11.1 Temperature, Heat, and Thermal Energy

Temperature, Heat, and Thermal Energy

​11.1

How is thermal energy transferred when you cook something?

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Focus Question

• ​Convection

• Radiation

• Specific Heat

• ​Thermal Conduction

• Thermal Equilibrium

• Heat

New Vocabulary

Review Vocabulary

​thermal energy

the sum of the kinetic and potential energies of the particles in a system

​Gas particles can collide with each other and the walls of their container, resulting in a transfer of energy

​Like macroscopic objects, particles can have kinetic and potential energy

Thermal Energy

Thermal Energy

• Because there are many particles and many collisions in a gas sample, it is convenient to discuss the total energy of the molecules and the average energy per molecule.

• The sum of the particles' energies is called thermal energy.

• The average energy per molecule is related to the temperature of the gas.

• A helium-filled balloon is kept inflated by the repeated pounding from helium atoms on the balloon wall.

• The size and the temperature of the balloon are affected by the average kinetic energy of the helium atoms.

• If you put a balloon in sunlight, energy absorbed from the sunlight makes each of the helium atoms move faster in random directions and bounce off the rubber walls of the balloon more often, causing the balloon to expand.

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​If you refrigerate a balloon, it shrinks, because the particles are moving more slowly. The refrigerator has removed some of their thermal energy.

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• On average, a particle in a hot object has more kinetic energy than a particle in a cold object. This does not mean that each of the particles that compose an object has the same amount of energy.

• Temperature is the measure of the average kinetic energy of the particles in the object.

Thermal Energy and Temperature

Multiple Choice

There are four beakers on a lab table. Each beaker contains 50 ml of water. The temperature of the water in each beaker is shown. Which beaker contains water molecules with the highest average kinetic energy?

• 25°C

• 50°C

• 75°C

• 100°C

• Temperature does not depend only on the number of particles that compose the object.

• The thermal energy of an object depends on both its temperature and the number of particles that make up that object.

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​Two muffins at the same temperature can have different thermal energies.

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Equilibrium and Thermometers

• Thermal conduction is the transfer of thermal energy that occurs when particles collide.

• As the particles in the glass of a thermometer gain more energy, they begin to give some of their energy back to the particles of the surroundings.

• At some point, the rate of transfer of energy between the glass and surroundings becomes equal.

At this point, the surroundings and the thermometer are said to have reached thermal equilibrium, the state in which the rate of energy flow between two objects is equal and the objects are at the same temperature.​

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Equilibrium

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Thermometers

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Multiple Choice

A bowl of hot soup is left on a kitchen counter for several hours. At which state will the soup, bowl, and surrounding air be?

• homeostatic equilibrium

• kinetic equilibrium

• thermal equilibrium

• conductive equilibrium

Objects with a wide variety of temperatures are present in the universe. Temperature does not appear to have an upper limit, but it does have a lower limit, known as absolute zero.

At absolute zero all thermal energy has been removed from the substance. There can be no temperature lower than -273.15°C or 0°K.

Heat and Thermal Energy Transfer

• The transfer of thermal energy, which occurs spontaneously from a hotter object to a cooler object, is called heat (Q).

• Heat, like energy and work, is measured in joules.

• If Q has a negative value, thermal energy has left the object; if Q has a positive value, the object has absorbed thermal energy.

• Recall that thermal conduction is the transfer of thermal energy that occurs when particles collide.

​Heat and Thermal Energy Transfer

• Heating caused by the motion of fluid in a liquid or gas due to temperature differences is called convection.

  • When heating a pot of water, thermal energy flows from the rising hot water and the descending cold water.

  • Thunderstorms and hurricanes are examples of large-scale convection.

​Heat and Thermal Energy Transfer

• Radiation is the transfer of energy by electromagnetic waves.

  • Radiation does not depend on the presence of matter.

  • The sun warms the Earth from more than 150 million km away via radiation.

Multiple Choice

Which is NOT a way that thermal energy can be transferred?

• conduction

• convection

• irradiation

• radiation

Specific Heat

​Some objects are easier to heat than others. The specific heat of a material is the amount of energy that must be added to the material to raise the temperature by one temperature unit. In SI units, specific heat, represented by C, is measured in J/(kg·K). The heat (Q) required to change the temperature of an object depends on the mass, the change in temperature, and the specific heat of the substance.

In a simple calorimeter, a hot test substance and a

known volume of cold water are placed in an isolated system and allowed to come to thermal equilibrium. The ideal calorimeter has perfect insulation and does not transfer thermal energy to or from the outside.  

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​Measuring Specific Heat

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Multiple Choice

What is the device shown used for?

• measuring thermal energy changes

• mixing solutions

• measuring temperature

• maintaining constant temperature