CH. 10 Sections 1-3CH 11

Temperature andThermal Physics

Temperature Scales

CelsiusFahrenheitKelvinRankine

Celsius Scale

Kelvin Scale

SI unit of temperature is the Kelvin.

Unlike Celsius and Fahrenheit, the Kelvin scale cannot go negative.

0 K is referred to as Absolute Zero.

We will later see that absolute zero is also related to a gas of particles having zero kinetic energy.Absolute Zero, or 0 K, is the related to having a perfect vacuum.

To convert from Celsius (TC) to Kelvin (T):T = TC + 273.15

Or from Kelvin (T) to Celsius (TC):TC= T - 273.15

In Kelvin:Water freezes at 273.15 K and boils at 373.15 K.

One very useful characteristic is that one degree change in Kelvin is the same as one degree change in Celsius.

Thermal ExpansionAs the temperature of a substance increases, the volume increases.

Atoms are separated from each other by some distance. As the temperature increases, this separation increases. Thus the entire object must expand as temperature increases.

The object expands in all dimensions.

This is important to consider when building structures such as bridges, building, railroad tracks,…Need to use thermal expansion joints to compensate for the changes in length.

See table 10.1 for some values of α

Thermostat example shows an application.

All dimensions of an object increase, when the object is heated.

Think of a metal washer. Both the outside circumference and the inner circumference increase when it is heated.

Take a glass jar with a lid where α for the lid is greater than α for the jar.

If the lid is stuck, you can loosen it by running it under hot water.

https://www.youtube.com/watch?v=ne8oPFTM_AU

Another potential problem caused by thermal expansion

The inside surface of the glass heats and expands.The outside surface is cooler and expands less.

Example: As the oceans get warmer from climate change, the volume of water in the oceans increases. This contributes to rising sea levels.

Think of possible consequences if ice sank…Figure 10.11

Chapter 11Energy in thermal processes

Heat is the transfer of energy between an object and its environment, due to temperature differences between them.

Units of heat (units of energy)CalorieJoule

1 cal = 4.186 J

Definition of a calorieThe calorie is defined as the energy necessary to raise the temperature of 1 gram of water from 14.5 to 15.5 degrees Celsius.

Related heat to mechanical energy or work.By converting an amount of heat into the amount of mechanical work that is produced, we can relate the calorie to the Joule.Defining the calorie in terms of Joules: 1 cal = 4.186 J

Water

Water has a relatively large specific heat.Table 11.1

CWater is almost 5 times as large as CAl.

It takes almost 5 times as much energy to change the temperature of a mass of water than to change an equal mass of aluminum by the same temperature difference.

For example: Mixing hot aluminum with colder water. By finding the temperature changes of the two substances, the specific heat of aluminum can be found.

Lead and other metals, can also go from the solid phase to the liquid phase. However, this happens at a much higher temperature.

Latent HeatThe energy, Q, needed to change the phase of a pure substance is:

𝑄𝑄 = ±𝑚𝑚𝑚𝑚m = massL = latent heat (Depends on the nature of the phase change and the substance.) Table 11.2

Phase change between solid and liquid phases: L = Lf

Lf = latent heat of fusionPhase change between liquid to vapor phases: L = Lv

Lv = latent heat of vaporization

𝑄𝑄 = ±𝑚𝑚𝑚𝑚

The plus or minus sign depends on which direction the phase transition is going.

For water (ice) to melt, the water must absorb heat.

When water freezes (liquid water turns to ice), the water loses heat.

1st Raise the temperature of the ice to 00C.2nd Melt the ice.3rd Raise the temperature of the water to 1000C.4th Change the liquid water to steam (vaporize).5th Heat the steam to the final temperature.

Types of Thermal Energy Transfer3 ways for thermal energy transfer to occur.

1st Thermal conduction, when two objects at different temperatures are in physical contact with each other, energy will be transferred from the hotter object to the colder object.

Rate of thermal conduction depends on the thermal conductivity of the material involved and the surface area of contact.

Table 11.3Example: A pot holder has poor thermal conductivity, so it keeps your hand from being burnt by a hot pot.

ConvectionConvection is the transfer of energy by the movement or circulation of a substance.

Examples: Air in a convection oven circulates, carrying the heat to different locations in the oven.Water cooling an engine by forcing cold water to pass by hot engine parts.Currents in the ocean, transfer heat to and from different locations on Earth.

Energy from the Sun

The Earth is not in contact with the sun, so it can’t be heated by conduction.

There is no substance such as air in between to circulate the energy from the sun to the Earth, so the Earth is not heated by convection.

How is the Earth heated?

Radiation

Radiation - Objects radiate energy in the form of electromagnetic waves due to the thermal vibrations of their molecules.

Those electromagnetic waves travel through space and deliver the energy.

Another example is the heat you feel when you put your hands near a hot light bulb. Atoms on the bulb are vibrating, which produces the waves that transmit the energy.