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**4.3.3 Thermal properties of materials**

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**(a) define and apply the concept of specific heat capacity**

Objective (a) define and apply the concept of specific heat capacity

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**(b) select and apply the equation E = mcΔθ**

Objective (b) select and apply the equation E = mcΔθ

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**Specific Heat Capacity**

If we heat matter so that its temperature rises, the amount of energy we must supply depends on three things: The mass m of the material The temperature rise Δθ we wish to achieve The material itself

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**Specific Heat Capacity**

ΔQ = mcΔθ where ΔQ = energy supplied (J) m = mass (kg) c = specific heat capacity (J kg-1 K-1) Δθ = change in temperature (°C or K)

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**Specific Heat Capacity**

The specific heat capacity of a substance is numerically equal to the amount of energy required to raise the temperature of 1kg of the substance by 1 K (or by 1 °C)

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**Specific Heat Capacity**

When J of energy is supplied to a 2kg block of aluminium, its temperature rises from 20 °C to 35 °C. Find the specific heat capacity of aluminium. c = ΔQ / mΔθ c = J / (2 kg x 15 K) c = 880 J kg-1 K-1

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**Specific Heat Capacity**

How much energy must be supplied to raise the temperature of 5 kg of water from 20°C to 100°C? Which requires more energy, heating a 2 kg block of lead by 30 K, or heating a 4 kg block of copper by 5 K? A well-insulated 1 kg block of iron is heated using a 50 W heater for 5 min. Its temperature rises from 22°C to 55°C. Find the specific heat capacity of iron.

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Objective c) describe an electrical experiment to determine the specific heat capacity of a solid or liquid

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**SHC Practical Run for 1200 seconds Take temperature every 30 s**

heater thermometer Run for 1200 seconds Take temperature every 30 s Draw graph Calculate c Repeat for different substance metal block

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**SHC Practical calculate gradient Δθ Δt E = mc Δθ Δt Δt**

P = m x c x gradient P = VI c = P m x gradient θ (°C) t (s)

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SHC Practical

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Objective d) describe what is meant by the terms latent heat of fusion and latent heat of vaporisation

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**Specific Latent Heat What is happening at: AB? CD?**

100 D θ AB: Melting – particles becoming disordered CD: Boiling – particles completely separating A B O t

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Specific Latent Heat At AB and CD, energy is being input, but the temperature isn’t rising The energy is being used to break the molecules free, not raise the temperature

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**Specific Latent Heat At AB (melting) and CD (boiling) energy input**

temperature does not change molecules become disordered (AB) or separate from each other (CD) little change in kinetic energy electrical potential energy increases

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**Specific Latent Heat At OA and BC and DE energy input**

temperature rises molecules move faster kinetic energy increases (temperature = average ke) little change in electrical potential energy

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Specific Latent Heat The energy needed to cause this change of state is Latent Heat (“Latent” means “hidden”) When a substance melts, this is the latent heat of fusion When a substance boils, this is the latent heat of vaporisation

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Definitions Latent heat of fusion is the energy which must be supplied to cause a substance to melt at a constant temperature Latent heat of vaporisation is the energy which must be supplied to cause a substance to boil at a constant temperature

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**Specific Latent Heat Remember:**

Temperature is a measure of the average kinetic energy of the molecules When a thermometer is put into water, the water molecules collide with the thermometer and share their kinetic energy with it. At a change of state, there is no change in kinetic energy, so no change of temperature

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Specific Latent Heat Why does it take more energy to boil a substance than it does to melt it? Melting – molecules still bonded to most of their neighbours – breaks one or two bonds Boiling – each molecule breaks free from all of its neighbours – breaks eight or nine bonds

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Specific Latent Heat The specific latent heat of a substance is the energy required per kilogram of the substance to change its state without any change of temperature

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Specific Latent Heat The specific latent heat of vaporisation of water is 2.26 MJ kg-1. Calculate the energy needed to change 2.0 g of water into steam at 100 °C. 1.0 kg (1000 g) of water requires 2.26 MJ of energy Therefore, energy = 2.0/1000 x 2.26 x 106 = 4520 J

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Specific Latent Heat The specific latent heat of fusion of water is kJ kg-1. Calculate the energy needed to change 2.0 g of ice into water at 0 °C. 1.0 kg (1000 g) of ice requires 330 kJ of energy Therefore, energy = 2.0/1000 x 3.30 x 105 = 660 J

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Practical

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Thermodynamics Notes Section: 10.4.

Thermodynamics Notes Section: 10.4.

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