)'s post “not sure of your exact qu...”

    (4 votes)
• Quang Tran

  8 years agoPosted 8 years ago. Direct link to Quang Tran's post “In the article it says " ...”

  In the article it says " temperature and internal energy are proportional", but I still don't understand. Internal energy is every kind of energy that exists in a system, including KE, PE and others. I understand that temperature is proportional to Kinetic energy, but KE is just part of what comprise total energy (Internal energy is every kind of energy that exists in a system, including KE, PE and others). So is temperature directly proportional to internal energy? And by saying A is proportional to B, is it necessarily equal to saying that A=kB, where k is a constant?

  Button navigates to signup page•Button navigates to signup page

  (4 votes)

  See Also

  1st Law of Thermodynamics

  • zhang jiachen

    7 years agoPosted 7 years ago. Direct link to zhang jiachen's post “I think the case that Int...”

    I think the case that Internal energy is proportional to kinetic energy is mostly based on monoatomic ideal gas, which means in the system there are no other forms of energy except kinetic energy.

    Button navigates to signup page

    (3 votes)
• bilalquetta457

  8 years agoPosted 8 years ago. Direct link to bilalquetta457's post “In example 2,Sample 1 I c...”

  In example 2,Sample 1 I cannot understand how does Heat Exits the system even when the work was done by the gas?
  Similary in Sample 4 the heat enters the system even when work is done on the gas.
  Explanations please anyone.

  Button navigates to signup page•Button navigates to signup page

  (3 votes)

  • Divya Varun

    8 years agoPosted 8 years ago. Direct link to Divya Varun's post “the most easy way to clar...”

    the most easy way to clarify this doubt is that consider yourself to be a candle. when it lights there is noble work done by the candle [ giving light to others (thermodynamically, work done by the gas ) ] but sadly the candle itself gets shorter and shorter( i.e thermodynamically heat loss) but when we add a little wax to it ( thermodynamically work done on the gas ) , the height increases ( heat enters the system)

    Button navigates to signup page

    (2 votes)
• Kairan

  6 years agoPosted 6 years ago. Direct link to Kairan's post “If you measure a liquid's...”

  If you measure a liquid's heat (coffee for example) with a thermometer. Do you also lose heat to the thermometer?

  Button navigates to signup page•Button navigates to signup page

  (2 votes)

  • Andrew M

    6 years agoPosted 6 years ago. Direct link to Andrew M's post “A hot liquid will lose he...”

    A hot liquid will lose heat to a cooler thermometer. The thermometer has to absorb thermal energy in order to measure it. Also a cold liquid might gain heat from a hot thermometer, right?
    But note that you don't measure "heat" when you use a thermometer. You measure temperature. They're not the same thing.

    Button navigates to signup page

    (2 votes)
• Archit Agarwal

  7 years agoPosted 7 years ago. Direct link to Archit Agarwal's post “while preparing for the S...”

  while preparing for the SAT subject tests physics from Barron's I read that they have taken the work done on the gas as positive.But we are told that W=P(change in volume). But if we apply force on the gas then its volume must decrease hence the work done must be negative please explain(its given on page 375)

  Button navigates to signup page•Button navigates to signup page

  (1 vote)

  • Teacher Mackenzie (UK)

    7 years agoPosted 7 years ago. Direct link to Teacher Mackenzie (UK)'s post “Its a good question... Yo...”

    Its a good question... You will see that W changing sign and meaning throughout your reading unfortunately.

    My answer to students is this: use your intelligence to define it within the context (or question) you are lookking at

    for example, if a gas is expanding, it is doing work ON the atmosphere. Work is being done BY the gas.

    If a piston squashes a gas, then it is doing work ON the gas. Work is being done BY the piston.

    When you know what is doing the work and what its being done ON, then you can fix the signage in your equaitons appropriately.

    Sorry to say, I have seen the negative sign in books given in different ways and not always clear; hence I am reluctant to give signage in equations on this topic : rather give clear understanding of the equaitons mean.

    Hope that helps

    Comment on Teacher Mackenzie (UK)'s post “Its a good question... Yo...”

    (3 votes)
• Zhiyu Wan

  2 years agoPosted 2 years ago. Direct link to Zhiyu Wan's post “If I move a bottle of gas...”

  If I move a bottle of gas from a low place to a high place (it seems I am doing work, so "W" is positive), according to "ΔU=Q+W", is there an increase in internal energy?

  Button navigates to signup page•Button navigates to signup page

  (1 vote)

  • Charles LaCour

    2 years agoPosted 2 years ago. Direct link to Charles LaCour's post “The gravitational potenti...”

    The gravitational potential energy of a system like the bottle is not part of the internal energy since it is not internal to the system, the bottle. Similarly the kinetic energy of the system from the motion of the entire system is not part of the internal energy. If you have two bottles of the same size, same amount of material in it at the same pressure and temperature but one is sitting on a table and the other is zipping by in a plane at 1000 meters above the table they have different kinetic energies but the internal energies are the same.

    Button navigates to signup page

    (3 votes)
• miARNr

  6 years agoPosted 6 years ago. Direct link to miARNr's post “i have a little confusion...”

  i have a little confusion , if we raise the pressure by decreasing the volume, the temperature would stay constant if we follow the ideal gas law , so how can the temperature raise according to the thermodynamic law ?

  Button navigates to signup page•Button navigates to signup page

  (1 vote)