Physics exam 4 (using energy, thermal properties of matter, fluids)

Isobaric

"barometric" pressure remains constant. change in temp and/or volume.

Two containers of the same (ideal) gas: box A: mass = 100 g, 0 degrees Celsius box B: m = 20 g, 50 degrees C 1. Which gas has atoms with the largest average thermal energy? 2. Which container has the largest thermal energy?

1. B 2. A

A machine uses 1000 J of electric energy to raise a heavy mass, increasing its potential energy by 300 J. Efficiency = ?

30%

Light bulbs are rated by the power that they consume, not the light that they emit. A 100 W incandescent bulb emits approximately 4 W of visible light. What is the efficiency of the bulb?

4W / 100W = 4%

Which of the following would allow a fridge to use less energy to run? A.) Increasing temp inside fridge B.) Increasing the temp in the kitchen C.) Decreasing the temp inside the fridge D.) Decreasing the temp of the kitchen

A and D. If it's colder outside the fridge, it doesn't have to use as much energy keeping the internal temp up.

Heat engine

A device that takes in energy by heat and expels a fraction of that energy through work, operating in a cyclic process. Ex: a steam engine (external energy heats water into steam, which pushes the piston to do work, steam turns back to water because it loses energy by working). Converts heat/thermal energy --> mechanical/electrical energy. Moves thermal energy from one place to another.

Ideal gas

A hypothetical gas whose particles are infinitely small and do not interact with each other. A simplification of how real gases behave. Does not exist.

Entropy

A measure of disorder/chaos at an atomic level. Helps us explain some basic observations about the world. Measure of energy unavailable to do work.

The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency? A. 300 °C, 30 °C B. 250 °C, 30 °C C. 200 °C, 20 °C D. 100 °C, 10 °C E. 90 °C, 0 °C

A.

A sample of nitrogen gas is in a sealed container with a constant volume. Heat is added to the gas. The pressure A. increases B. stays the same C. decreases D. can't be determined with the information given

A. increases

When the temperature of an ideal gas is increased, which of the following also increases? (1) The thermal energy of the gas (2) the average kinetic energy of the gas (3) the average potential energy of the gas (4) the mass of the gas atoms (5) the number of gas atoms A. 1, 2, and 3 B. 1 and 2 C. 4 and 5 D. 2 and 3 E. All 1 thru 5

B. 1 and 2

Suppose you have a sample of gas at 10°C that you need to warm up to 20°C. Which will take more heat energy: raising the temperature while keeping the pressure constant, or raising the temperature while keeping the volume constant? A. It takes more energy to raise the temperature while keeping the volume constant. B. It takes more energy to raise the temperature while keeping the pressure constant. C. The heat energy is the same in both cases

B. It takes more energy to raise the temperature while keeping the pressure constant.

When I do work on a gas in an adiabatic process, compressing it, I add energy to the gas. Where does this energy go? A. The energy is transferred as heat to the environment. B. The energy is converted to thermal energy of the gas. C. The energy converts the phase of the gas.

B. The energy is converted to thermal energy of the gas.

A refrigerator is an example of a A. reversible process. B. heat pump. C. cold reservoir. D. heat engine. E. hot reservoir.

B. heat pump.

The most efficient form of transportation after walking is

Bicycling

Which needs the most heat to bring to a final temperature of 50°C? A. 100 g of iron at 0°C B. 100 g of water at 0°C C. 100 g of ice at 0°C

C. 100 g of ice at 0°C

A sample of nitrogen gas is inside a sealed container. The container is slowly compressed, while the temperature is kept constant. This is a ________ process. A. constant-volume B. isobaric C. isothermal D. adiabatic

C. isothermal

Your body is a system that burns energy in food, but stays at a constant temp. For your body: A.) Q > 0 B.) Q = 0 C.) Q < 0

