Physics Unit 5: Heat and Temperature

Bellringer

1. In drawing 1, which bowl would feel warm to your hands? Which bowl would feel cool?

We use words like hot and cold, long and short, and heavy and light every day to describe the differences between things. In science, however, this is often not accurate enough and leads to confusion.

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Bellringer, continued

3. 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?

2. In drawing 2, which bowl would feel warm to your hands? Which would feel cool?

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TemperatureWhat does temperature indicate? Measurement of the average Kinetic

Energy of the moleculesNOT A MEASURE OF HEAT

Measuring Temperature• We use a Thermometer to

measure temperature– Thermometers rely on expansion – Most objects expand when their

temperature increases– Alcohol or Mercury

Thermometers use expansion and contraction to measure temperature.

• The warmer a liquid is, the more the particles move and thus the liquid expands (takes up more space) and rises up through the tube

• Liquid thermometers can only be used between certain temperatures, why?– Because the liquid will freeze at low temps– Because the liquid will boil at high temps

• So, some thermometers use a solid metal to measure temperature through expansion– Like a metal coil used in refrigerators

• Digital thermometers use electrical current to measure temperature.

Temperature Scales • There are three temperature scales1) Fahrenheit Scale: water boils at 212 oF and

freezes at 32 oF, normal body temp is 98.6 oF.2) Celsius Scale or Centigrade: like the metric

system, it is bases on powers of 10. - water boils at 100 oC and freezes at 0 oC.

3) Kelvin Scale (Science): water boils at 373 K and freezes at 273 K. (no degree symbol)- absolute zero (0 K): motion stops…the lowest possible temperature you can go. (-273 oC)

• Since the Celsius and Fahrenheit Scales can go below their zero mark, they have negative values.– This is why they are reported in degrees

• Since KELVIN starts at zero, and cannot go below that absolute zero, then you cannot have negative Kelvin– This is why Kelvin is not reported in degrees– Kelvin is always positive

Converting Temperature • Celsius to Kelvin: tK = t oC + 273

Ex: 23 oC to K

• Kelvin to Celsius: t oC = tK - 273

Ex: 338 K to oC

• Celsius to Fahrenheit: t oF = 1.8 (t oC) + 32

Ex: 33.5 oC to oF

• Fahrenheit to Celsius: t oC = (t oF – 32) Ex: 147 oF to oC 1.8

Bellringer

1. Why is it a bad idea to drink hot cocoa out of a tin cup? Explain the energy transfers on the atomic level.

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Bellringer, continued

2. What happens to your hand when you place it above a lighted candle? (Assume you are not touching the flame. Explain the energy transfers on the atomic level. Hint: Remember that warm air rises.)

3. When you sit near a fire, you can feel its warmth on your skin, even if you are in cool air. Does this sensation depend upon the fact that warm air rises?

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Temperature and Energy Transfer• When you hold an ice cube, the ice melts

because of the energy transfer. – The particles of your hand are moving faster than

those of the ice cube– The particles of your hand collide with the particles

of the ice cube and cause the particles of the ice cube to move faster

– When the ice cube particles move faster (higher kinetic energy) from this collision, the temperature of the ice cube rises and thus it melts.

• The energy transfer between particles of two objects due to a temperature difference between the two objects is called heat.

• The transfer of heat ALWAYS goes from higher temperature (faster moving) to lower temperature (slower moving)

• This happens until equilibrium is reached!• A seat may feel cool at first, but your body

will warm it up until they reach the same temperature (equal temp equilibrium)

• Heat gained = heat lost – Law of conservation of energy

Methods of Energy Transfer1. Conduction: involves objects in direct contact

– Fast moving molecule will collide with slow moving molecule, transferring energy

– Works for all 3 phases of matter• But gases are very poor conductors because the

particles are so far apart• Liquids are okay• Solids are best insulators, but vary by

substance– Heat Conductor: Substance that moves heat more

effectively (like metal skillets used to cook)– Insulator: Substance that will not conduct heat

well (like fiber glass insulation, wood on skillet handle)

Conductors and Insulators

• Any material through which energy can be easily transferred as heat is called a conductor.

• Poor conductors are called insulators.• Gases are extremely poor conductors.• Liquids are also poor conductors.• Some solids, such as rubber and wood, are good

insulators. • Most metals are good conductors.

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Methods of Energy Transfer, continued

Thermal Conduction• Conduction involves objects in direct contact.• Conduction takes place when two objects that are in

contact are at unequal temperatures.

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2. Convection: movement of warm fluids

–The fluid (air) transfers the heat–Molecules move in currents –Only in a fluid: (Liquid or Gas)–Heated portion speeds up and

becomes less dense, creating a current of heat

–Air currents are a result of convection

Methods of Energy Transfer, continued

Convection• Convection results from the

movement of warm fluids. • During convection, energy

is carried away by a heated fluid that expands and rises above cooler, denser fluids.

• A convection current is the vertical movement of air currents due to temperature variations.

