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Chapter 10 Temperature and Heat

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Chapter 10Temperature and Heat

Are they the same? What causes heat?

How do we measure temperature?

What are we actually measuring?

- How do we measure temperature?
- Thermometer: Device with a physical property that changes with temperature and can be easily measured quantitatively.
- If two objects are in contact with one another long enough, the two objects have the same temperature (thermal equilibrium).
- Two or more objects in thermal equilibrium have the same temperature.
Zeroth law of thermodynamics.

- The first widely used temperature scale was devised by Gabriel Fahrenheit.
Water freezing point: 32F

Water boiling point: 212F

- Another widely used scale was devised by Anders Celsius.
Water freezing point: 0C.

Water boiling point: 100C

- Conversion between two scales:
- E1. An object has a temperature of 45C. What is its temperature in degree Fahrenheit?
- E2. The temperature of a winter day is 14F. What is the temperature in degree Celsius?

- The zero point on the Fahrenheit scale was based on the temperature of a mixture of salt and ice in a saturated salt solution.
- The zero point on the Celsius scale is the freezing point of water.
- Both scales go below zero.
- Is there an absolute zero?

- If the volume of a gas is kept constant while the temperature is different, the pressure will be different.

- Absolute Temperature Scale (Kelvin Scale)
- Example
Water freezing point: 0C =273.2 K.

Water boiling point: 100C = 373.2 K

- Steel has a lower specific heat capacity than water.

- specific heat capacity (c): the quantity of heat needed to change a unit mass (1 g) of the material by a unit amount in temperature (1 C).
It is a property of the material, determined by experiment.

- The specific heat capacity of water is 1 cal/gC

Table 10.1 Specific capacity of some common substances

SubstanceSpecific Heat Capacity (in Cal/g/C)

Water1.0

Ice0.49

Steam 0.48

Ethyl alcohol0.58

Glass0.20

Aluminum0.215

- When a material’s temperature is changed, we can calculate how much heat absorbed/released by the material:
Q = mcT

whereQ = quantity of heat

m = mass

c = specific heat capacity

T = change in temperature

Example: E6

How much heat is required to raise the temperature of 70g of water from 20C to 80C

Heat:

Heat is the energy that flows from one object to another when there is a difference in temperature between the objects. Heat is the average kinetic energy of atoms or molecules making up the system.

Temperature:

Temperature is an indication

of whether or not and in which

direction, the heat will flow

(Temperature is an indication

of the average of kinetic energy

of atoms or molecules).

- When an object goes through a change of phase or state, heat is added or removed without changing the temperature. Instead, the state of matter changes: solid to liquid, for example.
- The amount of heat needed per unit mass to produce a phase change is called the latent heat (L)
- The latent heat of fusion of water is 80 cal/g (Lf = 80 cal/g is 80 cal/g): it takes 80 calorie of heat the melt 1 g of ice at 0C to become water at 0C.
- The latent heat of vaporizationof water is 540 cal/g (Lv = 540 cal/g): it takes 540 calories of heat to turn one gram of water at 100 C into steam at 100 C.

Example Box 10.1

If the specific heat capacity of ice is 0.5 cal/gC°, how much heat would have to be added to 200 g of ice, initially at a temperature of -10°C, to (a) raise the ice to the melting point?(b) complete melt the ice?

- Benjamin Thompson (1753-1814) noticed that cannon barrels and drill bits became hot during drilling.
- Joule performed a series of experiments showing that mechanical work could raise the temperature of a system.
1 cal = 4.19 J

- First law of Thermodynamics.
Energy Conservation - In an isolated system, the total amount of energy, including heat, is conserved.

- Example (Box 10.2) :A hot plate is used to transfer 400 cal of heat to a beaker containing ice and water. 500 J of work are also done on the contents of the beaker by stirring.
a) What is the increase in

internal energy of the

ice-water mixture?

b) How much ice melts in

this process?

- Caloriesin Physics and in food:
- In Physics: 1 calories is the amount of energy needed to raise the temperature of 1 g of water 1 C.
- In food: 1 Calories is the amount of energy needed to raise the temperature of 1 kg water 1 C
1 Cal =1,000 cal

- Normal body maintenance uses up about 15 calories per day for each pound of body weight.
- You must consume about 3500 calories to gain a pound of weight.
- To burn off 500 calories you would have to run 5 miles, bike 15 miles, or swim for an hour.

- Three basic processes for heat flow:
- Conduction
- Convection
- Radiation

- Conduction: heat flow when in contact

- A metal block at room temperature will feel colder than a wood block of the exact same temperature. Why?

- The rate of heat flow depends on:
a) the temperature difference between the objects.

b) the thermal conductivity of the materials, a measure of how well the materials conduct heat.

- Radiation, heat energy is transferred by electromagnetic waves.
- can take place across a vacuum.

Convection: heat is transferred by the motion of a fluid containing thermal energy.

Example: Suppose that you have 100 g of water at

the temperature of 20 C, and you have 50 g of a

metal at the temperature of 100 C. If you put the

metal into the water, and the final temperature is

30 C when the mix of water and aluminum reaches

a thermal equilibrium, what is the specific capacity

of the metal?