in the 1700s, most scientists thought heat was a fluid called caloric

16.1 Thermal Energy and Matter

In the 1700s, most scientists thought heat was a fluid called caloric.

Count Rumford (Benjamin Thompson) supervised the drilling of brass cannons in a factory in Bavaria.

From his observations, Rumford concluded that heat is not a form of matter, but was related to the motion of the drill.


A drill is a machine that does work on the cannon. No machine is 100% efficient. Some work done by the drill is useful, but some energy is lost due to friction.

Heat flows from the cannon to a surrounding water because the cannon is at a higher temperature than the water.

•Heat: the transfer of thermal energy from one object to another because of a temperature difference.

• Heat flows spontaneously from hot objects to cold objects.

Work and Heat


• Temperature: a measure of how hot or cold an object is compared to a reference point.

• On the Celsius scale, the reference points are the freezing and boiling points of water.

• On the Kelvin scale, absolute zero is defined as a temperature of 0 kelvins.

•Temperature is related to average kinetic energy of particles due to random motions through space.

Temperature


As an object heats up, its particles move faster The average kinetic energy of the particles and the temperature increase.

• Heat can flow by the transfer of energy in collisions.

• Overall, collisions transfer thermal energy from hot to cold objects.

Temperature


• Thermal energy: total potential and kinetic energy of all the particles in an object.

• Depends on the mass, temperature, and phase (solid, liquid, or gas) of an object.

Thermal Energy


Mass: a cup of tea and a teapot full of tea can have the same temperature.

• The average kinetic energy of the particles is the same in the cup and the pot.

• There is more thermal energy in the teapot because it contains more particles.

Thermal Energy


Temperature: compare a cup of hot tea with a cup of cold tea.

• In both cups, the tea has the same mass and number of particles.

• The average kinetic energy of particles is higher in the hot tea, so it has greater thermal energy.

Thermal Energy


Thermal energy depends on mass and temperature.

A. The tea is at a higher temperature than the lemonade.

B. The lemonade has more thermal energy because it has many more particles.

Thermal Energy


• Thermal expansion: increase in the volume of a material due to a temperature increase.

•Particles of matter move farther apart as temperature increases.

Thermal Contraction and Expansion


If you take a balloon outside on a cold winter day, it shrinks in a process of thermal contraction.

– As temperature decreases, the particles of the air inside the balloon move more slowly, on average.

– Slower particles collide less often & exert less force.– Pressure decreases and the balloon contracts.– If you bring the balloon inside, it expands.

• Gases expand more than liquids and liquids usually expand more than solids.

Thermal Energy


As temperature increases, the alcohol in a thermometer expands, and its height increases in proportion to the increase in temperature.

In an oven thermometer, strips of steel and brass expand at different rates as the coil heats up. The coil unwinds, moving the needle on the temperature scale.

Thermal Energy


• Specific heat: the amount of heat needed to raise the temperature of one gram of a material by one degree Celsius.

• The lower a material’s specific heat, the more its

temperature rises when a given amount of energy is

absorbed by a given mass. • When a car is heated by the sun, the temp. of the metal

door increases more than the temp. of the plastic

bumper.

•The iron in the door has a lower specific heat than the

plastic in the bumper.

Specific Heat


Specific Heat


In this formula, heat is in joules, mass is in grams, specific heat is in J/g•°C, and the temperature change is in degrees Celsius.

Specific Heat


Calculating Specific Heat

An iron skillet has a mass of 500.0 grams. The specific heat of iron is 0.449 J/g•°C. How much heat must be absorbed to raise the skillet’s temperature by 95.0°C?

Specific Heat


1. How much heat is needed to raise the temperature of 100.0 g of water by 85.0°C?

Answer:

Q = m * c * ∆T = (100.0 g)(4.18 J/g•°C)(85.0°C)

= 35.5 kJ

Specific Heat


2. How much heat is absorbed by a 750-g iron skillet when its temperature rises from 25°C to 125°C?

Answer:

Q = m * c * ∆T = (750 g)(0.449 J/g•°C)(125°C – 25°C)

= (750 g)(0.449 J/g•°C)(100°C)

= 34 kJ

Specific Heat


3. In setting up an aquarium, the heater transfers 1200 kJ of heat to 75,000 g of water. What is the increase in the water’s temperature? (Hint: Rearrange the specific heat formula to solve for ∆T.)

Answer: ∆T = Q / (m x c)= 1,200,000 J/(75,000 g × 4.18 J/g•°C)

= 3.8°C

Specific Heat


4. To release a diamond from its setting, a jeweler heats a 10.0-g silver ring by adding 23.5 J of heat. How much does the temperature of the silver increase?

Answer: ∆T = Q / (m x c)= 23.5 J/(10.0 g × 0.235 J/g•°C)

= 10.0°C

Specific Heat


5. What mass of water will change its temperature by 3.0°C when 525 J of heat is added to it?

Answer: m = Q / (∆T x c)= 525 J/(3.0°C × 4.18 J/g•°C)

= 42 g

Specific Heat


• A calorimeter is an instrument used to measure changes in thermal energy.

• Heat flows from a hotter object to a colder object until both reach the same temperature.

• According to the law of conservation of energy, the thermal energy released by a test sample is equal to the thermal energy absorbed by its surroundings.

• The calorimeter is sealed to prevent thermal energy from escaping.

Specific Heat


A calorimeter is used to measure specific heat.

1) A known mass of water is added

2) The mass of the sample is measured.

3) The sample is heated, placed in the water, and the calorimeter is sealed.

4) The temperature change is measured.

5) Thermal energy absorbed by the water is calculated using the specific heat equation.

6) Since the same amount of thermal energy was given off by the sample, the specific heat of the sample can be calculated.

in the 1700s, most scientists thought heat was a fluid called caloric

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