Internal Energy & Energy Transfers Flashcards

All 32 cards in this deck

What is internal energy?

The total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.

Which energy store of a system increases when it is heated?

The internal (thermal) energy store — the energy of the particles that make up the system.

What two things can heating a system do to it?

Raise its temperature, or produce a change of state.

True or false? Heating a system always raises its temperature.

False. It may instead produce a change of state, with the temperature staying the same.

What happens to the particles of a substance when its temperature increases?

Their mean kinetic energy increases.

What is the specific heat capacity of a substance?

The amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius. Unit: J/kg °C.

What three things determine how much the temperature of a substance rises when it is heated?

The mass of the substance, the type of material (its specific heat capacity) and the energy input.

What is the equation linking energy, mass, specific heat capacity and temperature change?

$\Delta E = mc\Delta\theta$ (energy = mass × specific heat capacity × temperature change)

What are the steps to calculate the energy needed to change a substance's temperature?

e.g. heat 3 kg of water by 10 °C, $c = 4200$ J/kg °C

1. Write $\Delta E = mc\Delta\theta$, mass in kg.
2. Substitute: $\Delta E = 3 \times 4200 \times 10$
3. Evaluate: $\Delta E = 126\,000$ J

What are the steps to find a specific heat capacity from measurements?

e.g. 8400 J raises the temperature of 2 kg of a liquid by 1 °C

1. Write $\Delta E = mc\Delta\theta$.
2. Rearrange: $c = \dfrac{\Delta E}{m\Delta\theta}$
3. Substitute: $c = \dfrac{8400}{2 \times 1} = 4200$ J/kg °C

True or false? In $\Delta E = mc\Delta\theta$ the mass must be converted into kilograms.

True. Masses given in grams must be divided by 1000 before substituting.

What is latent heat?

The energy needed for a substance to change state.

True or false? While ice is melting at 0 °C, supplying more energy makes its temperature rise.

False. During a change of state the energy supplied changes the internal energy but not the temperature.

What happens to the internal energy of a substance while it changes state at constant temperature?

It increases (when melting or boiling), because the energy supplied is stored by the particles rather than raising the temperature.

Is energy absorbed or released when a gas condenses to a liquid?

Released — energy must be removed for a substance to condense or freeze.

True or false? If some energy is transferred to the surroundings during an experiment to measure specific latent heat, the value obtained for $L$ is too high.

True. Less energy than measured reaches the substance, so less mass changes state and $E \div m$ comes out too big.

What is the specific latent heat of a substance?

The amount of energy required to change the state of one kilogram of the substance with no change in temperature.

Which change of state does specific latent heat of fusion refer to?

Solid to liquid (melting).

Which change of state does specific latent heat of vaporisation refer to?

Liquid to vapour (boiling/evaporating).

True or false? For a given substance, the specific latent heat of fusion and the specific latent heat of vaporisation are equal.

False. They are different values; for water the specific latent heat of vaporisation is much larger.

What are the steps to calculate the energy needed for a change of state?

e.g. melt 2 kg of ice, $L = 340\,000$ J/kg

1. Write $E = mL$, mass in kg.
2. Substitute: $E = 2 \times 340\,000$
3. Evaluate: $E = 680\,000$ J

What are the steps in an experiment to measure the specific latent heat of fusion of water?

e.g. 3400 J of energy melts 0.010 kg of ice

1. Place an immersion heater in a funnel of crushed ice, collecting melted water in a beaker on a balance.
2. Measure the energy supplied $E$ (joulemeter, or power × time) and the mass $m$ of water melted.
3. Calculate $L = \dfrac{E}{m} = \dfrac{3400}{0.010} = 340\,000$ J/kg

On a heating graph, what does a flat (horizontal) section show?

A change of state — energy is being supplied but the temperature is not changing.

True or false? During the flat section at the melting point, both solid and liquid are present.

True. The substance is part-melted until all of it has changed state.

On a heating graph, what does a sloping section tell you?

The temperature is rising, so the substance stays in one state and the mean kinetic energy of its particles is increasing.

On a heating graph, what does a steeper sloping section tell you about the specific heat capacity?

The specific heat capacity is lower — less energy is needed to raise the temperature by a fixed amount.

On a heating graph, what does a longer flat section tell you about the specific latent heat?

The specific latent heat is greater — more energy is needed to change the state of that substance.

What is the key difference between specific heat capacity and specific latent heat?

Specific heat capacity is the energy to raise 1 kg by 1 °C (temperature changes); specific latent heat is the energy to change the state of 1 kg with no change in temperature.

Which equation do you use for the energy transferred when water is heated from 20 °C to 90 °C?

$\Delta E = mc\Delta\theta$ — the temperature is changing with no change of state.

Which equation do you use for the energy transferred when water at 100 °C turns into steam?

$E = mL$ — a change of state at constant temperature.

True or false? A temperature change $\Delta\theta$ is needed to calculate the energy required to melt ice at 0 °C.

False. There is no temperature change during melting, so use $E = mL$.

True or false? Specific heat capacity and specific latent heat are both measured in J/kg.

False. Specific latent heat is in J/kg, but specific heat capacity is in J/kg °C.