Exam 2

You are bored during the summertime when school is not in session and you decide to do an experiment where you compare the properties of water and air. If the density of air is 1.25 kg/m³ and the density of water is 1000 kg/m³, what volume of air weighs the same as 1 liter of water?

800 liters

Mass = density x volume

1L = 0.001 m³

Mass of water = 1000 kg/m³ x 0.001 m³ = 1 kg

Volume = mass/ density

1/1.25 =0.8

0.8 × 1000 = 800 L

Or 1000 / 1.25 =800

You are a deep sea diver and are carrying out pressure tests on your air tanks, which are required by inspection rules. A gas cylinder is filled with a gas at a temp of 300K and pressure 200,000 Pa. The temp of the gas is then raised to 400K. What is the new pressure

266,667 Pa

P1/T1 =P2/T2

P2 = P1 x T2/T1

200,000 × (400/300)=266,667 Pa

You are in a hot air ballon and are hovering in the air so that the atmospheric pressure remains constant. At a temp of 300K your balloon hold 5×10^6 liters of air. The sun comes out and the temp rises to 330K. What is the new volume of the balloon? You may assume that no air can exit or enter the balloon

5.5×10^6 liter

V1/T1 =V2/T2

V2=V1 x T2/T1

(5×10^6) x (330/300)=5,500,000 pr 5.5×10^6 L

Atmospheric pressure is about 100,000 Pa. At what depth in water does this pressure double?

10m

P=Patm = pgh

100,000 = 1,000 × 10 x h

100,000 = 10,000h

100,000/10,000

H=10

Suppose you have an aquarium and you notice that you have a solid plastic decoration and a rock of about the same volume but twice as dense, and both are totally submerged in the water. The buoyant force on the plastic object is about ____ as that on the rock

The same

Archimedes’ Principle, the buoyant force acting on a submerged object depends only on the volume of the fluid the object displaces and the density of that fluid not the objects’s own weight or density

If the objects floated (not totally submerged) or sank, the buoyant force on the plastic object would be less than the buoyant force of the rock

If they are both held halfway down (maybe by a hand) it would be the same answer

You stopped for cappuccino at a coffee shop in knows that the tiny white bubbles of steamed milk remain on the surface of the coffee. These airfield bubbles stay where they are rather than descending into the coffee or rising into the air because they are

Less dense than the coffee but more dense than the air above the coffee

Suppose you have two containers with liquid. One has a density of 760 kg/m³ and the other object has a density of 910 kg/m³. If an object floats in one container and sinks in the other, the density of the object

Has a value between 760 and 910

Why are failures in dams more likely to occur closer to the bottom of the dam?

Water pressure is higher at the bottom

You work with a catering firm and have been given a challenge. By emptying several liquid beverage dispensers, you have to rank them in order of their content (which one is fullest, the next full list, etc.). Having had a physics class, you a scheme to compare the speeds of the fluid exiting the spout at the bottom of the container. The principal you base your prediction on is:

The pressure (and hence water level) is lower than the speed

You are camping in the breathtaking mountains of Colorado. You spy, an unopened diet soda, canned floating, motionless below the surface of a lake. What is the direction and amount of force the water exerts on it?

Up, equal to the cans weight

Lead sinkers actually float on the surface of liquid mercury. Which takes up more space 1 kg of mercury or 1 kg of lead

Lead

The popular drink is composed of pouring several types of beverages (one on top of the other) in order to get a layering effect. In what order should the individual beverages be poured into the glass so that your drink is stable?

Most dense first and then in order of decreasing density

If you were to pour the less dense liquid first, when you pour a more dense liquid, it would pass right through and ruin the layering effect

Water weighs about 10 N per liter a 4 L ball is pushed underwater. What is the buoyant force on the ball?

40 N

10×4

Water weighs about 10 N per liter a 4 kg ball floats on the water surface. What is the weight of the water being displaced by the ball?

