High School Physics : Thermodynamics

Study concepts, example questions & explanations for High School Physics

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Example Questions

Example Question #1 : Understanding Heat And Temperature

\(\displaystyle 0.75kg\) of soup at \(\displaystyle 80^\circ C\) cools down to \(\displaystyle 20^\circ C\) after \(\displaystyle 2300s\). If the specific heat of the soup is \(\displaystyle \small 3.67\frac{kJ}{kg^\circ C}\), how much energy does the soup release into the room?

Possible Answers:

\(\displaystyle 275.25kJ\)

\(\displaystyle 1651.5kJ\)

\(\displaystyle 0.72kJ\)

\(\displaystyle 27.53kJ\)

\(\displaystyle 165.15kJ\)

Correct answer:

\(\displaystyle 165.15kJ\)

Explanation:

The formula for heat energy is:

\(\displaystyle Q=mc\Delta t\)

We are given the initial and final temperatures, mass, and specific heat. Using these values, we can find the heat released. Note that the time is irrelevant to this calculation.

\(\displaystyle Q=(0.75kg)(3.67\frac{kJ}{kg^\circ C})(20^\circ C -80^\circ C)\)

\(\displaystyle Q=-165.15kJ\)

That means that the soup "lost" \(\displaystyle 165.15kJ\) of energy. This is the amount that it released into the room. The value is negative for the soup, the source of the heat, but positive for the room, which receives it.

Example Question #3 : Understanding Heat And Temperature

\(\displaystyle 0.75kg\) of soup cools down to \(\displaystyle 22^\circ C\)after \(\displaystyle 2800s\). If the specific heat of the soup is \(\displaystyle \small 3.67\frac{kJ}{kg^\circ C}\), and it released \(\displaystyle 64.11kJ\)of energy into the room, what was the initial temperature of the soup?

Possible Answers:

\(\displaystyle 45.29^\circ C\)

\(\displaystyle 1.29^\circ C\)

\(\displaystyle 0.03^\circ C\)

\(\displaystyle 24.58^\circ C\)

\(\displaystyle -1.29^\circ C\)

Correct answer:

\(\displaystyle 45.29^\circ C\)

Explanation:

The formula for heat energy is:

\(\displaystyle Q=mc\Delta t\)

We are given the final temperature, mass, specific heat, and heat released. Using these values, we can find the initial temperature. Note that the time is irrelevant to this calculation. Since heat is released from the soup, the net change in the soup's energy is negative. Since the soup is cooling, we expect our answer to be greater than \(\displaystyle 22^\circ C\).


\(\displaystyle -64.11kJ=(0.75kg)(3.67\frac{kJ}{kg^\circ C})(22^\circ C- T_i)\)

\(\displaystyle -64.11kJ=2.7525\frac{kJ}{^\circ C}*(22^\circ C - T_i)\)

\(\displaystyle \frac{-64.11kJ}{2.7525\frac{kJ}{^\circ C}}=(22^\circ C -T_i )\)

\(\displaystyle -23.29^\circ C=22^\circ C - T_i\)

\(\displaystyle -45.29^\circ C=-T_i\)

\(\displaystyle 45.29^\circ C=T_i\)

Example Question #2 : Understanding Heat And Temperature

\(\displaystyle 0.75kg\) of soup at \(\displaystyle 80^\circ C\) cools down after \(\displaystyle 2300s\). If the specific heat of the soup is \(\displaystyle \small 3.67\frac{kJ}{kg^\circ C}\), and it released \(\displaystyle 75.22kJ\) of energy into the room, what is the final temperature of the soup?

Possible Answers:

\(\displaystyle 0.023^\circ C\)

\(\displaystyle 52.67^\circ C\)

\(\displaystyle 107.33^\circ C\)

\(\displaystyle 27.33^\circ C\)

\(\displaystyle 43.67^\circ C\)

Correct answer:

\(\displaystyle 52.67^\circ C\)

Explanation:

The formula for heat energy is:

\(\displaystyle Q=mc\Delta t\)

We are given the initial temperature, mass, specific heat, and heat released. Using these values, we can find the final temperature. Note that the time is irrelevant to this calculation. Since heat is released from the soup, the net change in the soup's energy is negative. Since the soup is cooling, we expect our answer to be less than \(\displaystyle 80^\circ C\).

