61 terms

Chapter 16

science
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What are the two units of temperature?
degree Celsius and Kelvin
What did Count Rumford conclude about heat while he observed water boiling while a brass cannon was being drilled?
Heat cannot be a kind of matter, but instead it is related to the motion of the drill, that motion being work. Therefore heat is related to the work done.
Define: Heat
The transfer of thermal energy from one object to another because of a temperature difference.
Define: temperature
a measure of how hot or cold an object is compared to a reference point.
Ex: Celsius Thermometer - the reference points are the boiling and freezing of water
Ex: Kelvin Scale - the reference point is Absolute Zero = 0 K
What is temperature related to?
The average KE of the particles in an object due to their random motions through space.
What is the relationship between temperature and Average Kinetic Energy?
As an object heats up (increase in temperature), particles move faster on average, as a result, the average kinetic energy must increase.
T increase ---> Avg KE increase
What does thermal energy depend on?
Thermal energy depends on the mass, temperature, and phase (solid, liquid, gas) of an object
define: Thermal Expansion
the increase in volume of a material due to a temperature increase
Why does Thermal Expansion occur?
Particles of matter tend to move farther apart as temperature increases
How does a thermometer use thermal expansion?
In a glass thermometer, as temperature increases the alcohol in the tube expands and its height increases. Increase in height : Increase in temperature
define: Specific Heat
The amount of heat needed to raise the temperature of one gram of a material by one degree Celsius.
Explain the relationship between temperature and specific heat.
The lower a material's specific heat, the more its temperature increases when heat is absorbed.
Ex: In the sun, iron will heat up more than plastic, so iron has a lower specific heat than plastic.
What are the two units heat can be measured in?
Joules or Calories
define: Calorie
The energy needed to raise the temperature of one gram of water by one degree Celsius
Specific Heat formula:
Q = m x c x ΔT
m = mass (grams)
c = specific heat (J/g * °C)
ΔT = change in temperature (°C)
define: calorimeter
an instrument used to measure changes in thermal energy by directly measuring a change in temperature.
A sealed calorimeter is a "closed system" - what does this mean?
The seal prevents thermal energy from escaping and being transferred to the environment (outside of the calorimeter).
Why does heat flow from a high to a low temperature?
Heat flows by the transfer of energy in collisions. On average, high-energy particles lose energy and low-energy particles gain energy in collisions. Overall, these collisions transfer thermal energy from hot to cold objects.
define: Conduction
the transfer of thermal energy (through collisions between particles) without any overall transfer of matter.
Conduction occurs within a material or between materials that are touching.
Why is conduction in gases slower than in liquids or solids?
The particles in a gas collide less often, so thermal energy is transferred more slowly.
Why is conduction faster in metals?
In metals, some electrons are free to move about, so it is easier for collision among particles to take place allowing thermal energy to be transferred more quickly.
What is a thermal conductor?
a material that conducts thermal energy well.
Ex: metal - copper or aluminum (pots & pans)
Why doesn't a thermal conductor have to be hot?
Something that transfers energy rapidly away from your skin will feel colder to you.
Ex: tile floor
define: thermal insulator
a material that conducts thermal energy poorly
Ex: wood & air & argon gas & foam cups
define: convection
the transfer of thermal energy when particles of a fluid move from one place to another
What happens to hot air as it rises?
As air heats up, it expands & becomes less dense than cooler air. This lack of density allows the hot air to rise and as hot air rises, it cools and becomes more dense.
define: convection current
a current that occurs when a fluid circulates in a loop as it alternately heats up and cools down, transferring thermal energy
List natural cycles where convection currents are important:
ocean currents
weather systems
movements of hot rock in earth's interior
define: radiation
the transfer of energy by waves moving through space; all objects radiate energy
Ex: heat lamps warming food in a buffet
Explain: rate of radiation
1) the amount of energy radiated from an object increases as its temperature increases.
2) the farther away you are from a radiating object, the less radiation you receive.
define: thermodynamics
the study of conversions between thermal energy and other forms of energy
What did Joule discover in his paddle wheel experiment?
work done = thermal energy gain
(First Law of Thermodynamics)
First Law of Thermodynamics
energy is conserved
Second Law of Thermodynamics
thermal energy can flow from colder objects to hotter objects only if work is done on the system
(Ex: a refrigerator)
What is a heat engine?
any device that converts heat into work
Third Law of Thermodynamics
absolute zero cannot be reached
(matter can be cooled very near absolute zero)
define: waste heat
thermal energy that is not converted into work
Explain the efficiency of a heat engine
1) The efficiency of a heat engine is always less than 100% because there is waste heat (2nd Law) and because the temperature outside the engine cannot be cooled to Absolute Zero (3rd Law).
2) but efficiency increases with a greater difference between the high temperature inside the engine and a cold temperature outside the engine
3) work done = thermal energy added to the engine - waste heat
Why will disorder in the universe always increase?
