MR. PADILLAPHYSICS H

Temperature, Heat, and Expansion

Temperature

Quantity that tells how hot or cold something is compared with a standard.

Measured in degrees (°)We use devices such as a thermometer to

measure temperature.Temp. is proportional to the average kinetic

energy of an objects molecules.

3 Major Temperature Scales

Fahrenheit (F) – used only in the US Freezing = 32° Boiling = 212°

Celsius (C) – most widely used scale Freezing = 0° Boiling = 100°

To convert between F and C use the equation

FC 325

9

Kelvin (K)– scale used in scientific research Degrees are the same size as the Celsius degree Zero = absolute zero Absolute zero – lowest possible temperature, the

temperature at which all motion stops. Absolute zero about = -273.15° C.

K = C + 273.15 Freezing = 273K Boiling = 373K

Sample Problem 10A

What are the equivalent Celsius and Kelvin temperatures of 50.0oF?

Heat

Heat is the energy that transfers from one object to another because of temperature difference between them.

An object does not contain heat because heat is energy in transfer.

When objects touch to transfer heat they are in thermal contact.

Two objects in contact will reach the same temp. – they are in thermal equilibrium.

Which direction does heat move? Describe the heat

transfer in two cases: Hand on a burner ice in a glass of coke

Internal Energy

Every molecule contains potential and kinetic energy. The sum of all these energies is called internal energy.

When a substance takes in or gives off heat any of its internal energies can change.

The law of conservation of energy must be adapted to include this

ΔPE + Δ KE + Δ U = 0

The total energy remains the same

PEi + KEi + Ui =

PEf + KEf + Uf

Sample Problem 10B

A vessel contains water. Paddles that are propelled by falling masses turn in the water. The agitation warms the water and increases its internal energy. The temperature of the water is then measured, giving and indication of the water’s internal energy increase. If a total mass of 11.5 kg falls 1.3 m and all of the mechanical energy is converted to internal energy, by how much will the internal energy of the water increase? (Assume no energy is transferred as heat out of the vessel to the surroundings or from the surroundings to the vessel’s interior.)

Example

A .10kg ball falls 10.0m onto a hard floor, then bounces back up 9.0m. How much of it’s mechanical energy is transformed to the internal energy of the ball and the floor?

Let’s see how you do…PEi + KEi + Ui = PEf + KEf + Uf

PEi = PEf + Uf

(.10kg)(9.8 m/s2)(10.0m) = (.10kg)(9.8 m/s2)(9.0m) + Uf

Uf = 0.98 J

Measuring Heat

Heat is measured in calories.calorie – the amount of heat required to

raise the temperature of 1 g of water by 1°C

1000 calories = 1 kilocalorieThe unit for measuring a food,

the Calorie = 1 kilocalorie1 calorie = 4.186 Joules

Calculating Heat

Heat – Q measured in calories (cal) or Joules (J)

Specific Heat – c measured in cal/(g°C) or J/kg°C

Temperature – TCelsius – CKelvin – KFahrenheit - F

Specific Heat Capacity

An objects capacity for storing internal energy.

Quantity of heat required to raise the temperature of a unit mass of the substance by 1 degree.

Often referred to as specific heat.Water has a much higher specific heat than

most common materials.Table 10.4 in your book will give you some common heat

capacities.

Easy Problem

Calculating HeatQ = mcTHow many calories

are needed to raise the temperature of 1kg of water by 15°C?

c for water = 1cal/g°C

Q=(1000g)(1cal/g°C)(15°C)

Q = 15000 caloriesOr 15 kcal Or 15 Cal

How many joules is 15000 calories?

(15000cal)(4.186J/cal) = 63000 J

Harder Ex.

When heat is lost from one substance, an equal amount is gained by another substance(s).

What is the final temperature when a 3.0 kg gold bar at 99°C is dropped into 0.22kg of water at 25°C?

Heat lost = heat gained so…cgmgΔTg = cwmwΔTw

(1.29x102)(3.0kg)(99°C-Tf)=(4.186x103)(.22kg)(Tf-25°C)SolveTf = 47°C

Sample Problem 10C

A 0.050 kg metal bolt is heated to an unknown initial temperature. It is then dropped into a beaker containing 0.15 kg of water with an initial temperature of 21.0 oC. The bolt and the water then reach a final temperature of 25.0 oC. If the metal has a specific heat capacity of 889 J/kg x oC, find the initial temperature of the metal.

Change of Phase

Phases

Matter exists in three phases Solid Liquid Gas

Matter can change from one phase to another.

Changes involve a transfer of energy

Evaporation

Change in phase from a liquid to a gas that takes place at the surface of the liquid.

What are some examples of evaporation taking place?

Molecules at the surface gain enough kinetic energy to break free of the liquid

Become a vapor.

