Chapter 6: Thermal Energy

RECALL: Temperature is a measure of the average kinetic energy of the particles in the object.

As temperature increases, the average speed (and thus the average kinetic energy) of the particles increases.

Thermal energy is the sum of the kinetic energy and the potential energy of the particles in the object.

Note: potential energy is made up of chemical, electrical and nuclear potential energy and any other potential energy due to interactions between the particles.

Thermal energy, Temperature and Mass

100 g50 g

T = 25 °C T = 25 °C

Q: Is the thermal energy of both beakers the same?

A: No. While both beakers have the same average kinetic energy (same T), there are more particles in the 100 g sample of water so it must have more thermal energy.

More thermal E!

Heat: thermal energy that flows FROM a region of higher temperature TO a region of lower temperature.

As the temperature difference increases, the amount of heat transferred increases.

Hot Cool

VeryHot

Cold

HEAT

MOREHEAT

T1

T2

T = difference in temperature

T1T2 >

Specific Heat: The amount of heat needed to raise 1 kg of a substance by 1°C

Specific Heat: The amount of heat needed to raise 1 kg of a substance by 1°C

Water is often used as a coolant because of its high specific heat.

Q: What will heat up faster, water or iron?

Q: What will stay hot longer, copper or silver?

Iron: it has a lower specific heat

Copper: it has a higher specific heat

Calculating changes in thermal energy

Q = mCTQ = heat energy, units are Joules (J)m = mass, units are kgC = specific heat, units are J/(kg°C)T = change in temperature (Tf - Ti)

EX: A room containing 72 kg of air with a specific heat of 1010 J/kg°C is heated from 20°C to 25°C. How much thermal energy was used?

m = 72 kg C = 1010 J/kg°C T = 25°C - 20°C = 5°C

Q = mCT = (72 kg)(1010 J/kg°C)(5°C) = 363,600 J

Calorimetry: A technique used to measure the amount of heat transferred between substances.

0.025 kg of metal is heated to 100°C and then put into 0.1 kg of water that is at 20°C.The excess heat in the metal is transferred to the water warming it up to 23°C.

1. How much heat was transferred?

QH2O = mCT

QH2O =(0.1 kg)(4184 J/kg°C)(3°C)

AirWater

Metal pieces

ThermometerStirrer

QH2O = 1255 J

• If 1255 J of heat was gained by the water, that means that 1255 J of heat was lost by the metal (Conservation of Energy).

2. What is the specific heat of the 0.025 kg of metal?

Qmetal = 1255 JQmetal = 1255 JWhat is T?

• Metal started out at 100°C and ended up at the same temperature as the water, 23°C.

T = 100°C - 23°C = 77°CT = 100°C - 23°C = 77°C

Qmetal = mCT

1255 J = (0.025 kg) C (77°C)

1255 J = (1.93) C

C = 650 J/kg°CC = 650 J/kg°C

• Substitute the values into Qmetal = mCT and solve for C.

Thermodynamics: describe the inter-relationship between heat, thermal energy and work.

First law of thermodynamics: The increase in thermal energy (U) of a system is the sum of the work done on the system and the heat transferred to the system

U = Q + WU = Q + W

In closed systems, heat cannot enter or leave the system.

U = 0

In open systems, heat can be exchanged with the environment

U 0

Second law of thermodynamics: It is impossible for heat to flow from a cool object to a warm object unless work is done on the system.

HOT COLDHEAT

NO WORK = NO HEAT TRANSFER

Section 2: Transferring Thermal EnergyHeat

Conduction: heat is transferred through a material (glass, wood, metal etc.)

Gases: SLOW, good insulators

Rate of heat transfer depends upon the material.

Metals: FAST, good conductors

Convection: transfer of thermal energy in liquids and gases by movement of warmer and cooler regions from place to place.

• The warm water is less dense than the cooler water around it so it rises.

• The warm water cools (through conduction) as it rises.

• The cooled water sinks back to the bottom.

Radiation: transfer of energy by electromagnetic radiation. No matter is needed for transfer to occur.

• When solar radiation strikes a light colored object, most of the radiation is reflected. The small amount that is absorbed causes a small temperature increase.

• When solar radiation strikes a dark colored object, most of the radiation is absorbed. This causes a larger temperature increase.

• Because the molecules are far apart, gases do not absorb much solar radiation.

Heating systems

Forced-Air Steam/hot water Radiators

Electric Heating

Passive Solar Heating

Active Solar Heating

Limitations for both types:

• Doesn’t work at night • Doesn’t work well on cloudy days

Heat Engine: a device that converts heat into work

Example: An internal combustion engine

• Chemical energy of gasoline is converted into heat, which causes gases to expand and push (a force) against the piston heads. When the pistons move under the force of the expanding gases, work is being done.

• Only about 25% of the heat generated by the engine is converted into work. The rest is transferred out to the surroundings.

Q: Is an internal combustion engine a closed or open system?

A: Heat is transferred to the surroundings, so it is an OPEN system.

Heat movers: do work in order to move heat from a COLD area to a WARMER area.

Example: A refrigerator

• Electrical energy is used to compress the coolant into a liquid.

• The liquid coolant absorbs some of the the thermal energy from the inside of the refrigerator so it can expand into a gas. This causes the inside of the refrigerator to cool.

B

A

Air conditioners and heat pumps are also heat movers.Air conditioners and heat pumps are also heat movers.