\(\displaystyle -75.22kJ=(0.75kg)(3.67\frac{kJ}{kg^\circ C})(T_f -80^\circ C)\)

\(\displaystyle -75.22kJ=(2.7525\frac{kJ}{^\circ C})(T_f-80^\circ C)\)

\(\displaystyle \frac{-75.22kJ}{2.7525\frac{kJ}{^\circ C}}=(T_f-80^\circ C)\)

\(\displaystyle -27.33^\circ C=Tf-80^\circ C\)

\(\displaystyle 52.67^\circ C=T_f\)

Example Question #21 : Thermodynamics

An ice cube at \(\displaystyle 0^\circ C\) melts. As it melts, constant temperature readings are taken and the sample maintains the temperature of \(\displaystyle 0^\circ\) throughout the melting process. Which statement best describes the energy of the system?

Possible Answers:

Energy of the system remains the same

The form of water has no bearing on the system energy

Energy increases as the sample moves from a solid to a liquid

We would need to know the mass of the ice cube to draw a conclusion

Energy is being used to convert the ice to water

Correct answer:

Energy is being used to convert the ice to water

Explanation:

When an object changes phase, it requires energy called "latent heat." In this case, even though the temperature is remaining constant, the energy inside of the ice cube is decreasing as it expends energy to melt.

Example Question #3 : Understanding Heat And Temperature

\(\displaystyle 280K\) silver spoon is placed in a \(\displaystyle 366K\) cup of tea. If the spoon has a mass of \(\displaystyle 32g\) and the tea has \(\displaystyle 90g\) of mass, what is the final temperature of the spoon?

\(\displaystyle c_{tea}=1.00\frac{cal}{g*K}\)

\(\displaystyle c_{silver}= 0.0558\frac{cal}{g*K}\)

Possible Answers:

\(\displaystyle 353.43K\)

\(\displaystyle 288.32K\)

\(\displaystyle 323K\)

\(\displaystyle 364.33K\)

\(\displaystyle 301.21K\)

Correct answer:

\(\displaystyle 364.33K\)

Explanation:

The equation for two items reaching a thermal equilibrium is given by describing a heat transfer. The heat removed from one object is equal to the heat added to the other.

\(\displaystyle K_1=-K_2\)

\(\displaystyle K=mc\Delta T\)

\(\displaystyle m_1c_1\Delta T_1=-m_2c_2\Delta T_2\)

We are given the specific heat values of each substance, as well as their masses. We also know the initial temperature of each substance. Use these terms in the equation to solve for the final temperature. Remember that the final temperature will be the same for each substance, since they will be in thermodynamic equilibrium.

\(\displaystyle (32g)(0.0558\frac{cal}{g*K})(T_f-280K)=-(90g)(1.00\frac{cal}{g*K})(T_f-366K)\)

\(\displaystyle 1.7856\frac{cal}{K}*(T_f-280K)=-90\frac{cal}{g*K}*(T_f-366K)\)

\(\displaystyle (1.7856\frac{cal}{K}*T_f)-499.968cal=(-90\frac{cal}{K}*T_f)+32940cal\)

\(\displaystyle 1.7856\frac{cal}{K}*T_f=(-90\frac{cal}{K}*T_f)+33439.968cal\)

\(\displaystyle 91.7856\frac{cal}{K}*T_f=33439.968cal\)

\(\displaystyle T_f=\frac{33439.968cal}{91.7856\frac{cal}{K}}\)

\(\displaystyle T_f=364.33K\)

 

Example Question #7 : Understanding Heat And Temperature

A sample of \(\displaystyle 236g\) of water at \(\displaystyle 100^\circ C\) is placed in a \(\displaystyle 350g\) ceramic mug, which is at \(\displaystyle 23^\circ C\). What is the final temperature of the system?

