Physics: Ch. 10 Heat

10-1 Temperature and Thermal Equilibrium We can hold a “hot” object and a

“cold” object with our hands and describe the temperature as either “hot” or “cold.”

Our hands serve as qualitative indicators of temperature. However, this also depends on the temperature of our hands. This same object may feel hot or cold.

Quick Lab pg. 358 Sensing Temperature

Determining an object’s temperature with precision requires a standard definition of temperature and a procedure for making measurements that establish how “hot” or “cold” objects are.

Adding or removing energy usually changes temperature

Consider an electric range stove

A substance’s temperature increases as a direct result of added energy among the particles

Table 10-1: Different forms of energy

1. Translational2. Rotational3. Vibrational

Internal Energy(U)-energy associated with atomic motion and is proportional to the substance’s temperature. The energy of a substance due to the random motions of it component particles and equal to the total energy of those particles.

For an ideal gas, internal energy only depends on temperature. For gases with 2 or more atoms, as well as liquids and solids, other properties contribute to the internal energy.

Comparing Temperature and Internal Energy Compare the density, color,

temperature, and internal energy of a glass of milk at 20°C with these same properties of half a glass of milk

Do any of these properties change from full to half a glass of milk?

Demo 1: Temperature and Internal Energy Place one drop of food coloring in

each glass of cold and hot water. Describe what happens

Thermal equilibrium The state in which two bodies in

physical contact with each other have identical temperatures

Ex: a can of warm fruit juice in a large beaker of cold water

Thermal equilibrium is the basis for measuring temperature with thermometers. A thermometer in contact with an object measures the temperature when the mercury stops rising or falling. They are in thermal equilibrium with each other.

Conceptual Challenge pg. 360

Thermal expansion For solids, liquids, or gases,

increasing the temperature also increases its volume.

Why are there gaps several centimeters wide in concrete segments of a bridge?

Coefficient of volume expansion-indicates expansion characteristics Different substances have different

amounts of expansion for a temperature.

Gases have large values; liquids have much smaller values; solids have the smallest values

Ex: liquids in solid containers expand more than the container

Measuring Temperature In order for a device to be used as a

thermometer, it must make use of a change in some physical property that corresponds to changing temperature, such as volume of a gas or liquid, or the pressure of a gas at constant volume.

Ex: expansion of volume of mercury

Calibrating a thermometer Thermometer (thin, unmarked glass

tube of a liquid) Equilibrium with ice and water=ice

point (0°C) Equilibrium with steam and

water=steam point (100°C) Divide the distance between the

points into equally spaced degrees

Celsius-Fahrenheit Temperature Conversion

TF = 9/5 TC + 32.0

Celsius-Kelvin Temperature Conversion

T = TC + 273.15

0 K=absolute zero

Temperature conversion Sample problem 10A What are the equivalent Celsius and

Kelvin temperatures of 50°F?

Ch. 10-2: Defining Heat Energy is transferred from warm objects

to colder objects because of a difference in temperatures.

Ex: can of juice in a beaker of water (fig. 10-7)

The energy is heat. Heat is transferred from the higher

energy object (hotter) to the less energy object (colder)

How is the energy transferred at the atomic level? Fig. 10-8 The average kinetic energy of the juice

molecules is the highest. They transfer energy to the can and then the can transferes energy to the water molecules surrounding it.

The water molecules’ energy will increase while the juice molecules’ energy will decrease until equilibrium is reached.

Energy can travel both ways.

Thermal equilibrium may be understood in terms of energy exchange between two objects at equal temperature.

Fig. 10-9 Equal Temperature Energy transferred from the can to the

water is the same as the energy transferred from the water to the can.

The net energy transferred between the objects = zero

The amount of energy transferred depends on the difference of temperatures.

The greater the temperature difference between two objects, the greater the amount of energy that is transferred between them as heat.

Quick Lab Explanation The nerves in our hands detect the

energy change passing through the skin of our hands.

If you place a hand in cold water, energy is transferred from the hand to the cold water. When the hand is placed in a higher temperature water (warm), energy is transferred from the water to the cool hand. The energy into the skin causes the water to feel hot.

Likewise, the hand that has been in hot water gains energy from the water. The loss of energy to the lukewarm water makes that water feel cold.

Units of Heat-Table 10-3 Because heat is a form of energy, all heat

units can be converted to joules, the SI unit for energy.

PE=Potential Energy KE=kinetic energy U=Internal Energy W=Work Q=Heat

Heat and Work Hammer a nail into wood. Pry the nail

loose from the wood and touch the side of the nail. It should feel warm.

Work is done to the nail to pull it out of the wood. The nail encounters friction with the wood and most energy to overcome this friction is transferred to internal energy. The internal energy increase raises the temperature of the nail.

Increasing Internal Energy Friction In solids, deforming their structureEx: stretching a rubber band or

bending a piece of metal

Demo 2-Show the conversion of work into internal energy

QuickLab pg. 368

Total Energy is conserved Not all energy from work is transferred

into mechanical energy and not all kinetic energy in inelastic collisions remains kinetic energy.

Some energy is absorbed by objects as internal energy (the nail in the wood)

If internal energy is taken into account, the total energy is a universally conserved property.

Conservation of Energy ΔPE + ΔKE + ΔU = 0

Sample Problem-Conservation of Energy pg. 369

Conservation of Energy A 0.10 kg ball falls 10.0 m onto a

hard floor and then bounces back up to a 9.0 m. How much of its mechanical energy is transformed to the internal energy of the ball and the floor?

10-3 changes in temperature and phase We will explore a property of all

substances that causes their temperatures to vary by different amounts when equal amounts of energy are added to or removed from them.

