1

Physics 1

Thermal Physics

2

What do you think? know?

1. Why does popcorn pop?

2. On a camping trip, your friend tells you that fluffing up a down sleeping bag before you go to bed will keep you warmer than sleeping in the same bag when it is still crushed. Why?

3

3. Why is it difficult to build a fire with damp wood?

4. Why does steam at 100oC cause more severe burns than liquid water does at 100oC?

4

5. Until refrigerators were invented, many people stored fruits and vegetables in underground cellars. Why was this more effective than keeping them in the open air?

6. In the past, when a baby had a high fever, the doctor might have suggested gently sponging off the baby with rubbing alcohol. Why would this help?

5

7. Why does water expand when it freezes?

8. Why, during the final construction of the St. Louis arch, was water sprayed on the previous sections as the last section was put in place?

6

1.Relate temperature to the kinetic energy of atoms and molecules

• Temperature scales• The Celsius scale is based on the temperatures at which

water freezes and boils. 0°C is the freezing point of water, and 100° C is the boiling point. Room temperature is about 20° C, a hot summer day might be 40° C, and a cold winter day would be around -20° C.

• To convert between Fahrenheit and Celsius, use these equations:

7

• Convert 72 oF to oC

• C = 5/9 (F-32)

• C = 5/9 (72-32) = 22oC

• Convert -10 oC to oF

• F = 9/5 C + 32

• F = 9/5(-10) + 32 = 14oF

8

Temperature degree scales comparison 

                                                                                                                                                      

Objective 3:

Temperature Scales

9

Celsius to Kelvin: T = Tc + 273.15

Problem:

1. The lowest outdoor temperature ever recorded on Earth is -128.6 o F., recorded at Vostok Station, Antarctica, in 1983. What is this temperature on the Celsius and Kelvin scales?

Answers: -89.22oC, 183.93 K

10

• Obj. #2 - Hotter temperature means

• – more heat present in a substance

• – the faster the molecules of the substance move.

• Obj #5 – Relate heat and temperature

• Heat units: calorie or joule (amount of heat energy present in a substance)

• Temperature units: degree (proportional to heat energy present in a substance)

11

2.Describe changes in temperatures of two objects reaching thermal equilibrium

• The temperature of the hotter substance will decrease. The temperature of the colder substance will increase. Each change will stop when the temperatures are the same – thermal equilibrium. In other words, thermal energy travels from hot to cold.

• Obj. #4 - Heat energy can be transferred by– Convection (motion of fluid),

conduction (touching), or radiation (electromagnetic waves)

12

ConvectionHeat transfer in fluids generally takes place via

convection. Convection currents are set up in the fluid because the hotter part of the fluid is not as dense as

the cooler part, so there is an upward buoyant force on the hotter fluid, making it rise while the cooler, denser,

fluid sinks. Birds and gliders make use of upward convection currents to rise, and we also rely on

convection to remove ground-level pollution.

ConductionWhen heat is transferred via conduction, the substance itself does not flow; rather, heat is transferred internally, by vibrations of atoms and molecules. Electrons can also carry heat, which is the reason metals are generally very good conductors of heat.

13

RadiationThe third way to transfer heat, in addition to convection and conduction, is by radiation, in which energy is transferred in the form of electromagnetic waves. More about electromagnetic waves in a lot more detail in a later chapter; an e-m wave is basically an oscillating electric and magnetic field traveling through space at the speed of light.

14

Specific heat capacity

The amount of energy that must be added to raise the temperature of a unit mass of a substance by one temperature unit.

• Units: J/kg K

• For Water: 4180 J/kg K

• For Aluminum: 897 J/kg K

• Which one heats faster?

15

This table is on page 318.

16

Heat Transfer Q = mCΔT = mC (Tf – Ti)

• Heat Transfer

• Q = mCΔT = mC (Tf – Ti)

• Q, quantity of heat in joule

• m, mass of substance in kg

• c, specific heat for water in 4180 j/kg K

• t, temperature in Celsius

17

See table 12-1 on page 318

Find the amount of heat needed to change the temperature of 5.0 g of liquid water from 8.0oC to 100oC.

Q = mct = .005kg(4180 j/kgoC) (92oC) = 1.9 x 103 j

Again,

specific heat is the amount of heat necessary to change one kg of a substance 1 degree Celsius or Kelvin.

18

• 12/3 When you turn on the hot water to wash dishes, the water pipes have to heat up. How much heat is absorbed by a copper water pipe with a mass of 2.3 kg when its temperature is raised from 20.0oC to 80.0oC? (Cpof Cu = 390J/kgoC )

• Q = mcpt

• Q = (2.3kg)(390J/kgoC)(60.0oC)

• Q = 53820 J or 5.4x104J

19

20

12/4b