\(\displaystyle c_{ceramic}=1100\frac{J}{kg*^\circ C}\)

\(\displaystyle c_{water}=4.2\frac{J}{g*^\circ C}\)

 

Possible Answers:

\(\displaystyle 91^\circ C\)

\(\displaystyle 123^\circ C\)

\(\displaystyle 78.43^\circ C\)

\(\displaystyle 108^\circ C\)

\(\displaystyle 84^\circ C\)

Correct answer:

\(\displaystyle 78.43^\circ C\)

Explanation:

For this question, we must recognize that the system going to end up in equilibrium. That means that:

\(\displaystyle m_1c_1\Delta T_1=-m_2c_2\Delta T_2\)

We are given the initial temperatures, masses, and specific heats of both the water and the ceramic. This will allow us to solve for the final temperature of the system; this value will be equal for both components. Notice that the specific heat given to us in the problem for the ceramic is in terms of kilograms, not grams. Convert to grams.

\(\displaystyle (350g)(1.1\frac{J}{g*^\circ C})(T_f-23^\circ C)=-(236g)(4.2\frac{J}{g*^\circ C})(T_f-100^\circ C)\)

\(\displaystyle (385\frac{J}{^\circ C})(T_f-23^\circ C)=-(991.2\frac{J}{^\circ C})(T_f-100^\circ C)\)

\(\displaystyle \frac{(385\frac{J}{^\circ C})(T_f-23^\circ C)}{385\frac{J}{^\circ C}}=\frac{(-991.2\frac{J}{^\circ C})(T_f-100^\circ C)}{385\frac{J}{^\circ C}}\)

\(\displaystyle T_f-23^\circ C=(-2.57)(T_f-100^\circ C)\)

\(\displaystyle T_f-23^\circ C=-2.57T_f+257^\circ C\)

\(\displaystyle -23^\circ C=-3.57T_f+257^\circ C\)

\(\displaystyle -280^\circ C=-3.57T_f\)

\(\displaystyle \frac{-3.57T_f}{-3.57}=\frac{-280^\circ C}{-3.57}\)

\(\displaystyle T_f=78.43^\circ C\)

Example Question #8 : Understanding Heat And Temperature

A \(\displaystyle 78g\) vial of an unknown liquid is \(\displaystyle 9 8^\circ C\). Julie adds \(\displaystyle 3g\) of the same liquid at \(\displaystyle 40^\circ C\) to the vial. What is the final temperature? 

Possible Answers:

We need to know the freezing/boiling points of the liquid in order to solve.

\(\displaystyle 95.85^\circ C\)

\(\displaystyle 94.74^\circ C\)

We need to know the specific heat of the liquid in order to solve.

\(\displaystyle 7 7 . 6 4 ^\circ C\)

Correct answer:

\(\displaystyle 95.85^\circ C\)

Explanation:

The equation for change in temperature is \(\displaystyle m_1c_1\Delta T_1=-m_2c_2\Delta T_2\)

Plug in our given values.

\(\displaystyle m_1c_1\Delta T_1=-m_2c_2\Delta T_2\)

\(\displaystyle 0.78kg*c*(T_f-98^\circ C)=-0.03kg*c*(T_f -40^\circ C)\)

Notice that the specific heats will cancel out.

\(\displaystyle 0.78kg*(T_f-98^\circ C)=-0.03kg*(T_f -40^\circ C)\)

\(\displaystyle (0.78kg*T_f)-(0.78kg*98^\circ C)=(-0.03kg*T_f)+(0.03kg*40^\circ C)\)

\(\displaystyle (0.78kg*T_f)-(76.44kg*^\circ C)=(-0.03kg*T_f)+(1.2kg*^\circ C)\)

Combine like terms.

\(\displaystyle (0.78kg*T_f)-(-0.03kg*T_f)=(1.2kg*^\circ C)+(76.44kg*^\circ C)\)\(\displaystyle 0.81kg*T_f=77.64kg*^\circ C\)

\(\displaystyle T_f=\frac{77.64kg*^\circ C}{0.81kg}{}\)

\(\displaystyle T_f=95.85^\circ C\)

Example Question #21 : Thermodynamics

A \(\displaystyle 2g\) disc of copper is dropped into a \(\displaystyle 12g\) glass of water. If the copper was at \(\displaystyle 85^\circ C\) and the water was at \(\displaystyle 70^\circ C\), what is the new temperature of the mixture?