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3. Radiation: does not require direct contact– Heat is transferred through space– Does not involve the movement of matter– Travels as waves– Electromagnetic radiation (Gamma, UV, Visible, X-

rays, Microwaves, Radio, Infrared,)

Measuring Heat• calories (cal) or joules (j) are used to

measure heat• calorie: Amount of heat needed to raise

the temperature of one gram of liquid one degree Celsius

• Since heat is related to kinetic energy, the unit of joules is often used

• 1 cal = 4.2 joules• Convert 350 cal to kJ

What Are Food Calories?• Calories are really

kilocalories• 2000 Cal diet is really

2,000,000 calories• We use capital C (kilo)

because it is easier

Specific Heat (s) :

Specific Heat (s) : amount of energy required to change the temperature of one gram of a substance 1 oC

• How well a substance conducts heat• Varies from one substance to another• Heat always travels from high concentration

to low concentration!!• Heat lost = Heat gained

• Water has a specific heat = 1 cal/goC or 4.184 J/goC– Water has the second highest specific heat capacity

of all known substances. So it requires high amounts of heat energy to raise water temperature.

– water also has a high energy/heat requirement for evaporation

• SIRON = 0.449 J/goC– Which would heat up faster, 5.00 grams of iron or

5.00 grams of water?– Which would cool down faster, 5.00 grams of iron

or 5.00 grams of water?– Which is a better thermal conductor?– Which is a better insulator?

Q = s x m x TQ = energy (heat) required (J) or (cal)s = specific heat capacity (J/goC) or (cal/goC)m = mass of the sample in gramsT = change in temperature in oC• A 2.8 g sample of a pure metal requires 10.1 J of

energy to change its temperature from 21 oC to 36 oC. What is the specific heat of the metal?

s = Q = 10.1 J = 0.24 J/goC m x T (2.8 g x 15oC)

Bellringer

1. You have learned that there is an energy change when a liquid evaporates. Will the area near the liquid get hotter or cooler as evaporation occurs? (Hint: Compare and contrast the molecular motion of particles as liquids and as gases.)

One extremely cold winter day, the thermostat in the science classroom was set too low and the room was cold. The science teacher did not have the right tool to reset the thermostat, so she made a thin cloth cover for the thermostat, wet it, and placed it over the thermostat. Soon the room was comfortably warm.

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Bellringer, continued3. Imagine a different situation in the same class, during the

week before summer vacation. This time, it is very hot outside, but the thermostat is set so high that the air conditioner does not come on. Which of the following might help the thermostat trigger the air conditioner more frequently? a. use another wet cloth on the thermostatb. point a fan at the thermostatc. wrap the thermostat in a dark cloth that has been sitting

by the windowsilld. wrap the thermostat in a dark cloth that has been kept

in the refrigeratore. redirect air from the air conditioner vent away from the

thermostat

Applications• Heating Systems:

– Work can increase ave kinetic energy, like when lighting a fire by using the friction of two sticks

– Our bodies act like a heating system to regulate our body temp to stay at 37 oC or 98.6 oF, we burn stored calories and nutrients to provide the energy we need to raise our temp in the cold.

– The sun can be used to heat a system• by cold-blooded animals to help maintain their

temp

Most heating systems use a source of energy to raise the temperature of a substance such as air or water.

Heating Systems

• The human body is a heating system. Some of the energy from food is transferred as heat to blood moving throughout the human body to maintain a temperature of about 37°C (98.6°F).

• In central heating systems, heated water or air transfers energy as heat.

• Solar heating systems also use warmed air or water.

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• In the solar system shown here, a solar collector uses panels to gather energy radiated by the sun.

• This energy is used to heat water that is then moved throughout the house.

• This is an active solar heating system because it uses energy from another source, such as electricity, to move the heated water.

Heating Systems, continuedChapter 13

• In a passive solar heating system, energy transfer is accomplished by radiation and convection.

• In this example, energy from sunlight is absorbed in a rooftop panel.

• Pipes carry the hot fluid that exchanges heat energy with the air in each room.

Heating Systems, continuedChapter 13

• When energy can be easily transformed and transferred to accomplish a task, such as heating a room, we say that the energy is in a usable form.

• After this transfer, the same amount of energy is present, according to the law of conservation of energy. Yet less of it is in a form that can be used.

• In general, the amount of usable energy always decreases whenever energy is transferred or transformed.

• Insulation minimizes undesirable energy transfers.

Heating Systems, continuedChapter 13

• Cooling systems: – Liquids can be evaporated or condensed to

allow for transfer of energy in either direction so as to cool or heat a system.

– Evaporation causes a cooling effect because gases are farther apart and thus cannot transfer energy as well through physical contact. Also particles gain energy as they evaporate, removing it from surroundings.

One example is an air conditioner. An air conditioner does work to remove energy as heat from the warm air inside a room and then transfers the energy to the warmer air outside the room.

Air ConditionerChapter 13

• In all cooling systems, energy is transferred as heat from one substance to another, leaving the first substance with less energy and thus a lower temperature.

• A refrigerant is a material used to cool an area or an object to a temperature that is lower than the temperature of the environment. During each operating cycle, the refrigerant evaporates into a gas and then condenses back into a liquid.

Cooling SystemsChapter 13

• A heat engine is a machine that transforms heat into mechanical energy, or work.

• Internal combustion engines burn fuel inside the engine.

• An automobile engine is a four-stroke engine, because four strokes take place for each cycle of the piston.

• The four strokes are called intake, compression, power, and exhaust strokes.

• Internal combustion engines vary in number of pistons.

Heat Engines

Internal Combustion Engine