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Heat and Temperature

Section 1: Temperature

•What does temperature have to do with energy?

•What three temperature scales are commonly used?

•What makes things feel hot or cold?

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Intro: Discussion

A person from Seattle tells his friend from Florida that the weather in Seattle is somewhat warm. When the friend arrives for a visit, he finds that he is uncomfortably cool wearing the shorts he packed. What would be a more effective way for the person from Seattle to explain the weather?

Temperature

- The degree of “hotness” or “coldness” of an object,

- related to the average kinetic energy of an object’s atoms or molecules

•What makes something hot?•Particles that make up matter are in constant motion

•They have kinetic energy

•When you heat something the particles move faster So, what kind of energy

does temperature measure?

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Kinetic Energy

•transferred as particles collide

•Particles are always in motion no matter what state they are in.

Kinetic Energy Temperature

What is happening to the coffee?What else is being affected?

• Thermometers- mechanical or electrical device for measuring temperature.

• Early thermometer was invented by Galileo.

• How thermometer works:• Most materials expand when heated.

• Liquid thermometers have a large bulb hooked to a narrow tube.

• When the liquid expands it rises up the tube.

• Won’t work if too hot or cold

Measuring Temperature

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•Metals expand when heated

•Different metals expand different amounts

•Hook two metals together to make a thermometer that turns

•Bimetallic Thermometers•Used in thermostats

IronCopper Hot

Cold

Measuring Temperature

Limits of Temperature

-Upper Limit

- No upper limit exists.

- Plasma found in starts= millions of degrees C

-Lower Limit

- Definite limit called absolute zero.

- Molecules will slow down SO much, they will essentially stop

moving

- Out of energy, so they can’t get any colder.

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•Three different scales:1. Fahrenheit- the one we use

2. Celsius- metric standard

3. Kelvin- starts at absolute zero but same degree size as Celsius

FahrenheitFahrenheitFahrenheitFahrenheit scale.In the US we know at 32 ⁰F water freezes and 212 ⁰F water boils.

Temperature Scales

- The absolute temperature

scale is called the Kelvin

scale.

-Absolute zero is 0 K.

-The melting point of ice is

273 K, and the boiling point

of water is 373 K.

-There are no negative

numbers on the Kelvin

scale.

Measuring Temperature

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0⁰C 32⁰F 273 K

0⁰C 32⁰F 273 K

100⁰C 212⁰F 373 K

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0⁰C 32⁰F 273 K

100⁰C 212⁰F 373 K

-273⁰C -492⁰F 0 K

Notice anything else about the scales?

Heat, T and Energy Video

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Converting Temperature

Fahrenheit to Celsius 0C = (0F – 32)1.8

Celsius to Fahrenheit 0F = (1.8 x 0C) + 32

Celsius to Kelvin K = 0C + 273

1. Water freezes at 32°F, what is this in Celsius? In Kelvin?

0C = (0F – 32)1.8 0C = (320F – 32)1.8 0 0C K = 0C + 273 K = 0 0C + 273 273 KPractice Problems: Temperature Scales

2. Water boils at 100°C. What is this in Fahrenheit?

In Kelvin?

0F = (1.8 x 0C) + 320F = (1.8 x100 0C) + 32 0F = 180 + 32 212 0FK = 0C + 273K =100 0C + 273

373 K

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Practice Problems: Temperature Scales

3. Lead melts at 600 K, what is this in Celsius?

In Fahrenheit?

K = 0C + 2730C= K - 273 0C= 600 - 273 327 0C0F = (1.8 x 0C) + 320F = (1.8 x327 0C) + 320F = 588.6 + 32620.6 0F

4. Body temperature is 98.6°F, what is this in Celsius?

In Kelvin?

5. Methanol boils at 75°C, what is this in Fahrenheit?

Practice Problems: Temperature Scales

In Kelvin?

0C = (0F – 32)1.80F = (1.8 x 0C) + 32K = 0C + 273

K = 0C + 273

0C = (98.60F – 32)1.8 37 0CK =37 0C + 273 310 K0F = (1.8 x 75 0C) + 32167 0F

K =75 0C + 273 348 K

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Heat• We know that temperature is the hot or cold nature of

something (based on the kinetic energy of its molecules),

So what is heat? • Heat is the thermal energy transferred from one thing to Heat is the thermal energy transferred from one thing to Heat is the thermal energy transferred from one thing to Heat is the thermal energy transferred from one thing to

another due to a temperature difference. another due to a temperature difference. another due to a temperature difference. another due to a temperature difference. • Always moves from high

temperature to low temperature.

• Faster molecules (high

temperature) hit slower molecules

(low temperature) and speed them

up

- If you touch a hot stove,

thermal energy enters your hand

because the stove is warmer than

your hand. When you touch a

piece of ice, thermal energy

passes out of your hand and into

the colder ice.

Heat

Clip

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Section 14.2 Energy Transfer

•How does energy transfer happen?

