Changes of State (OCR GCSE Physics A (Gateway)): Flashcards

Internal energy is the total energy stored inside a system by the particles that make up the system, due to their motion (kinetic energy) and positions (potential energy).

True.

The higher the temperature of a substance, the higher the kinetic energy of its molecules — meaning they move faster. Heating a substance increases its kinetic energy and therefore its internal energy.

When a substance is heated, its internal energy increases. This can either cause the temperature to rise (increased kinetic energy of particles) or produce a change of state (increased potential energy, breaking bonds).

Specific heat capacity is the amount of energy required to raise the temperature of 1 kg of a substance by 1 °C.

When a substance changes state, the potential energy of molecules increases while their kinetic energy stays constant, so the temperature does not change.

Temperature remains constant during a flat section because the energy supplied is used to break bonds between particles, increasing potential energy rather than kinetic energy. The change of state occurs at this point.

Specific latent heat is the amount of thermal energy required to change the state of 1 kg of a substance with no change in temperature.

False.

Specific heat capacity applies when there is a change in temperature (in the same state). Specific latent heat applies when there is a change of state but no change in temperature.

Complete the table showing the two types of specific latent heat.

A substance with a high specific heat capacity requires more energy to raise its temperature by 1 °C per kilogram, so for a given energy input, the temperature rise is smaller and it heats up more slowly.

Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1 °C. It is measured in joules per kilogram per degree Celsius (J/kg °C).

The equation for change in thermal energy is:

ΔQ = mcΔθ

Where ΔQ is the change in thermal energy (J), m is mass (kg), c is specific heat capacity (J/kg °C), and Δθ is the change in temperature (°C).

True.

Specific heat capacity is defined as the energy required to raise the temperature of 1 kg of a substance by 1 °C. It differs between substances and is measured in J/kg °C.

In the equation ΔQ = mcΔθ, the symbol c represents specific heat capacity and is measured in J/kg °C.

Step 1: Identify the values: m = 0.48 kg, Δθ = 0.7 °C, c = 4200 J/kg °C

Step 2: Apply the equation: ΔQ = mcΔθ

Step 3: ΔQ = 0.48 × 4200 × 0.7 = 1400 J (2 s.f.)

A change in internal energy is usually caused by the transfer of thermal energy. When a substance is heated, thermal energy is transferred to it, increasing its internal energy.

False.

A substance with a low specific heat capacity requires less energy to change its temperature — it heats up and cools down more quickly. A high specific heat capacity means more energy is needed per kilogram per degree.

To find the mass of a substance using the thermal energy equation, you would rearrange ΔQ = mcΔθ to give m = ΔQ ÷ (cΔθ).

The units of specific heat capacity are joules per kilogram per degree Celsius, written as J/kg °C. This reflects that it measures energy per unit mass per degree of temperature change.

Latent heat is the energy required to change the state of a substance. The word latent means hidden, because this energy does not cause a temperature change.

The equation for thermal energy required for a change of state is:

Q = mL

Where Q is the thermal energy (J), m is mass (kg), and L is the specific latent heat (J/kg).

Specific latent heat is the energy required to change the state of 1 kg of a substance with no change in temperature. It is measured in joules per kilogram (J/kg).

False.

During a change of state, the temperature remains constant even as energy is supplied. The energy goes into breaking bonds between particles (increasing potential energy) rather than raising the temperature.

In the equation Q = mL, the symbol L stands for specific latent heat and has units of J/kg.

The specific latent heat of fusion is the energy needed to change 1 kg of solid to liquid (or vice versa) with no temperature change. The specific latent heat of vaporisation is the energy needed to change 1 kg of liquid to gas (or vice versa) with no temperature change.

True.

The specific latent heat of vaporisation of water (2.26 MJ/kg) is roughly seven times greater than the specific latent heat of fusion (330 kJ/kg), so far more energy is needed to evaporate water than to melt ice.

Complete the table to show which type of specific latent heat applies to each process.

Step 1: Identify values: m = 0.60 kg, L = 199 000 J/kg

Step 2: Apply the equation: Q = mL

Step 3: Q = 0.60 × 199 000 = 119 400 J

The aim of the specific heat capacity experiment is to determine the specific heat capacity of water by measuring the energy required to increase the temperature of a known mass of water by 1 °C.

The electrical energy supplied is calculated using:

Energy (J) = voltage (V) × current (A) × time (s)

The voltage and current are recorded from the voltmeter and ammeter, and time is measured with a stopwatch.

True.

When energy supplied is plotted against (temperature change × mass), the gradient of the straight-line region equals the specific heat capacity of the water. This method is more reliable than using a single data point.

In the SHC experiment, the ammeter is connected in series with the immersion heater, and the voltmeter is connected in parallel with the immersion heater.

The flat section shows that temperature remains constant during the change of state (melting). Energy is being used to break bonds between ice molecules rather than raise the temperature.

A systematic error is that the immersion heater itself absorbs some energy as it heats up, so the energy measured is not all transferred to the water. This means the calculated specific heat capacity will be larger than the true value.

True.

The immersion heater must be connected to a direct current (DC) supply. Using an AC supply would be unsafe. The heater also becomes very hot, so it should not be touched and a heatproof mat should be used.

Taking a gradient averages out random errors across many data points, giving a more reliable result. A single data point may be affected by an anomaly or random error, making the calculated specific heat capacity less accurate.

Complete the table showing the measuring instruments used in the SHC experiment and their resolutions.

You can reduce random error by stirring the ice water constantly while heating. This ensures an even temperature throughout the mixture and reduces variation in temperature readings.