C.) Q < 0

The 3 Types of Energy Transfer

Conduction Convenction Radiation

When you walk at a constant speed on level ground, what energy transformation is taking place? A. Echem --> Ug B. Ug --> Eth C. Echem --> K D. Echem --> Eth E. K --> Eth

D. Echem --> Eth

What type of heat transfer can happen thru empty space? A. conduction B. evaporation C. convection D. radiation

D. radiation

Which one of the following gas molecules would be most likely to be moving at a speed high enough to escape the earth's atmosphere? A. Carbon dioxide B. Oxygen C. Nitrogen D. Water vapor E. Hydrogen

E. hydrogen

2nd Law of Thermodynamics

Energy and matter move towards chaos. Transformations lead to increased entropy. Entropy increases in an isolated system. Recycling uses energy.

T/F: A container with fewer moles will have a higher pressure compared to a container of the same size with more moles.

False

T/F: Efficiency is limitless.

False

T/F: Incandescent lightbulbs are more efficient than LEDs.

False

T/F: It is possible to reach maximum efficiency.

False

T/F: Lower entropy states are more likely, and systems naturally evolve to states of lower entropy. The Universe forces a decrease in entropy.

False

T/F: Pressure, temp and volume are unrelated concepts. Temp, volume, and number of moles are also unrelated.

False

Operation of a heat engine

Heat energy is transferred from hot reservoir to the system. Part of the energy is used to do work. The remaining energy is exhausted to the cold reservoir as waste heat.

Zeroth Law of Thermodynamics

If two thermodynamic systems are in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.

3rd Law of Thermodynamics

In order for motion to stop we have to reach absolute zero (0 K temp), but this is not possible. All vibration, rotation and translation would stop completely. "The entropy of a pure crystalline substance at absolute zero is zero."

Charles's Law

In order to keep pressure constant, volume must be increased if you increase temperature. Volume and temp are directly proportional. V1/T1=V2/T2

The 3 types of changes to an ideal gas system (gas in closed container)

Isothermal Isobaric Isochoric

The unit used to measure temperature in physics

Kelvin. Starts at absolute zero of the Universe, 273 degrees Celsius.

1st Law of Thermodynamics

Matter and energy cannot be created or destroyed (in any closed system). They can be interchanged. It is impossible to make something from nothing.

In a geothermal powerplant, electricity is generated by using the temperature difference between the 15 °C surface and 240 °C rock deep underground. What is the maximum possible efficiency? What happens to the energy that is extracted from the steam that is not converted to electricity?

Max efficiency = 1 - Tcold / Thot 1 - 15/240 =94%

Boyle's Law

PV = k P1V1 = P2V2 The basis law of thermal changes. Describes relationship between the pressure and volume of a gas. Squeeze gas --> gas pressure increases. Pressure * temperature is constant for gas that's as at a constant temp.

Ideal Gas Law

PV=nRT (absolute pressure)(volume of container)=(number of moles)(gas constant)(temp)

The ideal gas model tells us what of the relationship between energy and temperature?

Proportional relation btwn temp, # of molecules, and system's overall energy. Energy (E) is proportional to temp (T) and the # of particles (N). Kinetic energy of gas molecules increases with temperature. Energy is required to raise temp, this energy must go somewhere after. It goes into making the molecules move faster with more KE. More molecules, more energy needed to raise temp.

The symbol for thermal energy

Q

Thermodynamics

The study of the relations between heat, work, temperature, and energy, and their effects on a system. Only concerned w/large-scale observations.

Conduction

Transfer of energy by collisions of particles. Mostly occurs in solids.

Convection

Transfer of energy by currents moving thru liquids or gases. Caused by density differences.

Radiation

Transfer of energy thru empty space. No molecules needed.

T/F: A larger container with less moles will have a lower temperature than a smaller container with more moles.

True

T/F: At an atomic level, solids and liquids are like a bunch of balls connected by springs called molecular bonds. Heating something makes the ball shake, and if heated up enough the springs will break.

