First Law of Thermodynamics question

In summary, the quantity of gas in a cylinder receives 1600J of heat from a hot plate and 800J of work are done on the gas by outside forces pressing down on a piston. The change in thermal energy of the gas is 2400J.
  • #1
in10sivkid
36
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a quantity of gas in a cylinder receives 1600J of heat from a hot plate. At the same time 800J of work are done on the gas by outside forces pressing down on a piston. Calculate the change in thermal energy of the gas


Input heat Q = 1600J
The amount of work done on the gas is, W = 800J

now I'm not exactly sure how I'm supposed to set this up

do I use U = Q - W
or Q = W + U

so i either get 800J or 2400J for the answer for U

i know its a simple question but i think I'm just a bit confused conceptually. any help would be great :)
 
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  • #2
It should be [itex]\Delta U = Q + W[/itex], where Q is the heat input to the system and W is the work done on the system. In [itex]\Delta U = Q - W[/itex], W is the work done by the system.
 
  • #3
in10sivkid said:
a quantity of gas in a cylinder receives 1600J of heat from a hot plate. At the same time 800J of work are done on the gas by outside forces pressing down on a piston. Calculate the change in thermal energy of the gas


Input heat Q = 1600J
The amount of work done on the gas is, W = 800J

now I'm not exactly sure how I'm supposed to set this up

do I use U = Q - W
or Q = W + U

so i either get 800J or 2400J for the answer for U

i know its a simple question but i think I'm just a bit confused conceptually. any help would be great :)
you are correct.
it's the first form:
ΔU = Q - W,
where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. of course, when work is done to the system (like your problem), W is negative, so that -W is positive, resulting in an increase in internal energy (like your problem):
ΔU = (1600) - (-800) = 2400 J
check here for more info:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html
 
Last edited:

1. What is the First Law of Thermodynamics?

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another.

2. How does the First Law of Thermodynamics relate to heat and work?

The First Law of Thermodynamics states that the total energy of a closed system remains constant. This means that any external work done on the system will result in a change in internal energy, either through an increase or decrease in heat.

3. What is an example of the First Law of Thermodynamics in action?

An example of the First Law of Thermodynamics is a car engine. The energy from the fuel is converted into motion (work) and heat. Even though the fuel is being consumed and transformed, the total energy of the system remains constant.

4. How does the First Law of Thermodynamics affect the efficiency of energy use?

The First Law of Thermodynamics dictates that energy cannot be created or destroyed, only transferred or converted. This means that no energy can be completely converted into work, and some will always be lost as heat. Therefore, the efficiency of energy use is limited by this law.

5. Can the First Law of Thermodynamics be violated?

No, the First Law of Thermodynamics is a fundamental law of physics and has been proven through numerous experiments and observations. It is a fundamental principle that applies to all processes and systems, and cannot be violated.

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