Ex: why the water in a swimming pool is cool on a hot, sunny day

This property affects the motion of atoms and molecules in a substance, which determines how much the substances’ temperature changes for a given amount of energy added or removed

Specific heat capacity-the energy required to raise the temperature of 1 kg of a substance by 1ºC at constant pressure

Specific Heat Capacity at constant pressure

Cp = Q

mΔT

When temperature increases, T and Q are taken to be positive, which corresponds to energy transferred into the substance.

When the temperature decreases, T and Q are negative and energy is transferred from the substance.

Table 10-4 Specific Heat Capacities Specific heat capacities depends on

phase Ex: water, ice, and steam all have

different heat capacities

Calorimetry By the equation, we need to find the

mass, temperature change, and energy transferred as heat to find the specific heat capacity of a substance

The measurement of heat is difficult

If a hot substance is placed in an insulated container of cool water, energy conservation requires that the energy that substance gives up must equal the energy absorbed by the water.

Qw = Qx

cpwmwΔTw = cpxmxΔTx

Calorimetry-a procedure used to measure the energy transferred from one substance to another as heat

Calorimeter-device used for making this measurement; a thermometer is also used for measuring the final temperature when the substances are at thermal equilibrium and a stirrer to ensure the uniform mixture of energy throughout the water

Fig. 10-12

Sample problem 10C

Calorimetry practice You are preparing to take a bath.

The cold water faucet supplies water at 20°C, and the water from the hot water faucet is 60°C. Each faucet has poured 25.0 kg of water into the tub. What is the temperature of the bath?

Latent Heat Look at the graph and chart on pg.

376 These show the temperature change

of 10 g of ice as it is heated from –25°C in the ice phase to steam above 125°C at atmospheric pressure

When substances melt, freeze, boil, condense, or sublime, the energy added or removed changes the internal energy of the substance without changing its temperature.

These changes in matter are called phase changes.

Heat is the energy that is exchanged between two objects at different temperatures or between two objects at the same temperature when one of them is undergoing a phase change

Phase changes involve potential energy between particles

If the particles are far enough apart, the bonds between them can break. The work needed to increase potential energy and break a bond is provided by collisions with energetic atoms or molecules as shown in fig. 10-14

New bonds can be formed if atoms or molecules are brought close together. This involves the collection of particles going from a high potential energy to a lower potential energy. This decrease involves a release of energy in the form of increasing kinetic energy of nearby particles.

Energy required to melt a substance goes into rearranging the molecules

Phase changes result from a change in the potential energy between particles of a substance.

Energy is added or removed from a substance undergoing a phase change and so the particles rearrange themselves to make up for the change in energy.

This occurs without a change in the average kinetic energy of the particles

For example, energy is absorbed if ice is melting and breaking the bonds of the solid. However, new bonds will form between the liquid molecules and release some (not all) energy again.

Fig. 10-15 The difference between the PE of the

broken bonds and the newly formed bonds is equal to the net energy added to the ice. (no energy is available to increase the kinetic energy of the molecules)

As a result, there is no increase in temperature of the ice-and-water mixture

Net energy=heat of fusion The energy per unit mass

transferred in order to change a substance from solid to liquid or from liquid to solid at constant temperature and pressure

The energy required to vaporize a substance mostly goes into separating the molecules, not increasing the kinetic energy of the molecules.

Fig. 10-16 The net energy added to the liquid

to vaporize it equals the difference in the potential energy of attraction between the particles of a liquid and the potential energy of attraction between the gas particles.

This is the heat of vaporization=energy per unit mass

The energy needed to vaporize a substance is higher than to melt the substance

Therefore, the heat of vaporization values are higher than the heat of fusion values

Latent heat=heat of fusion and the heat of vaporization

Latent heat = the energy per unit mass that is transferred during a phase change of a substance

Q = mL Table 10-6 Lf = heat of fusion

Lv = heat of vaporization

Sample problem 10D Heat of phase change How much energy is removed when

10 g of water is cooled from steam at 133°C to liquid at 53°C?

How much energy is needed to melt a 100 g sample of aluminum whose initial temperature is 20°C?

10-4 Controlling Heat Thermal conduction-the process by

which energy is transferred as heat through a material between two points at different temperatures.

Ex: heating a metal skillet The atoms gain energy and pass

energy along to their neighbors by collisions

Thermal conductors-substances that rapidly transfer energy as heat

Ex: metals Thermal insulators-substances that

slowly transfer energy as heat Ex: cork, ceramic, cardboard,

fiberglass, gases

Convection & Radiation Convection-displacement of cold matter

by hot matter, such as when hot air over a flame rises upward (also involves conduction & buoyancy)

The air particles are heated(conduction), causing expansion and the density to decrease. The warm air is then displaced by cold air.

Electromagnetic radiation-no transfer of matter in this type; objects radiate energy by wavelengths

Body Temperature Body Temp is around 37°C Body must work to maintain this temp in

very cold or very hot surrounding air temperatures

Proper insulation is needed to keep the cold out

Hypothermia slows down pulse, blood pressure, and respiration and could be fatal at 25.6°C

Insulating Materials for Cold The energy given off by the body

must be kept in to prevent hypothermia

An insulating material surrounds the body, such as air.

Clothing is made to provide two layers of clothing with air trapped in between

Evaporation aids in hot temperatures In hot climates, clothing is worn to

protect the skin from direct sunlight and prevent excessive loss of body water from evaporation while also cooling the wearer.

Heat exhaustion or heat stroke can result of body temperature too high

Allowing air flow over the skin allows any perspiration on the skin to evaporate, causing a great deal of energy to be given off to the gas phase

This causes the skin to cool Head wraps can help evaporate

perspiration during hot climates or help insulate during cold temperatures