Energy transfers with the surroundings

In thermodynamics we will use the following sign convention: $Q > 0$ when the system absorbs heat from the surroundings and $Q < 0$ when it gives off heat; $W > 0$ when the surroundings do work on the system (compression) and $W < 0$ when the system does work on the surroundings (expansion). With this convention, the first law is written as $\Delta U = Q + W$.

Write with its sign the value of $Q$ or $W$ in each case:

1. a) The system absorbs $400\ \text{J}$ of heat.
2. b) The system gives off $250\ \text{J}$ of heat to the surroundings.
3. c) The surroundings compress the system, doing $100\ \text{J}$ of work.
4. d) The system expands and does $150\ \text{J}$ of work on the piston.

A gas inside a cylinder absorbs heat $Q = +600\ \text{J}$ from a flame while the piston compresses it, doing work $W = +150\ \text{J}$ on it. The diagram shows the energy transfers to the gas.

1. a) Calculate the change in internal energy $\Delta U$ of the gas.
2. b) Interpret the result: has the internal energy of the gas increased or decreased?

State the first law of thermodynamics in your own words.

1. a) Explain what each of the three terms in the expression $\Delta U = Q + W$ represents.
2. b) Reason why this law is, at heart, the principle of conservation of energy applied to thermodynamic systems.

An electric kettle heats $m = 2\ \text{kg}$ of water and raises its temperature by $\Delta T = 10\ \celsius$. The specific heat capacity of water is $c = 4180\ \text{J}/(\text{kg}\cdot\celsius)$.

Calculate the heat absorbed by the water using the expression $Q = m \cdot c \cdot \Delta T$.

The specific heat capacity of water is $c_{\text{agua}} = 4180\ \text{J}/(\text{kg}\cdot\celsius)$ and that of ethanol is $c_{\text{etanol}} = 2400\ \text{J}/(\text{kg}\cdot\celsius)$.

1. a) Explain in your own words what the value $4180\ \text{J}/(\text{kg}\cdot\celsius)$ means: how many joules are needed and to achieve what, exactly.
2. b) For equal masses and the same temperature increase, reason using formula $Q = m \cdot c \cdot \Delta T$ which of the two substances requires more heat.

An ice pack containing $m = 0,5\ \text{kg}$ of ice at $0\ \celsius$ is applied to a sports injury until all the ice melts, remaining at $0\ \celsius$ throughout the process. The latent heat of fusion of ice is $L_{\text{f}} = 3,34\cdot10^{5}\ \text{J/kg}$.

1. a) Calculate the heat absorbed by the ice using $Q = m \cdot L_{\text{f}}$ and express the result in standard form.
2. b) State where that heat comes from and, therefore, explain the cooling effect on the treated area.

A pot of boiling water remains on the hob, but the thermometer reads $100\ \celsius$ throughout the process, even though the flame continues to supply energy. The graph shows how the temperature of the water changes over time.

1. a) Does the water continue to receive energy while it is boiling? Justify your answer.
2. b) If the temperature does not rise, what is that energy being used for at a microscopic level?
3. c) Explain, using the concept of latent heat, why the temperature remains constant during a change of state.

We want to convert $m = 0,5\ \text{kg}$ of ice at $-10\ \celsius$ into liquid water at $40\ \celsius$. Data: $c_{\text{hielo}} = 2100\ \text{J}/(\text{kg}\cdot\celsius)$, $c_{\text{agua}} = 4180\ \text{J}/(\text{kg}\cdot\celsius)$ and $L_{\text{f}} = 3,34\cdot10^{5}\ \text{J/kg}$.

1. a) Calculate the heat required to heat the ice from $-10\ \celsius$ to $0\ \celsius$.
2. b) Calculate the heat required to melt all the ice at $0\ \celsius$.
3. c) Calculate the heat required to heat the resulting water from $0\ \celsius$ to $40\ \celsius$.
4. d) Calculate the total heat of the process, express it in standard form and draw the graph of temperature as a function of heat supplied in your notebook, marking the different stages.

