Lesson 17: Heat Energy

Download here: Ontario Curriculum Expectations

Matter is made up of particles that are always in motion. The hotter a material is, the faster its particles will be moving. Thermal energy is a measure of the amount of kinetic energy an object possesses due to the random vibration and movement of molecules within the object. As the molecules vibrate more quickly, the temperature of the object increases. This extra kinetic energy tends to get dissipated to the object’s surroundings.

When there is a temperature difference between two places, thermal energy will flow from the hotter region to the cooler region.

This can occur by three different means:

1. Conduction
2. Convection
3. Radiation

Conduction, Convection, and Radiation:

Conduction is aprocess by which heat is transferred from one substance to another through atomic collisions.  As the atoms collide with each other, heat energy is passed from one atom to neighbouring atoms. Conduction occurs mainly in solids.

Convection is the process by which heat is transferred by the atoms themselves as they move from hotter regions to cooler regions. Convection occurs mainly in gases and fluids.

Radiation is the process by which energy is transferred by electromagnetic waves.  Anything that is hot or warm is able to radiate heat. If you place your hand above, but not touching, a red-hot stove element, your hand will be warmed as it absorbs some of the infrared radiation being emitted by the element.

Specific Heat Capacity:

It takes about 4 500 J of energy to increase the temperature of 1 kg of steel by 10 °C. In order to increase the temperature of 2 kg of steel, twice the mass, by the same amount, we would need about 9 000 J of energy, about twice as much energy. That all seems pretty logical! However, in order to increase the temperature of 1 kg of water by 10 °C we would need about 42 000 J of energy…almost ten times as much energy. This is because different materials have different abilities to absorb heat. Some materials, like steel, absorb and release heat quite easily. Other materials, like water, are much more resistant to temperature changes.

The specific heat capacity of a substance allows us to calculate how easily its temperature can be changed.

The specific heat capacity of a substance is the amount of energy that must be added to raise the temperature of 1.0 kg of the substance by one degree Celsius.

The equation

allows us to determine how much energy is needed to change the temperature of a substance.

Q = the amount of heat energy lost or gained, measured in Joules (J)

m = the mass of the substance, measured in kilograms (kg)

c = the specific heat capacity of the substance, measured in Joule per kilogram per degree Celsius ()

ΔT = the change in temperature of the substance, measured in degrees Celsius (°C)

Example 1

To make a cup of tea, you need to boil about 0.50 kg of tap water which has a temperature of 12 °C. If the heat capacity of the water is 4 200 J/kg·°C, how much heat energy is needed to bring the water to a boil?

Given:

m = 0.50 kg
ΔT = 100 °C – 12 °C = 88 °C
c = 4 200 J/kg·°C

Required:

Q = ?

Solution:

Q = (0.50 kg)(4 200 J/kg·°C)(88 °C)
Q = 1.85 x 10⁵ J
It will require 1.85 x 10⁵ J of energy to boil the water for a cup of tea.

Example 2

You have been handed a 0.45 kg sample of metal by your teacher and given the task of determining the heat capacity of the sample. You heat the sample to a temperature of 80 °C then quickly transfer it into a beaker that contains 0.70 kg of water at 15 °C; the specific heat capacity of water is 4 200 J/kg·°C. The highest temperature of the sample plus the water is 21 °C. What is the heat capacity of the sample of metal?

Given:

m_metal = 0.45 kg
ΔT_metal = T₂ – T₁ = 21 °C – 80 °C = -59 °C
c_water = 4 200 J/kg·°C
m_water = 0.70 kg
ΔT_water = T₂ – T₁ = 21 °C – 15 °C = 6 °C

Required:

Q_water = ? (to get Q_metal)
Q_metal = ? (to get c_metal)
c_metal = ?

Solution:

Find the heat energy gained by the water.

Q = (0.70 kg)(4 200 J/kg·°C)(6 °C)
Q = 17 640 J
The heat energy gained by the water will equal the heat energy lost by the metal.
Q_metal = -Q_water
Q_metal = -17 640 J


c_metal = 664 J/kg·°C
The specific heat capacity of the metal is 664 J/kg·°C.

In this multimedia activity you will conduct an experiment to determine the specific heat capacity of a substance. Be sure to return here when you are finished.

Changes of State

Evaporation: In a liquid, the hottest particles are moving the fastest. When these fast moving particles approach the surface of the liquid, they are likely to break free of the liquid and evaporate leaving the slower moving cooler particles behind. This lowers the average energy of the liquid left behind, which becomes cooler. This is the same process that occurs when you sweat.

The graph below shows the heating curve for water. In section A of the graph, the temperature is below 0 °C and the water is in the form of a solid, or ice. As more thermal energy is added the temperature rises. When the temperature reaches 0 °C (section B), it stops rising. All of the additional added thermal energy is being used to melt the ice. It changes state from solid (ice) into liquid (water). During section C, the water remains a liquid and continues to get hotter.

In section D, the temperature stops rising again. Here, all of the additional added thermal energy is being used to evaporate the water. It changes state from liquid (water) into gas (water vapour, or steam). In section E, the temperature of the water vapour continues to rise.