Physics Unit 3 Cont.: thermal energy Flashcards Preview

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Flashcards in Physics Unit 3 Cont.: thermal energy Deck (24):

What causes warmth and coldness?

The vibrations of atoms and molecules

-Kinetic Molecular theory: describes motion of molecules in terms of kinetic energy

(therefore on a molecular scale, kinetic energy is the motion of molecules)
(potential energy is the force of attraction between these particles)

-increase in motion = warmer
-decrease in motion = colder


What is thermal energy?

the total quantity of kinetic and potential energy possessed by the ATOMS OR MOLECULES of a substance. Measured in joules

(differentiate thermal energy from mechanical energy)

• Kinetic energy →motion of the particles
• Potential energy →atom bonds

• The amount of thermal energy determines how fast its particles move/vibrate →how hot or cold the object is (temperature)

o Absorb thermal energy = warms/increase in temperature (increase in kinetic energy)
o Releases thermal energy = cools/decrease in temperature (decrease in kinetic energy)
**Because a large component of the thermal energy is kinetic energy of its particles


How can you transfer thermal energy?

• Can transfer thermal energy from a warmer object to a colder object only. Thermal energy is transferred until both objects have the same temperature (thermal equilibrium)

When the objects are at the same temperature, the particles inside will still move, but the energy transfer from A to B is the same as from B to A. There is no net energy transfer.

Thermal energy can also be transformed into other forms of energy


What is Temperature?

a measure of the average kinetic energy of the particles in a substance (NOT TOTAL Ek)
• It is impossible to measure the kinetic energy and potential energy of EVERY particle
• Some particles are moving faster; others moving slower than average

The majority of particles in a warmer object move faster than the particles in a colder object. Therefore the temperature of a warmer object is higher than the temperature of a colder object

The unit of temperature depends on the scale used
Ex: Celcius, Kelvin, Fahrenheit (use of thermometer)

Thermometers are based on the principle of thermal expansion. As the temperature of the liquid increases, its volume increases -->change in height in the container.
There is a linear relationship between temperature and the height of the liquid

Centigrade scale = celcius


Conversion between Celcius and Kelvin

K = C + 273

0 K = -273 C (absolute zero: no motion of particles)


Relationship between Thermal energy and temperature

Temperature: average kinetic energy
Thermal energy: total quantity of Ek and Eg (depends on the mass and size of object)

Ex: objects of different masses at the same temperature will have different thermal energy (more massive object has more thermal energy due to more atoms).

Different objects of the same mass at the same temperature also will have different thermal energy (thermal energy includes both Ek and Eg, so part of the energy absorbed will increase Ek and part will increase. The proportion of the absorbed energy that goes towards each energy is different in different substances)


Heat vs. thermal energy

Heat: the TRANSFER of thermal energy from a substance with a higher temperature to a substance with a lower temperature (warmer object to colder object)

In science, heat is commonly used as a verb i.e. the substance was heated using a hot plate (hot plate transferred thermal energy)


Methods of transferring thermal energy

1. Thermal Conduction
• The transfer of thermal energy that occurs when warmer objects are in PHYSICAL CONTACT (PARTICLE COLLISION) with colder objects (occurs in solids, liquids and gases)
o Fast moving particles of a warmer material collide with slower-moving particles of a colder material → the slower-moving particles in the cold object to speed up (gain kinetic energy) and the faster-moving particles in the warm object to slow down (lose kinetic energy)
o Result: warmer object cools as the colder object warms

2. Convection
• The transfer of thermal energy through a FLUID that occurs when colder, denser fluid falls and pushes up warmer, less dense fluid
o Ex: Water being heated
• Water particles nearest to the heat source absorb the thermal energy (NOT CALLED HEAT) and move faster and spread farther apart. As it gets farther away from the heat source, it cools down, increases in density, and falls into the warmer, less dense water below.
• Repeating process (Cycle) →Continuous convection current: colder liquid moves downward (towards heat source) as warmer liquid move upward (away from heat source)
• Allows thermal energy to spread throughout the liquid

3. Radiation
• The movement of thermal energy as electromagnetic waves ex: sun, flames, lamps
• All particles that have kinetic energy emit some radiant energy
the sun is the largest source of radiant energy

Conduction always transfers toward the colder object (one-direction).
Convection moves up (density).
Radiation "radiates" outward in all directions (not touching)
*think about how the energy is transferred. Is the object receiving the energy TOUCHING?


Thermal conductors and thermal insulators

Thermal conductors: a material that is a good conductor of thermal energy
• Feels cold when touch thermal conductor like metal because thermal energy is easily transferred from your hand to the object
-metals have mobile electrons

Thermal insulator: a material that is a poor conductor of thermal energy
• Feels warm when touch thermal insulator like plastic because thermal energy stays in your hand longer
• Objects with pockets of air are good insulators (make it difficult for thermal energy to pass)
o Ex: gases, hair, fur, feathers, plastic, vacuums (no particles or very few particles)

**Shiny reflected surfaces reflect radiation


The Specific Heat capacity (c)

Specific Heat Capacity (c): the amount of energy (J) required to increase the temperature of 1 kg of a substance by 1 degrees Celsius.
Units: J/(Kg*degreesCelsius)

Unique to each substance (therefore usually given, unless asked to calculate c and identify the substance)


Quantity of Heat (Q)

Quantity of Heat (Q): the amount of thermal energy transferred from one object (Warmer) to another (colder).

