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Law of Conservation of Energy
• States that energy can neither be created or destroyed or that the total amount of energy in a system remains constant.
• We can measure the energy in terms of calories.
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Calories
• One calorie is defined as the amount of energy needed to raise the temperature of 1 gram of water 1oC
• Abbreviated as (cal)
• Dietary calories (C) = 1000cal or 1kcal = amount of energy released from foods
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Heat Energy• Heat - a flow of energy from a hot object
(higher energy) to a cold object (lower energy). – The most familiar form of energy – All matter contains some heat energy.– The net energy transferred from or to an
object as a result of a temperature difference which results in a change in internal energy
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Review Heat Energy
The heat energy of a substance is determined by how active its particles (atoms and molecules) are.
A hot object is one whose atoms and molecules are excited and show rapid movement.
A cooler object's molecules and atoms will be less excited and show less movement.
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What is Heat Energy?
• As heat is added to an object the temp. increases
• The added heat becomes part of the object’s internal energy
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Heat travels from a hot object (higher energy) to a cold object (lower energy).
So how does heat travel from one object to another?
By Convection, Conduction and Radiation.
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Heat Transfer by Convection
• Convection occurs only in liquids and gases.
• We call liquids and gases fluids.
• It cannot happen in solids. • It needs particles to be free
to move about.
• A radiator in a room heats up the room by convection (NOT radiation).
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Heat Transfer by Convection
• Look at the engine at the left.
• Water carries heat from a hot engine through a pump and delivers it to a radiator, whose duty is to give up heat to the air.
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• The process in which heat passes through a solid substance is called conduction.
• Metals are good conductors of heat. • Non-metals are generally bad conductors
of heat. • Liquids and gases are bad conductors of
heat as well. • A bad conductor of heat is called an
insulator.
Heat Transfer by Conduction
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Ability of a metal to conduct heat?
• What is an element of metal made of?
Atoms that vibrate
Free electrons that move around
• They are constantly vibrating or oscillating against each other.
• Through these collisions, the heat energy is conducted
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• The flow of heat from a hot part of a body to a cooler part, without transfer of matter.
• Can also take place from one body to another, provided the two bodies are in contact, and a temperature difference exists between them.
• A pot on a hot plate is heated by conduction from the stove
surface, via the underside
of the pot.
Heat Transfer by Conduction
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Heat Transfer by Radiation
• The easiest way to explain heat transfer by RADIATION is by the heat we feel from the sun or a light bulb.
• RADIATION of heat is the transfer of heat energy through matter or space as electromagnetic waves.
• Examples :– Electromagnetic waves called infra-red radiation that is
emitted from earth.– Ultraviolet radiation that is emitted from the sun and
causes sunburns
– Microwaves that cook food
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Review
• Heat flows from hot objects to cold objects.
• Heat flow depends on the temperature difference.
• Heat is not temperature.
• Heat passes by conduction, convection, or radiation.
• Radiation is caused by infra-red radiation
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Determining the amount of heat gained or lost by a substance
Step 1• You will need the following formulas for
your calculations:– where T refers to the change in
temperature
– Tf refers to the final temperature and
– Ti refers to the initial temperature
T = T - Tf i
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Determining the amount of energy lost or gained
Step 2– where Qcal refers to the heat gained (or lost)
expressed in calories– mass of water is expressed in grams – (1 mL water = 1 gram) T refers to the change in temperature
– Answers are expressed in calories or cal
Q ca l (m a ss o f wa ter)( T)
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Q ca l (m a ss o f wa ter)( T)
45 gm sample of water is heated from 24.0ºC to 82.0 ºC
How much heat was gained by the cold water?
∆T = 82.0 – 24.0 = 58.0 ºC
Qcal = 45 g x 58.0 ºC = 2610 = 2600cal
T = T - Tf i
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• If a sample of hot water and cold water are mixed together…
• Based on the Law of Conservation of Energy what would you expect?
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Q ca l (m a ss o f wa ter)( T)
110.5 gm sample of hot water has an initial temp of 61.8ºC. 250.0 gm of 1.8ºC water is added to it and the final temperature is 6.8ºC
1 - How much heat was gained or lost by the by the cold water?
2 - How much heat was gained or lost by the warm water?
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How Do We Measure the Changes in Heat Energy?
Temperature
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What is Temperature?
• The measure of the average kinetic energy of the particles (atoms and molecules) of a substance– Kinetic energy is the energy an object posses
because of it’s motion
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How do we measure temperature?
With a thermometer.
1. A thermometer is an instrument that utilizes the physical property of thermal expansion for the purpose of accurately determining temperature
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What is thermal expansion?
1. The property of a substance to expand with an increasing temperature and
contract with a decreasing temperature
2. The expansion and contracting allows for the quantitative measure of temperature by using a thermometer.
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Thermometer
• Most common is the bulb type
• As the substance gets hotter, the particles in the liquid mover faster and spread out
• This causes the red liquid to move up.
• Gives us a quantitative reading.
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3 Different Temperature Scales