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Thermal energy - specific heat capacity: 2.1 Explaining and defining the specific heat capacity of materials and the calculation formula with units

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2.2 Worked out practice questions involving specific heat

INDEX for my physics notes on specific heat capacity


2.1 Explaining and defining the specific heat capacity of materials

Whenever any material is heated to a higher temperature you increase the thermal energy store of the material.

A measure of how much energy is needed to raise the temperature of a given amount of material to a specific temperature is called the heat capacity of the material.

The specific heat capacity of a substance can be defined as the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius.

From the specific heat capacity of a material, the amount of material and the temperature change the material experiences, you can calculate the increase or decrease of that material's thermal energy store.

It's a good idea to read Examples of energy store conversions in systems first and

Specific latent heat is dealt with in a separate section

Whenever you get an increase in temperature of a system, energy must be transferred from one energy store to another.

However, for the same quantity of heat energy transferred, the temperature rise will vary.

The temperature rise will depends on the amount of material heated and its structure.

Don't confuse heat and temperature!

When some object is heated, the thermal energy ('heat') transferred increases the thermal energy store of the object.

The temperature increases, but the temperature only indicates how hot or cold the object is.

When you heat a material, thermal energy is absorbed and its internal energy is increased due to an increase in its thermal energy and potential energy stores.

At a particle level this is due to:

(i) An increase in the kinetic energy store caused by increased vibration of solid particles or increased kinetic energy of the free movement of liquid and gas particles from one place to another.

From kinetic particle theory, a temperature value is a measure of the average kinetic energy of the particles - much of the average internal energy of the material.

(ii) An increase in the potential energy caused by the increase in kinetic energy opposing the inter-particle forces of attraction - the particles on average a bit further apart with increase in temperature.

The internal energy store is the sum of the kinetic energy store plus the potential energy store - the latter can often be ignored in the situations described here concerning heat capacity.

The energy transferred to a given material acting as a thermal energy store to raise its temperature by a specific amount can vary quite widely.

e.g. you need over four times more heat energy to raise a given mass of water to specified temperature than that for the same mass of central heating oil or aluminium (they have different specific heat capacities - but more on this later).

Application: Solar panels may contain water that is heated by radiation from the Sun.

Water has a high heat capacity and can store a lot of thermal energy.

This water may then be used to heat buildings or provide domestic hot water.

Water is the usual conveyer of thermal energy in central heating systems.

Water is a very good thermal energy store in a hot bottle for cold winter nights in bed.

Different substances store different amounts of energy per kilogram for each C temperature rise.

To put it another way, different materials require different amounts of heat energy to raise a given amount of material by the same increase in temperature.

This is called the specific heat capacity and varies from material to material, whether it be a gas, liquid or a solid - its all to do with the nature and arrangement of the particles - atoms, ions or molecules.

Materials with a high heat capacity will release lots of heat energy when cooling down from a higher to a lower temperature.

The specific heat capacity (SHC or just c) of a substance is the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius.

This is a way of quantifying an increase or decrease in a material's thermal energy store.

The formula for expressing the amount of heat transferred between energy stores is given by the equation.

change in thermal energy store (J) = mass (kg) x specific heat capacity (J/kgoC) x change in temperature (oC)

∆E = m x c x ∆θ

E = energy transferred in Joules (change in thermal energy)

m = mass of material in kilograms kg

c = SHC = specific heat capacity J/kgoC,

θ = ∆T = temperature change in Celsius oC

The specific heat capacity of water is 4180 J/kgoC (Joules per kilogram per degree),

this means it takes 4180 J of heat energy to raise the temperature of 1 kg of water by 1oC.

See 2.2 for Worked out practice questions involving specific heat ....

... where you have to use the formula and correct units described above and you MUST be able to rearrange the equation.

The amount of energy stored in (transferred to) or released from a system as its temperature changes can be calculated using the above equation.

Other specific heat capacity values (J/kgoC):

ice 2100, aluminium 902, concrete 800, glass 670,  steel 450, brass 380, copper 385, lead 130

Because each material has a different heat capacity, although you can heat the same mass of substance from one temperature to another, you cannot assume they store the same amount of thermal heat energy per kilogram.

The materials with the highest heat capacity will store the most thermal energy per kilogram for the same increase in temperature - they are effectively a more concentrated thermal energy store.

Conversely, when allowing materials to cool, the materials with the highest specific heat capacity will release more thermal energy per kilogram for the same decrease in temperature.

Be able to evaluate different materials according to their specific heat capacities.

The heat specific heat capacity in simple terms is how much energy (J) is needed to heat a specific mass (1 kg) by one degree oC.

Examples may have studied include the use of water, which has a very high specific heat capacity, oil-filled radiators and electric storage heaters containing concrete or bricks.

 

INDEX for my physics notes on specific heat capacity


Keywords, phrases and learning objectives for specific heat capacity - definition and an equation for heat transfer

Be able to explain and define what we mean by the specific heat capacity of a materials

Be able to know and use the formula for energy transfer in calculations  involving specific heat capacity i.e. know how to use the equation ∆E = m x c x ∆θ and be able to rearrange it too.


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