About 40N

Equal to buoyant force

The brake system in most cars makes use of the hydraulic system. This system consist of a fluid filled to connected at each end to a pistol assume that the pistol attached to the brake pedal has a cross-sectional area of ½ a square inch and the piston attached to the brake pad has a cross-section area of 2 in.² when you apply a force of 10 pounds to the piss on attached, the brake pedal the force at the brake pad will be

40 pounds

P= force/ area

10/.5=20

F2=pressure xA2=

20×2=40

A block of hardwood is floating easily on the surface of a lake as a storm approaches the density and pressure of the air above the lake begins to decrease as a result of this decrease in air pressure and density, the block of wood

Moves downward slightly and float lower in the water

if the density and pressure of the air above the lake began to increase, it would move upward slightly and float higher in the water

For an object that is floating on a fluid

The weight of the fluid displaced equals the objects weight

For an object that sinks in the fluid

The weight of the fluid displaced is less than its weight

For an object that is neutrally buoyant in a fluid

The weight of the fluid displays equals the object weight

Suppose you have an aquarium and you notice that you have a solid plastic decoration and rock of about the same mass the decoration is twice as large as the rock both are totally submerged in the water. The buoyant force of the plastic object is____ as that on the rock

Twice as big

If the decor is the same size than the buoyant force of the plastic object would be the same as that on the rock

If the decor is smaller it would be half the buoyant force

Water weights about 10 N per liter on earth. A 4 liter ball is pushed underwater on the space shuttle, where gravity was 1/10 that on earth. What is the buoyant force on tb ball?

4N

10 × 1/10 = 1

4 × 1 = 4N

Gravity creates

Pressure gradients in fluid

The pressure gradient set up in water standing is a glass is an example of _____ in pressure, while variation in pressure due to water flowing faster in the smaller parts of a pipe are a type of ____ in pressure

Static variation; dynamic variation

Suppose you have a very long straw that could reach from a cup of water on the ground level to the top of a very tall building. When you suck on the straw to bring the water up

You could bring the water up to only a certain height in the straw bc the atmosphere pushes down on the surface of the water by a given amount

Cities have to use water towers to supply water to bc

Water pressure at the bottom of the tower is much higher than at the top

Pressure potential energy

Has to do with describing moving water and is only an abstract concept

If the water pressure at a pump station at sea level 4,000,000 Pa, how high can the water be pumped uphill and how fast will it flow out of a facet at a washing station at sea level

408m; 90 m/s

H = P / pg

4,000,000 / 1,000 × 9.8 = 408.16m

P(pump)= ½ pv²

4,000,000 = ½ (1000)v²

4,000,000 = 500 v²

V² = 4,000,000/500 =8,000

V= 90m/s

Why does an ice cube feel cold?

Heat from your hand enters the cube by conduction

Wind is one of natures way’s of transferring energy through

Convection

Why are good thermal conductors generally good electric conductors also?

Electron motion is responsible for determining both properties

Free moving electrons (think of busy highway (plugging outlet or heating one end of metal spoon) and racers getting the green light and racing off (free electrons)

The surface of the moon is exposed to full solar radiation because it has no atmosphere. Why then does the moon not heat up endlessly until it disintegrates?

It’s temp rises until it is able to radiate heat away into space as fast as it arrives from the sun

As the object gets hotter, it releases more heat back into space through thermal radiation

Thermal equilibrium: the amount of heat it absorbs form the sun is the same as the amount of heat it radiates

Soil heats up much faster than water when the two are exposed to sunlight. Use that fact and your understanding of heat transfer to predict which way the wind will blow near the surface of the earth as the sun rises near the seashore

The surface wind will blow from the water toward the land

Both the land and ocean receive the same amount of solar radiation but soil has a lower specific heat capacity than water, the land heats up rapidly while the ocean temp changes very little.