• Takes into account the mass, specific heat capacity, and change in temperature
-greater mass, greater energy
-greater change in temperature, greater energy
*Visualize water boiling (first need to heat up to 100 degrees Celcius, then the water can start changing state/boiling)
• Units: joules

Q = m*c*changeT

Change in T = T2 - T1

If object absorbs energy → change in T & Q is positive
If object releases energy → change in T & Q is negative

**label Q as Qabsorbed or Qreleased


Principle of thermal energy exchange

when thermal energy is transferred from a warmer object to a colder object, the amount of thermal energy released by the warmer object is EQUAL to the amount of thermal energy absorbed by the colder object

Qreleased + Qabsorbed = 0
^equal in magnitude but opposite in sign
(assumption: thermal energy remains in the two objects and is not released to the surroundings)

Remember that Q = m*c*change in T


Using the principle of thermal energy exchange problems

To determine mass, temperature, and Specific heat capacity

Question situations:
-metal is heated before placed in a liquid at a temperature. Thermal energy is transferred. The metal and liquid have the same final temperature. Set Qabsorbed + Qreleased = 0 and rearrange to solve for the unknown

Must distribute when calculating final temperature (change in T = T2 - T1)

*Mass of water can be calculated by using the density:
(1 g/1 ml) -->cancels mL
then convert to kg


Thermal expansion and contraction

The absorption or release of thermal energy results in...

Thermal expansion: the expansion of a substance (volume) as it warms up (spreads apart)

Thermal contraction: the contraction of a substance (volume) when it cools down (comes closer)


What causes phase/state changes?

Phase changes occur when solids, liquids, or gases ABSORB (heated) or RELEASE (cooled) enough thermal energy/heated or cooled to a temperature that corresponds to its melting/freezing or boiling/condesnation point.

-->change in thermal energy of A SINGLE substance

Absorb thermal energy: particles move faster and further apart. S to L to G
-have enough energy to overcome the intermolecular forces holding the particles together

Release thermal energy: particles move slower and closer together. G to L to S
-the attractions of the particles cause them to arrange in fixed positions


Processes of state changes

Fusion: the process by which a solid changes to a liquid (melting).
-Thermal energy continues to be absorbed during the melting process (both solids and liquids present), but the temperature does not change

Evaporation or vaporization: liquid changes to a gas

Condensation: gas changes to a liquid

Freezing: liquid changes to a solid

Sublimation: solid changes to a gas

Deposition: gas changes to a solid


Why doesn't temperature change during the state change?

• Thermal energy is being used to change its potential energy of the particles (not kinetic energy, therefore temperature does not change)
o Absorbed energy is required to break the bonds that hold the particles together
o Released energy is required to allow the particles to move closer


Heating and Cooling graphs

Heating graph: a graph that shows the temperature changes that occur while thermal energy is ABSORBED by a substance
• Melting and boiling point

Cooling graph: a graph that shows the temperature changes that occur while thermal energy is being REMOVED from a substance
• Condensation and freezing point

Y axis: temperature
X axis: amount of thermal energy absorbed or released
*flat parts represent when more than one state is present (temperature remains constant during phase change)
Temperature changes only when one state is present (represented by angled parts of each graph)

*Melting point and freezing point occur at the same temperature, and boiling and condensation occur at the same temperature


Latent heat (Q)

the total thermal energy absorbed or released when a substance CHANGES STATE (Joules)

• “Latent” = hidden (no measurable change in temperature and thermal energy remains hidden until the opposite change of state occurs)

Latent heat of fusion: the amount of thermal energy required to change a solid into a liquid or a liquid into a solid (amount of energy absorbed = the amount of energy released)

Latent heat of vaporization: the amount of thermal energy required to change a liquid into a gas or a gas into a liquid


Specific latent heat (L)

the amount of thermal energy required to 1 kg of a substance to CHANGE from one state into another (J/Kg)

→varies depending on the substance because every substance is composed of different particles. Also depends on mass/amount of the substance, and the phase change.

Specific latent heat of fusion (Lf): the amount of thermal energy required to melt or freeze 1 kg of a substance (J/Kg)

Specific latent heat of vaporization (Lv): the amount of thermal energy required to evaporate or condense 1 kg of a substance (J/Kg)


To calculate the latent heat (Q) DURING A CHANGE OF STATE

What is the difference between Latent Heat and Quantity of Heat?

Q = mLf (for substances that are melting/freezing)

Q = mLv (for substances that are boiling or condensing)

*If not change state, use quantity of heat equation (Q = mcT)
Q represents both latent heat and quantity of heat

Difference is latent heat = the amount of thermal energy required for a substance to change its state (temperature is constant)

Quantity of heat = the amount of thermal energy absorbed or released when a substance in a particular state is warming up or cooling down (temperature changes)


How is thermal energy transferred?

particles with a lot of kinetic energy of a warmer object bump/collide into particles of a colder object. There is a transfer of kinetic energy (highly energetic particles will lose kinetic energy while the nonenergetic particles will gain kinetic energy).

*remember that warm objects have particles that are moving slower than the colder object. Therefore, although there is a net energy transfer from the warmer object to the colder object, there will be some collisions from the high energetic particles in the colder object to the nonenergetic particles in the warmer object.


Evaporation vs. Vaporation/boiling?

Evaporation occurs when a FEW molecules in a liquid gain enough kinetic energy to overcome the intermolecular forces and converts to a gas BELOW the boiling point (and only at the surface)

Boiling occurs when ALL the molecules have sufficient kinetic energy to escape the liquid phase (when Vapour pressure equals or is greater than the atmospheric pressure).


Application of methods of thermal energy transfer:
Thermos bottles

Conduction and convection are heat transfer methods which depend upon the presence of materials to transfer heat. By lining a thermos bottle with a vacuum lining, energy cannot escape the contents of the bottle by two of the three forms of heat transfer.