Condensation

Changing of a gas to a liquid.

Molecule loses kinetic energy.

It is a warming process.

Saturated: air contains its limit of water vapor for a temp.

Relative Humidity: indicates how much water vapor is in the air compared to the max for that temp.

Condensation in the Atmosphere

When particles move slowly they are more likely to stick together.

This is when condensation occurs.Clouds form when water vapor condenses

upon other particles or ions.Fog is basically a cloud near the ground.

Occurs when moist air near the ground cools.

Evaporation & Condensation Rates

When evaporation and condensation take place at the same time, at equal rates they are in equilibrium.

If there is more evaporation than condensation, a liquid is cooled.

If there is more condensation than evaporation, a liquid is warmed.

This heat is usually transferred to the environment.

Boiling & Freezing

Bubbles of gas from below the surface escape into the surrounding air.

The pressure in the bubbles must be great enough to resist the pressure of the surrounding liquid.

Liquid gives way to the solid phase

Foreign molecules may interfere with freezing, making it more difficult.

Water freezes together to form a 6 sided crystal structure.

Boiling & Freezing

At the same time…When water boils it takes energy, leaving the

remaining water cooler.Lessening the air pressure causes water to

boil at a lower temperature.With low enough air pressure, the water’s

surface will freeze while it is still bubbling.

Energy

When a material changes from a solid to a liquid to a gas it gains energy

When it changes from a gas to a liquid, to a solid to loses energy.

Energy & Phase ChangeHeat is required to change matter from one phase to another.

This heat is absorbed without changing the temp.

Water Latent Heat of Fusion –

80 cal/g (3.33x105 J/kg)

Latent Heat of Vaporization – 540 cal/g (2.26 x 106 J/kg)

Q = mcT + mLv + mLf

How much energy is released from 50 g of water at 20° freezing to -10° ice?

How much energy is required to raise 30g of water at 40°C to steam at 110°C?

Q = (50g)(1cal/g°C)(20°C) + (80cal/g)(50g) + (50g)(0.50cal/goC)(10oC)Q = 5250 cal

Q = (30g)(1cal/g°C)(60°C) + (540 cal/g)(30g) + (30g)(0.48cal/goC)(10oC)

Q = 18144 cal

Sample Problem 10D

How much energy is removed when 10.0 g of water is cooled from steam at 133.0 oC to liquid at 53.0 oC?

Assignment

Practice 10D – all

Thermal Expansion

When the temperature of a substance is increased, its molecules tend to move farther apart.

This results in expansion.Almost all forms of matter will expand

when heated and contract when cooled.

More Thermal Expansion

Normally, a gas will expand/contract more than a liquid and a liquid will expand/contract more than a solid.

The amount of expansion of a substance depends on its change in temperature.

Different substances expand at different rates.

Ex. Jar lid, thermostat, roads

Expansion of Water

Water at a temperature of 0°C contracts when heated.

Water is most dense at 4°C.

Ice, then, is less dense than water.

This is why ice floats.

Conduction

Materials that conduct (or transmit) heat well are called conductors.

This means energy is moving from one place to another.

Metals are the best conductors.If you touch a piece of metal and a piece of

wood that have been outside, which will feel colder? Which is really colder?

The metal will feel colder because it is a better conductor (heat moving out of you)

Wood is a poor conductor.Poor conductors are said to be good

insulators.In general liquids and gasses are good

insulators.

Convection

Convection occurs by currents in a fluid.

A fluid will move from one place to another, releasing energy as it goes.

Convection occurs in all fluids, liquid or gas.

Convection follows Archimede’s principle A substance that is less

dense will rise to the top.

Cold air will move toward areas of hot air

Convection is responsible for producing winds.

Wind

Convection currents can form in areas of uneven heating, such as a shoreline.

Rising Warm Air

As warm air rises, the atmospheric pressure around it decreases.

Since there is less pressure, it will expand, and cool.

When there is more pressure it will become compressed and heat up.

Ex. Blowing air, Candle

Radiation

The sun transmits heat through radiation. Radiation is neither convection or

conduction.Any energy that is transmitted by radiation

is called radiant energy.Radiant energy is in the form of

electromagnetic waves (radio, microwaves, light, x-ray)

Cooling

If you want to cool off a warm can of Coke, would it cool faster in the fridge or in the freezer?

Why?

Newton’s Law of Cooling

The rate of cooling of an object is approx. proportional to the temp. difference between the object and its surroundings.

Greenhouse Effect

What is the greenhouse effect?Short wavelengths of energy enter in and get

absorbed. Long wavelengths get emitted from inside but get reflected back in.

Energy the Earth radiates = terrestrial radiation

pg 387-389: 5-19 odd, 20, 34-40 even