Physics 1

Salah El-Sheikh

Section 1

How a Straw Work

Section 2

PressurePressure is defined as force per unit area. It is usually more convenient to use pressure rather than force to describe the influences upon fluid behavior. The standard unit for pressure is the Pascal, which is a Newton per square meter.For an object sitting on a surface, the force pressing on the surface is the weight of the object, but in different orientations it might have a different area in contact with the surface and therefore exert a different pressure

1 atm = 101,325 Pa (by definition) = 760 torr (by definition) = 763.43… mm Hg (approximately) = 1.03… kg/cm2 (approximately) = 10.3… tonnes/m2 (approximately) = 14.7… psi (approximately) = 1.06… tons/ft2 (approximately)

The atmospheric pressure…the pressure exerted by the weight of the atmosphere, which at sea level has a mean value of 101,325 pascal

The atmosphere as a unit.

⎡⎣ Pa = 

N =

kg m/s2

=kg

⎤⎦

m2 m2 m s2

The unit of pressure is the (pascal)

Pressure= p x g x h p = force/A . Force =m x g

M=p x v V= A x h

Force= p x a x h x g Pressure= p x a x h x g / a

Pressure= p x g x h

• P = final Pressure p = density of liquid g = acceleration of gravity (9.8 m/s^2 9.8 m/sec² at the earth's surface) h = height of the liquid

depends only upon the depth of the fluid, the density of the fluid, and the acceleration of

gravity.

How to convert to Pascal ??

• Bar * 10^5 = PascalKPa * 10^3 = PascalM.Hg * P * g = Pascalcm.Hg *10^-2 * p * g = Pascalmm.Hg *10^-3 * p * g = Pascal

Practice Problems

• 1. Calculate the pressure produced by a force of 800 N acting on an area of 2.0 m2.

2. A swimming pool of width 9.0 m and length 24.0 m is filled with water to a depth of 3.0 m. Calculate pressure on the bottom of the pool due to the water.(density of water 1000 kg/m^3)

Answers.

• 1. F = P A • F = (300 * 0.5 )• F = 150 N

2. ∆ P = P h g • ∆ P = (1000 kg / m^3) (3.0 m) (9.8 m / s2)• ∆ P = 29400 kg m^2 / m^3 /s^2

∆ P = 29400 pa

In 1643, Evangelista Torricelli invented the barometer

• The mercury in the tube pushes down with its weight.

• The bottom of the tube is open to the atmosphere.

• The air pushes on the open surface of the mercury.

• On an average day, the pressure of the air equals the pressure exerted by a column of mercury 760 mm high.

• Above 760 mm, there is a vacuum in the tube.

Weight of mercury

Barometer

Manometer

• A manometer is comprised of a bulb containing a gas and a U-shaped tube.

• The U-shaped tube is partially filled with mercury. The weight of the mercury puts pressure on the gas.

• If the U-tube is OPEN there is also air pressure acting on the gas.

• The gas molecules put pressure on the mercury.

PHg

Closed Manometers• There is a balance

between the weight of the mercury on the left (PHg) and the pressure of the gas on the right (Pgas).

• The difference between the heights of the mercury on each side of the tube is a measure of the pressure of the gas.

Pgas = Dh

vacuum

PHg

Open Manometers• When gas pressure is

greater than atmospheric pressure, the mercury is pushed toward the open end.

• The balance is between the gas on the right, and the air plus mercury on the left.

Pair + PHg = Pgas

• The weight of the mercury is measured as the height difference:

PHg = Dh

So Pgas = Pair + Dh

Pair

PHg

Open Manometers• When gas pressure is less

than atmospheric pressure, the mercury is pushed toward the gas reservoir.

• The balance is between the air on the left and the gas plus mercury on the right:

Pair = Pgas + PHg

• The weight of the mercury is measured as the height difference:

PHg = DhSo Pair = Pgas + Dh

Or Pgas = Pair- Dh

PHgPair

Sample Problems

PAIR = 765 mm

h = 27 mmh = 13 mm

PAIR = 753 mm

h = 20 mm

Pair = 790 mm

Find the pressure of the gas in each manometer.Pay attention to whether the manometer is open or closed!

1. 2. 3.

Answers

1. Pgas+ Dh = Pair

Pgas= 790 mm - 20 mm = 770 mm Hg

2. Pgas = vacuum + 13 mm = 13 mm Hg

3. Pgas = Pair + Dh

Pgas = 765 + 27 = 792 mm Hg

10. Atmospheric pressure at sea level is greater than on top of mountains. This is because:

A )there is more air in a given area at sea levelB )there is less at sea level due to sea breezes

C) water evaporates to a vapor increasing the density of the air at sea level.

• 8. Ships are made from iron. The reason why they do not sink is because:A) iron has a density less than waterB) they have a large surface area of the hullC) they are filled with air

15. A person whose mass is 50 kg (weight about 500 N) has a surface area on one foot of 0.01 square meters. What pressure does the person exert on the ground if

standing on both legs?A )50 000 newtons per square meterB )25 000 newtons per square meterC )5 newtons per square centimeter

Latent Heat

When a solid melts or a liquid boils, energy must be added but the temperature remains constant! (This can be explained by considering that it takes energy to break the bonds holding the material together.)

The amount of energy it takes to melt or boil a certain amount of material is called a latent heat.

Latent Heat

For water, the latent heat of fusion (heat needed to melt ice to water) is 3.34x10^5.

For water, the latent heat of vaporization (heat needed to boil water) is 2.25x10^6

Copyright 1999, PRENTICE HALL

Chapter 11 25

Phase ChangesHeating Curves

3. How much heat does a refrigerator need to remove from 1.5 kg of water at 20.0 °C to make ice at 0°C?

PH 0101 UNIT 4 LECTURE 9 28

CONSTANT VOLUME GAS THERMOMETER

When we heat a gas keeping the volume constant, its pressure increases and when we cool the gas its pressure decreases.

The relationship between pressure and temperature at constant volume is given by the law of pressure. According to this law, the pressure of a gas changes by

of its original pressure at 0oC for each degree centigrade (or Celsius) rise in temperature at constant volume.

273

1

PH 0101 UNIT 4 LECTURE 9 29

CONSTANT VOLUME GAS THERMOMETER

If Po is the pressure of a given volume of a gas at 0oC and Pt is the pressure of the same volume of the gas (i.e., at constant volume) at toC, then

i.e., (or) (1)

Where is constant and is known as coefficient of

increase of pressure

tP

PP oot 273

)273

1(t

PP ot

)1( tPP vot

273

1v

PH 0101 UNIT 4 LECTURE 9 30

CONSTANT VOLUME GAS THERMOMETER

PH 0101 UNIT 4 LECTURE 9 31

MERITS

Gas thermometers are more sensitive when compared to other thermometers.They work uniformly and regularly over a wide range of temperature.They can be used to measure low and high temperatures and have a wide range of operation. (–200oC to 1600oC)Since the expansion of glass is negligible when compared to the expansion of the gas the error due to expansion of glass is negligible.

PH 0101 UNIT 4 LECTURE 9 32

DEMERITS

It takes time to measure the temperature and is not a quick thermometer.These thermometers can be used only in some cases where the bodies can be brought in contact with the bulb.They are not easily portable.

Made By…..

• Salah Mohamed El-sheikh