Specific Heat CapacitySpecific Heat Capacity

Thermal Physics Lesson 1Thermal Physics Lesson 1

Learning ObjectivesLearning Objectives

Define specific heat capacity.Define specific heat capacity.

Perform calculations using ∆Q=mc Perform calculations using ∆Q=mc ∆∆θθ..

Describe how specific heat capacity Describe how specific heat capacity can be measured in the lab.can be measured in the lab.

HomeworkHomework

Complete worksheet to practice Complete worksheet to practice calculations we will look at today.calculations we will look at today.

Why do we careWhy do we care

The large SHC of water is particularly The large SHC of water is particularly important for the development and important for the development and maintenance of life on Earth.maintenance of life on Earth.

Why are we learning this?Why are we learning this?

Historical ContextHistorical Context

Images of Joule, KelvinImages of Joule, Kelvin

Steam enginesSteam engines

Thermal physics in starsThermal physics in stars

Which is hotter? Bath water or a Which is hotter? Bath water or a lit match?lit match?

Which needed more energy to Which needed more energy to heat it?heat it?

Heat vs. TemperatureHeat vs. Temperature

Heat as water and temperature as Heat as water and temperature as wetness analogywetness analogy

‘‘Heat’ is not an entity but a short Heat’ is not an entity but a short hand name for a process (heating as hand name for a process (heating as oppose to working).oppose to working).

State vs. PhaseState vs. Phase

Solids, liquids and gases are three of the Solids, liquids and gases are three of the different phases of matter (superfluids different phases of matter (superfluids and plasmas are two others. Thus melting, and plasmas are two others. Thus melting, boiling etc are changes of boiling etc are changes of phasephase. .

Each phase can exist in a variety of states Each phase can exist in a variety of states depending upon e.g. the temperature and depending upon e.g. the temperature and pressure. pressure.

However, the exam board uses “state” However, the exam board uses “state” to mean phase.to mean phase.

Specific Heat CapacitySpecific Heat Capacity

Definition:-Definition:-

The specific heat capacity (c) of a The specific heat capacity (c) of a substance is the amount of energy substance is the amount of energy needed to raise the temperature of needed to raise the temperature of 1kg of the substance by 1K (or 1 1kg of the substance by 1K (or 1 ººC) C)

Specific Heat CapacitySpecific Heat Capacity

mcQ

kJ.210K10KkgJ4200kg0.5 11 mcE

where:-∆Q is the energy change in Jm is the mass in kgc is the specific heat capacity in J K-1 kg-1

∆θ is the temperature change in K

Equation:

Worked Example 1Worked Example 1A bucket containing 11.5 litres of cold A bucket containing 11.5 litres of cold water at 10°C is taken into a house at a water at 10°C is taken into a house at a warmer temperature and left inside until it warmer temperature and left inside until it has reached thermal equilibrium with it has reached thermal equilibrium with it new surroundings. new surroundings.

If 504 kJ of energy is absorbed from the If 504 kJ of energy is absorbed from the surroundings to heat the water, what is the surroundings to heat the water, what is the temperature of the room?temperature of the room?

Worked Example 1Worked Example 1A bucket containing 11.5 litres of cold water at 10°C is taken A bucket containing 11.5 litres of cold water at 10°C is taken into a house at a warmer temperature and left inside until it into a house at a warmer temperature and left inside until it has reached thermal equilibrium with it new surroundings. has reached thermal equilibrium with it new surroundings. If 504 kJ of energy is absorbed from the surroundings to heat If 504 kJ of energy is absorbed from the surroundings to heat the water, what is the temperature of the room?the water, what is the temperature of the room?

∆Q =504 kJm=11.5 kgc = 4200 J kg-1 °C°C-1 ∆θ = ?

Using Using ∆ θ = 504,000 J / (11.5kg = 504,000 J / (11.5kg ×× 4200 4200 J K-1 °C°C-1 ) = 10.4 °C (to 3 s.f.) ) = 10.4 °C (to 3 s.f.)

So temperature of the water = So temperature of the water = 10°C + 10.4 °C = 20.4 °C 10°C + 10.4 °C = 20.4 °C Temperature of the room = T of water in thermal eqm. = Temperature of the room = T of water in thermal eqm. = 20.4 20.4

°C °C

mcQ

mc

Q

Worked Example 2Worked Example 2A 60 W immersion heater takes 2.5 minutes to heat 0.5 kg A 60 W immersion heater takes 2.5 minutes to heat 0.5 kg of water from 21 to 25, what is the specific heat capacity of of water from 21 to 25, what is the specific heat capacity of the water.the water.

P = 60 W∆ t = 2.5 minutes = (2.5 × 60)s = 150 s(2.5 × 60)s = 150 s∆θ = θ2 - θ1 = 4 = 4 ooC = 4 KC = 4 Km=0.5 kgc =?

∆Q = P = P ∆ t = 60 W × 150 s = 9,000 J= 60 W × 150 s = 9,000 Jc = 9,000 J / (0.5 kg × 4 K) = 4,500 c = 9,000 J / (0.5 kg × 4 K) = 4,500 J kg-1 KK-1

Why is it higher than the accepted value of Why is it higher than the accepted value of c = 4200 J kg-1

KK-1?

t

EnergyP

m

Qc

NoteNote

If a question includes heating and then If a question includes heating and then melting a substance then use:-melting a substance then use:-

mlmcQ