Order can only be applied at a local level, but it always produces waste heat because work is being done to create order. The waste heat contributes to the disorder of the universe, causing disorder to increase (2nd Law).
When using a calorimeter to determine specific heat, what measurements must be taken?
1) the temperature of the water
2) the temperature of the sample
3) the temperature of the water and sample together once they have reached the same temperature within the calorimeter
Explain why there is a difference in thermal expansion between solids, liquids, and gases.
Thermal expansion depends on the forces of attraction between particles. Gases expand more easily because the attraction between particles is weaker. And, in general, liquids expand more than solids.
Why does the tile floor feel cooler than the bath mat when both the tile and the mat are the same temperature?
The tiles conduct heat much better than the fluffy mat, which means they suck heat out of your feet much faster. This is not just a psychological effect. Your feet actually get colder on the tiles. But it's because of high thermal conductivity, not low temperature. Granite tiles have a thermal conductivity that is 100-150 times higher than that of cotton bathroom mats.
How does a steam engine work?
- it is an external combustion engine, so it burns fuel outside the engine
- hot steam enters the cylinder & a valve traps it in the cylinder (heat)
- hot steam expands & cools as it pushes the piston to the left (work)
- In this way, heat is converted to work
What are the two main types of heat engines?
external combustion engine &
internal combustion engine
define: external combustion engine
an engine that burns fuel outside the engine
Ex: steam engine to run a pump
define: internal combustion engine
a heat engine in which the fuel burns inside the engine
Ex: car gasoline engine
How does an internal combustion 4-stroke engine work?
1) in the intake stoke: a mixture of air and gasoline vapor enter the cylinder
2) in the compression stroke: the piston compresses the gas mixture
* at the end of compression: the spark plug ignites the mixture, which heats the gas under pressure
3) in the power stroke: the hot gas expands and drives the piston downward
4) during the exhaust stroke: gas leaves the cylinder and the cycle repeats
In an internal combustion engine, what is the linear motion of each stroke converted into?
the linear motion is converted into the rotary motion by the crank shaft, turning the wheels (the rotary motion).
define: central heating system
a heating system that heats many rooms from one central location
List the 4 most common energy sources used for central heating systems:
1) electrical energy
2) natural gas
3) oil
4) coal
define: Hot-water heating system
- at the boiler, heating oil or natural gas burn and heat the water
- the circulating pump carries the hot water to the radiators in each room
- the hot water transfers thermal energy to the radiator by conduction
- as the pipes heat up they heat the room air by conduction & radiation
- hot air rises and sets up a convection current in each room
- after the heat from the water is transferred to the room, the cooled water returns to the boiler, and the cycle begins again
How is temperature controlled in a hot-water heating system?
by a thermostat - the heat of the room expands an interior coil which causes it to rotate and the coil trips a switch that turns the heat off. The reverse occurs as the room cools, the coil contracts, turns the opposite direction, and turns the switch on.
How does steam heating work?
It is very similar to hot water heating, but steam replaces the hot water.
It is often used in older buildings or when many buildings are heated from one central location
How does electric baseboard heating work?
- it uses electrical energy to heat a room
- a conductor, like a heating element on a stove, is heated with electrical energy, which converts to thermal energy
How does forced air heating work?
- fans are used to circulate warm air through ducts to the rooms of a building
- convection currents circulate air in each room
- cool room air returns to the furnace through a vent with a filter, cleaning the air
Ex: the furnace heating system in our home
define: heat pump
a device that reverses the normal flow of thermal energy by circulating a refrigerant through tubing.
Ex: a refrigerator
define: refrigerant
a fluid that vaporizes and condenses inside the tubing of a heat pump.
When the refrigerant absorbs heat, it vaporizes (turns to gas).
When the refrigerant gives off heat, it condenses (turns back to a liquid)
How does the heat pump reverse the normal flow of thermal energy (heat flowing from hot to cold)?
Heat pumps must do work on a refrigerant.
How does a refrigerator work?
- it is a heat pump: it transfers thermal energy from the cold food compartment to the warm room
- a motor moves refrigerant through tubing in the refrigerator walls
- it draws heat out of the food compartment and releases it into the room through the exterior coil (under or behind the refrigerator)
How does an air conditioner work?
- a compressor raises the temperature and pressure of the refrigerant (a chemical inside the evaporator coils), turning it into a hot, high pressure gas
- the temperature of the compressor coils is higher than the outside air temperature, so heat flows from the coil to the outside air. A fan increases the rate heat flows
- as thermal energy is removed from the coil, the refrigerant cools and condenses into a liquid
- the liquid refrigerant flows through the expansion valve and decreases in temperature
- as the cold refrigerant flows through the coils, it absorbs thermal energy from the warm room air
- the fan sends cold air back into the room
- the refrigerant becomes a vapor & the cycle begins again
Explain: a spontaneous flow of heat
heat flows spontaneously from hot objects to cold objects