A heat engine undergoes closed cycles. In each cycle it absorbs heat $Q_1 = +3000\ \text{J}$ from the hot reservoir and delivers work of $900\ \text{J}$ to the surroundings, that is, $W = -900\ \text{J}$ with the standard sign convention. The diagram of the engine is shown in the figure.

1. a) Justify why the change in internal energy of the engine in a complete cycle is zero.
2. b) Apply $\Delta U = Q + W$ to the cycle and calculate the net heat $Q$ exchanged per cycle.
3. c) Taking into account that $Q = Q_1 + Q_2$, calculate the heat $Q_2$ exchanged with the cold reservoir, with its sign, and interpret it.
4. d) Calculate the efficiency $\eta = W_{\text{useful}}/Q_1$ of the engine.

A refrigerator operates as a heat engine in reverse: in each cycle, it extracts heat $Q_2 = +600\ \text{J}$ from the inside and the electric motor does work $W = +200\ \text{J}$ on the system. In a complete cycle, $\Delta U = 0$. The diagram of the refrigerator is shown in the figure.

1. a) Apply the first law of thermodynamics to the cycle and calculate the net heat $Q$ exchanged.
2. b) Calculate the heat $Q_1$ that the refrigerator releases to the kitchen, including its sign, and interpret it: why is it greater than the heat extracted from the inside?
3. c) Use the result to explain why leaving the refrigerator door open does not serve to cool down the kitchen, but actually ends up heating it up.

Two heat engines work between the same reservoirs. Engine A absorbs $Q_1 = 5000\ \text{J}$ from the hot reservoir and delivers useful work of $1500\ \text{J}$. Engine B absorbs the same $5000\ \text{J}$ and releases heat of $2500\ \text{J}$ to the cold reservoir.

1. a) Calculate the efficiency of engine A.
2. b) Calculate the useful work of engine B from its energy balance and then its efficiency.
3. c) Compare both engines: which one makes better use of the fuel's heat?
4. d) An inventor claims that they can build an engine with an efficiency of $100\ \%$, which releases no heat to the cold reservoir. Using the energy balance and the concept that every real heat engine must release heat to a cold reservoir, explain why such an engine is not possible.

In an insulated container there is $0,3\ \text{kg}$ of water at $60\ \celsius$ and $0,3\ \text{kg}$ of ice at $0\ \celsius$ is added. Data: $c_{\text{agua}} = 4180\ \text{J}/(\text{kg}\cdot\celsius)$ and $L_{\text{f}} = 3,34\cdot10^{5}\ \text{J/kg}$.

1. a) Calculate the maximum heat that the hot water can give off if it cooled down to $0\ \celsius$.
2. b) Calculate the heat needed to melt all the ice.
3. c) Comparing both results, predict whether all the ice will melt and deduce the final temperature of the mixture.
4. d) Calculate the mass of ice that melts and the mass that remains unmelted.

A gas absorbs heat $Q = +1000\ \text{J}$ from a burner and its internal energy increases by $600\ \text{J}$ while pushing the piston of a mechanism.

1. a) Calculate the work $W$ exchanged by the gas, including its sign, and state how much work the gas has done.
2. b) The piston experiences friction, so out of that work only $300\ \text{J}$ is delivered as useful work to the external mechanism. Explain what happened to the rest.
3. c) Verify that the balance $\Delta U = Q + W$ still holds and explain why friction does not "break" the conservation of energy even though it reduces useful work.

On a clear day, the Sun transfers the same energy $Q = 4180\ \text{J}$ to $1\ \text{kg}$ of sea water ($c_{\text{agua}} = 4180\ \text{J}/(\text{kg}\cdot\celsius)$) and to $1\ \text{kg}$ of beach sand ($c_{\text{arena}} = 836\ \text{J}/(\text{kg}\cdot\celsius)$).

1. a) Calculate the temperature increase of the water.
2. b) Calculate the temperature increase of the sand.
3. c) Use the results to explain why the sand is burning hot at midday while the sea water is barely lukewarm.
4. d) Relate this to daytime sea breezes: reason over which surface the air heats up first and in which direction the breeze blows.