•What do Conductors and Insulators do?

•What makes something a good conductor of heat?

Energy Transfer

Heat can be transferred 3 different ways:1. Conduction2. Convection3. Radiation

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Energy Transfer: Conduction

• Transferred of energy by direct contact• Works well in some solids, then liquids, and least in gases. • ConductorsConductorsConductorsConductors- materials that

allow heat to pass through them

• Most metals

• InsulatorsInsulatorsInsulatorsInsulators- materials that don’t let heat pass through them well

• Rubber, plastics, glass, air

Energy Transfer: Convection

• Transferring energy by moving fluids• Liquids and gases are fluids• When heated they expand, become less

dense• They rise, replaced by cooler denser fluids• Make a circular flow called a convection

current

Clip

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Convection Current

Hot Air Rises

Cool Air Sinks

Same in waterClipConvection Current

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Why is it windy at the seaside?

The third method of heat transfer

How does heat energy get

from the Sun to the Earth?There are no particles

between the Sun and the

Earth so it CANNOT

travel by conduction or

by convection.

?RADIATION

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Energy Transfer: Radiation

• Energy transferred by electromagnetic waves• Ex: infrared radiation, visible light, ultraviolet

rays• Can travel through empty space• When wave hit object they make the molecules

move faster.

Radiation

Radiation travels in straight lines

True/False

Radiation can travel through a vacuum

True/False

Radiation requires particles to travel

True/False

Radiation travels at the speed of light

True/False

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Specific Heat

• Some materials heat up easily• Others require a large amount of energy

to change their temperature• Specific heat measures the amount of

energy required to raise the temperature 1 kg of a substance by 1 degree Kelvin

• Table on page 485 (Learn to use it)• Water has a high specific heat of

4186 J/kg K

Specific Heat

• The amount of energy required to change the temperature of substance identifies type of conductor.

• Metals have a low specific heat

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Calculating Specific Heat

• Energy = Specific heat x mass x change in temp• Energy (Q) = cm∆T• mass= kg• energy= Joules• Temperature= K• c= specific heat

Q

c m ∆T

Specific heat is the “price” to change 1 kg of a substance.

1. How much energy must be transferred as heat to 200 kg of water in a bathtub to raise the water’s temperature from 25 °C to 37 °C?energy = cm∆T

Practice Problems: Specific Heat

∆T= 37 °C- 25 °C ∆T= 12 Km= 200 kgc= 4,186 J/kg x Kenergy = ?

energy = 4186 x 200 kg x 12 kenergy = 10,000,000 J or 1.0 x 107 J

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2. How much heat does it take to change the temperature of 3 kg of water by 75 K?

Practice Problems: Specific Heat

energy = cm∆T∆T= 75 Km= 3 kgc= 4,186 J/kg x Kenergy = ?

energy = 4186 x 3 kg x 75 kenergy = 900000 J or 9.0 x 105 J

3. How much energy is needed to increase the temperature of 0.755 kg of iron from 283 K to 403 K?

Practice Problems: Specific Heat

energy = cm∆T∆T= 403 K -283 K∆T= 120 Km= 0.755 kgc= 449 J/kg x Kenergy = ?

energy = 449 x 0.755 kg x 120 kenergy = 40,700 J

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Adding energy either raises T or changes state, not both at the same time.

Temperature vs. Time

Section 3 Using Heat

•Laws of Thermodynamics

•First Law of Thermodynamics: the total Energy used in any process is conserved whether that energy is transferred as a result of work, heat, or both.

•Second law of Thermodynamics: energy transferred as heat always moves from an object at a higher temperature to an object at a Lower temperature.

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1st Law of Thermodynamics

-The word thermodynamics stems from Greek for

“movement of heat.”

• When thermal energy transfers as heat, it does so without

net loss or gain. The energy lost from one place is gained

by the other.

• Whenever heat flows into or out of a system, the gain or

loss of thermal energy equals the amount of heat

transferred. (You can’t get something from nothing

because energy and matter are always conserved).

First Law of Thermodynamics:First Law of Thermodynamics:First Law of Thermodynamics:First Law of Thermodynamics:

2nd Law of Thermodynamics

Energy transfers as heat always moves from a hot to cold

objects. (You can’t break even; you can’t return to the

same energy state because entropy always increases).

• Entropy is the tendency for natural

systems to become disorganized over

time.

• Any system that is left to itself will fall

apart. This means Entropy increases.

• Usable energy decreases with every

transfer or energy transformation.

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Thermodynamics

• Work increases average kinetic energy• Mechanical Process is the process in

which energy is transferred by work.

Bow drill

Heat Engines

Automobile Cylinder Animation

• Heat engines convert chemical energy to mechanical energy through the process of combustion.

• Internal combustion engines• Fuel burns inside the engine• A carburetor is the part of the engine in which liquid

gasoline becomes vaporized.• 4 strokes: Intake, Compression, Power, and exhaust