PHYS 12C Lecture

What is a phase change?

A transition between solid, liquid, and gas in which the physical state of a substance changes.

What is melting?

The phase change from solid to liquid.

What is freezing?

The phase change from liquid to solid.

What is vaporization or boiling?

The phase change from liquid to gas.

What is condensation?

The phase change from gas to liquid.

What is sublimation?

The phase change directly from solid to gas.

What is heat capacity C?

The amount of heat required to change the temperature of an entire object by one degree.

What is specific heat c?

The heat required to raise the temperature of 1 kg of a particular substance by 1°C or 1 K.

What is the equation for heating or cooling without a phase change?

Q = mcΔT = mc(Tf - Ti).

What are the SI units of specific heat?

J/(kg·K).

What does positive Q mean in Q = mcΔT?

The substance gains heat and its temperature increases.

What does negative Q mean in Q = mcΔT?

The substance loses heat and its temperature decreases.

Do Celsius and kelvin temperature changes have the same numerical value?

Yes. A change of 1°C equals a change of 1 K.

Which requires more energy to heat by the same temperature: water or an equal mass of metal?

Water because it has a higher specific heat.

Why does water heat and cool more slowly than most metals?

Water has a much higher specific heat, so it requires more energy for the same temperature change.

How do large bodies of water affect nearby climates?

Their high specific heat moderates temperature changes and contributes to land and sea breezes.

What is latent heat or heat of transformation L?

The energy required per unit mass to complete a phase change without changing temperature.

What equation is used during a phase change?

Q = mL.

What are the SI units of latent heat?

J/kg.

What is latent heat of fusion LF?

The energy per unit mass required to melt a solid or released when a liquid freezes.

What is latent heat of vaporization LV?

The energy per unit mass required to vaporize a liquid or released when a gas condenses.

What happens to temperature during a phase change?

It remains constant even though energy is being absorbed or released.

Where does added energy go during a phase change?

It changes the molecular arrangement and potential energy instead of raising temperature.

When should Q = mcΔT be used?

When the substance stays in one phase while its temperature changes.

When should Q = mL be used?

When the substance changes phase at constant temperature.

How do you solve a heating problem involving multiple phases?

Divide it into separate temperature-change and phase-change steps, calculate each Q, and add them.

How much energy heats 1 g of ice from -30°C to 0°C in the lecture example?

62.7 J using Q = mciceΔT.

How much energy melts 1 g of ice at 0°C?

333 J using Q = mLF.

How much energy heats 1 g of water from 0°C to 100°C?

419 J using Q = mcwaterΔT.

How much energy boils 1 g of water at 100°C?

2,260 J using Q = mLV.

How much energy heats 1 g of steam from 100°C to 120°C?

40.2 J using Q = mcsteamΔT.

How much total energy converts 1 g of ice at -30°C into steam at 120°C?

Approximately 3,110 J.

Which step requires the most energy when converting ice at -30°C to steam at 120°C?

Boiling the liquid water at 100°C because the latent heat of vaporization is very large.

What is calorimetry?

A method of measuring heat transfer by observing temperature changes in an isolated system.

What is a calorimeter?

A device designed to reduce energy exchange between a system and its surroundings.

What conservation equation applies in an ideal isolated calorimeter?

Qlost + Qgained = 0.

If a hot sample is placed into cooler water, what occurs?

The sample loses heat while the water gains the same amount of heat until equilibrium is reached.

What final-temperature condition applies in calorimetry?

All objects in thermal contact eventually reach the same final equilibrium temperature.

State the First Law of Thermodynamics using the course convention.

ΔEint = Q + W, where W is work done on the system.

For an ideal gas, what determines internal energy?

Internal energy depends directly on temperature.

What does an isothermal process imply for an ideal gas?

Temperature is constant, so ΔEint = 0.

For an isothermal ideal-gas process, how are Q and W related?

Q = -W.

What is an isochoric process?

A constant-volume process.

What are W and ΔEint for an isochoric process?

W = 0, so ΔEint = Q.

What is an adiabatic process?

A process in which no heat enters or leaves the system.

What are Q and ΔEint for an adiabatic process?

Q = 0, so ΔEint = W.

What is the general equation for work done on a gas?

W = -∫ from Vi to Vf p dV.

What does the area under a pV curve represent?

The magnitude of the work; work done on the gas is the negative of the signed area under the curve.

What is the sign of W during gas expansion?

W is negative because the gas performs work on its surroundings.

What is the sign of W during gas compression?

W is positive because the surroundings perform work on the gas.

What is the work during a constant-volume process?

W = 0 because ΔV = 0.

What is the work during a constant-pressure process?

W = -pΔV.

What is the work during an isothermal ideal-gas process?

W = -nRT ln(Vf/Vi).

During isothermal expansion, what are the signs of ΔEint, W, and Q?

ΔEint = 0, W < 0, and Q > 0.

Why must heat enter a gas during isothermal expansion?

The gas does work on the environment, but its temperature and internal energy remain constant, so heat must replace the lost energy.

During isothermal compression, what are the signs of ΔEint, W, and Q?

ΔEint = 0, W > 0, and Q < 0.

How is work calculated for a multistep path on a pV diagram?

Calculate the work for each segment and add the values, including their signs.

What is the equation for temperature at a point on a pV diagram?

T = pV/(nR).

For a fixed amount of ideal gas, what do points on the same isotherm share?

The same value of pV and therefore the same temperature.

What shape is an isotherm on a pV diagram?

A downward-curving hyperbola described by p = nRT/V.

An ideal gas has p = 1.5 atm, V = 3,000 cm³, and n = 0.1 mol. What is its temperature?

After converting to SI units, T = pV/(nR) ≈ 549 K.

What is a common mistake in multistep heating problems?

Using one Q = mcΔT calculation across a phase change instead of separating each stage.

What is a common mistake with pV work?

Forgetting the negative sign because the course defines W as work done on the gas.