\(\displaystyle c_{Cu}=0.092\frac{cal}{g^oC}\)

\(\displaystyle c_{water}=1.00\frac{cal}{g^oC}\)

Possible Answers:

\(\displaystyle 69.77^oC\)

\(\displaystyle 70.23^oC\)

\(\displaystyle 79.77^oC\)

\(\displaystyle 83.76^oC\)

\(\displaystyle 75.22^oC\)

Correct answer:

\(\displaystyle 70.23^oC\)

Explanation:

The relationship between mass and temperature, when two masses are mixed together, is:

\(\displaystyle m_1c_1\Delta T=-m_2c_2\Delta T\)

Using the given values for the mass and specific heat of each compound, we can solve for the final temperature.

\(\displaystyle (2g)(0.092\frac{cal}{g*^\circ C})(T_f-85^\circ C)=-(12g)(1\frac{cal}{g*^\circ C})(T_f-70^{\circ} C)\)

We need to work to isolate the final temperature.

\(\displaystyle (0.184\frac{cal}{^\circ C})(T_f-85^\circ C)=-(12\frac{cal}{^\circ C})(T_f-70^{\circ} C)\)

Distribute into the parenthesis using multiplication.

\(\displaystyle (0.184\frac{cal}{^\circ C})T_f-15.64cal=(-12\frac{cal}{^\circ C})T_f+840cal\)

Combine like terms.

\(\displaystyle (12.184\frac{cal}{^oC})T_f=855.64cal\textup{}\)

\(\displaystyle T_f=\frac{855.64cal}{12.184\frac{cal}{^oC}}\)

\(\displaystyle T_f=70.23^\circ C\)

Example Question #1 : Understanding Effects Of Heat On Volume

An ideal gas is inside of a container with a pressure of \(\displaystyle 2Pa\). If it starts with a volume of \(\displaystyle 1L\)  and is compressed to \(\displaystyle 0.5L\), what is the new pressure if the temperature remains constant?

Possible Answers:

\(\displaystyle 10Pa\)

\(\displaystyle 2Pa\)

\(\displaystyle 8Pa\)

\(\displaystyle 0.5Pa\)

\(\displaystyle 4Pa\)

Correct answer:

\(\displaystyle 4Pa\)

Explanation:

We will need to use Boyle's Law to solve:

\(\displaystyle P_1V_1=P_2V_2\)

Boyle's Law allows us to set up a relationship between the changes in pressure and volume under conditions with constant temperature. Since the equation is a proportion, we do not need to convert any units.

We can use the given values to solve for the new pressure.

\(\displaystyle (2Pa)(1L)=(P_2)(0.5L)\)
 

\(\displaystyle \frac{(2Pa)(1L)}{0.5L}=(P_2)\)

\(\displaystyle \frac{2Pa\cdot L}{0.5L}=P_2\)

\(\displaystyle 4Pa=P_2\)

Example Question #611 : High School Physics

An ideal gas is inside of a tube at \(\displaystyle 60^oC\). If the pressure remains constant, but the volume decreases from \(\displaystyle 5m^3\) to \(\displaystyle 3m^3\), what will be the final temperature in the tube?

Possible Answers:

\(\displaystyle 18^oC\)

\(\displaystyle 36^oC\)

\(\displaystyle 120^oC\)

\(\displaystyle 180^oC\)

\(\displaystyle 24^oC\)

Correct answer:

\(\displaystyle 36^oC\)

Explanation:

For this problem, use Charles's Law:

\(\displaystyle \frac{V_1}{T_1}=\frac{V_2}{T_2}\)

In this formula, \(\displaystyle V\) is the volume and \(\displaystyle T\) is the temperature. Charles's Law allows us to set up a proportion for changes in volume and temperature, as long as pressure remains constant. Since we are dealing with a proportion, the units for temperature are irrelevant and we do not need to convert to Kelvin.

Using the given values, we should be able to solve for the final temperature.

\(\displaystyle \frac{V_1}{T_1}=\frac{V_2}{T_2}\)

\(\displaystyle \frac{5m^3}{60^oC}=\frac{3m^3}{T_2}\)

Cross multiply.

\(\displaystyle 5m^3* T_2=3m^3* 60^oC\)

\(\displaystyle 5m^3* T_2=180(^oC\cdot m^3)\)

\(\displaystyle T_2=\frac{180(^{\circ}C\cdot m^3)}{5m^3}\)

\(\displaystyle T_2=36^{\circ}C\)

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