True

T/F: Heat causes both change in temp and changes of phase (solid to liquid, liquid to gas, etc.).

True

T/F: Heat flows from hot to cold.

True

T/F: In a system where only the thermal energy changes, the change in thermal energy is equal to the energy transferred into or out of the system as work (W), heat (Q), or both.

True

T/F: Pumping out the air in a vacuum jar will reduce pressure, and the ball inside will increase in volume to keep PV/T constant.

True

T/F: Temperature is a measure of the average kinetic energy of the particles.

True

T/F: The Second Law of Thermodynamics states that the state of entropy of the entire universe, as an isolated system, will always increase over time. Thus, energy always flows from order to disorder.

True

T/F: The pressure and volume of a container increase as you add more particles (n) and/or raise the temperature.

True

T/F: Non-random energy (mechanical, chemical, radiant) cannot be changed without some degradation into heat.

True (2nd Law of Thermodynamics)

Boltzmann's constant

a physical constant relating the average kinetic energy of particles in a gas with the temperature of the gas; it is the gas constant R divided by Avogadro's number (1.38x10^-23 J K^-1)

The equation for the theoretical maximum efficiency of a heat engine

emax = 1 - Tc / Th

When liquids and solids are heated, they ________ because of the increased motion of their atoms or molecules.

expand

Heating something up causes it to _________ while cooling it down makes it _________. The amount depends on how much heat is put in and the type of material.

expand, contract

On an atomic level, when the change of a state of substance occurs, the substances _________ _____ are _______.

intermolecular bonds, broken

The number of particles in exactly 12g of Carbon-12 is called a

mole # of moles = # of particles / Avagadro's number

pressure

p = force / area The physical force exerted on an object per unit area.

the unit used for pressure in physics us

pascals one pascal is one newton of force pushing on one square meter of a surface

When substances are heated, they sometimes change

state (of matter)

Isothermal

temperature remains constant. change in volume and/or pressure.

The theoretical maximum efficiency of a heat engine depends on

the temp difference between the hot and cold resevoirs.

If an object is heated, its ________ ________ increases. The atoms and molecules jiggle around at higher speeds and move further apart.

thermal energy

Isochoric

volume remains constant. change in temp and/or pressure.

When a gas expands, it does _______. When a gas is compressed _______ is done on it.

work

A 75 kg person climbs 248 steps, a total climb of 59 m. How many joules of chemical energy (food) does he burn?

work = force * distance work = (weight=75kg*9.8) * 59m = 43.465 kJ

Volume thermal expansion equation Linear thermal expansion equation

ΔV = β * V initial * ΔT ΔL = alpha * L initial * ΔT

A sample of gas is in a cylinder with a moveable piston. The force on the piston can be varied, altering the pressure and volume. A sample of gas is taken from an initial state to a final state following vertical line on the pV diagram, which depicts an increase in pressure from 2 to 6 atm (y axis), and a constant volume (x axis). Another sample's graft depicts a horizontal graph line, with an increase in volume from 100 to 300 cm^3 (x axis), and constant pressure (y axis). A third sample depicts a curve on the graph, with a decrease in pressure from 6 to 2 (y axis), and an increase in volume from 100 to 300 (x axis). Each sample has 0.02 moles. The final temperature is: A. higher than the initial temperature. B. the same as the initial temperature. C. lower than the initial temperature.

Sample 1: A. higher than the initial temperature. Sample 2: C. Lower than the initial temperature. Sample 3: B. The same as the initial temperature.

Specific heat vs latent heat

Specific heat raises temp Latent heat causes a state transition

Absolute zero

The lowest temperature possible, where no more thermal energy can be removed from matter. -273 Celsius or -273 Kelvin.

Energy transfer

The movement of energy from one form to another. Ex: using electricity to heat water. Chemical energy from food --> body moving.

Efficiency

The ratio of the energy you put in, over the energy you get as a result. = Output/input. = Bang/buck.