Why does the handle of a metal spoon submerged in boiling soup feel hot

Heat from the soup is transferred though the spoon by conduction

Th Gulf Stream is one of natures ways of transferring energy through

Convection

When cooking a potato in a conventional oven it is useful to stick a nail in it, which makes it cook faster. This is because

The nail is a good conductor of heat. Since the nail made of metal (good conductor), it will cook the potato faster through conduction

Without the nail it would relay on convention (hot air circulating around the potato)

In addition to heat energy, another product of fires is dangerous smoke and other chemicals. This fact is the reason for a ______ in a furnace

Heat exchanger

Heat exchangers are sealed metal chambers that allow heat to pass through but block gases from crossing over (toxic gas vents outside and clean air blows inside house)

Which temp scale has a 373 as the boiling point of water

Kelvin

You are hired as an engineer at a thermometer factory. Suppose you built a water thermometer to replace the mercury in a mercury thermometer with water. You want to measure the temp above freezing with it. How would it behave as the temp drops from 5*C to 1*C

The reading will steadily decrease

In making ice cream, the flavored cream is in a container in contact with ice. In order to freeze the ice cream why is it essential to make the ice melt at a very low temperature?

The low temp of the ice as well as its melting removes energy from the flavored cream, causing it to freeze

The water and salt mixture allows it to drop below normal freezing point and naturally hot (cream) rushes to cold (sub-zero slush)

In making ice cream, the flavored cream is in a container in contact with melted ice. When the ice cream freezes it gives energy up to the ice/water mixture. What happens to the temp of the ice/water mixture.

It will not change because the energy is going into its melting the ice

It can’t do more than one thing at a time. It either increase energy via transfer or change phases and because its changing its phase from ice to water than that is why it stays the same (no change)

A popular classroom demonstration involves placing a paper cup with water in it on a burner, and boiling the water in the cup. Although part of the cup may burn, the part containing the water does not because

The water absorbs most of the heat from the flame

Thermal conduction + high heat capacity; the water rapidly draw the heat away from the paper cup, keeping the paper below its ignition emo

Ice is

Crystalline (because of hydrogen bond)

Which temp scale has a 273 as the boiling point of water

none

100 = Celsius

373 = Kelvin

212 = Fahrenheit

Melting is a ___ that requires energy ___ for the melting system

Phase transition; input

Freezing is a ____ that requires energy ___ for the melting system

Phase transition; release

In studying tornado formation, it is thought that water condensing is a very important driving mechanism for the violet storm bc

Water condensing releases a lot of energy into the environment

When gas changes to liquid (condensation; phase transition) it releases lots of stored heat energy (latent heat of condensation) into the storm cloud

2 objects in thermal equilibrium with a 3rd object are in thermal equilibrium with each other. Which law of thermodynamics is this?

Zeroth

The entropy of an isolated system never decreases. Which law of thermodynamics is this?

Second

Which law of thermodynamics relates change in internal energy to heat added to a system and work done by it

First

What does a heat pump no do

Perform work on its surrounding

Which of the following quantities is not conserved

Entropy

An ideally efficient freezer cools food to 210K. If room temp is 300Km and the machine consumes 75J of work, how much heat was removed from the food?

175J

210/300-210 =

210/90=2.333

2.33 × 75 = 175

An ideally efficient heater adds 1400J of heat to a wam room. How much work does it perform if it removes 1100J of energy from the outdoor

300J

Qh = Qc + W

1400 = 1100 + w

1400-1100 =300

Suppose you stir 100g of water and do 419J of work on the system. Assuming no leakage of energy, what will the resulting change in water temperature be?

+1*C

Delta T= Q/mxc

419 / 100×4.19 =

Suppose you try to cool your house in the summer by letting heat flow from your house to your neighbors home. You will

Not get far, because of the zeroth law of thermodynamics

The first law of thermodynamics is a re-statement of the law of

Conservation of energy

Suppose 100g of water does 419J of work on the system. Assuming no leakage of energy, what will the resulting change in water temp be

-1*C

Latente heat is the heat

Absorbed or released during a phase change at constant temp

Formula for heat during a phase change is

Q=mL

Specific latent heat of fusion of water is approx

334 kJ/kg

Latent heat of vaporization of water at 100*C is approximately

2260 kj/kg