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4.3.3 Thermal properties of materials
Objective
(a) define and apply the concept of specific heat capacity
Objective
(b) select and apply the equation E = mcΔθ
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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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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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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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When 26 400 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 = 26 400 J / (2 kg x 15 K)
c = 880 J kg-1 K-1
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a) How much energy must be supplied to raise the temperature of 5 kg of water from 20°C to 100°C?
b) 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?
c) 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.
Objective
c) describe an electrical experiment to determine the specific heat capacity of a solid or liquid
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metal block
thermometerheater
12 V
Run for 1200 seconds Take temperature every 30 s Draw graph Calculate c Repeat for different substance
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t (s)
θ (
°C)
calculate gradient Δθ Δt
E = mc Δθ Δt Δt
P = m x c x gradient
P = VI
c = Pm x gradient
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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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O
AB
C
D
E
100
0
θ
t
What is happening at: AB? CD?
AB: Melting – particles becoming disordered
CD: Boiling – particles completely separating
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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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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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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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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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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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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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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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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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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. 1.0 kg (1000 g) of water requires 2.26 MJ of energy
2. Therefore,
energy = 2.0/1000 x 2.26 x 106
= 4520 J
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The specific latent heat of fusion of water is 330 kJ kg-1. Calculate the energy needed to change 2.0 g of ice into water at 0 °C.
1. 1.0 kg (1000 g) of ice requires 330 kJ of energy
2. Therefore,
energy = 2.0/1000 x 3.30 x 105
= 660 J
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