Lecture 7Temperature and Heat. Buoyancy.

Chapter 4.1 4.6

Outline

• Temperature and Heat• Density and Pressure• Buoyancy

Temperature

Atoms are constantly moving have energy

Temperature is a physical quantity that is related to sensations of hot and cold.It is related to energy of an object that we feel.

All material objects in the Universe consist of small particles called atoms.

The amount of energy they have depends on their mass:Kinetic energy KE=mv2/2Rest energy E0=m0c2

Temperature and Heat

Thermometers

Thermometer is a device that measures temperature.It uses the property of substances to expand when heated and shrink when cooled.

The most common thermometers use mercury or alcohol.

Thermometers for high temperatures use pairs of metal strips, which disconnect above a certain temperature.

Temperature scales

Any temperature measurement makes sense if it is done on a scale.

There are 3 major temperature scales currently used in the world: Celsius (Centigrade), Kelvin, and Fahrenheit.

The Celsius scale set its zero-point (0o C) at the water freezing point and 100o C at the water boiling point.The Kelvin scale has a zero-point at the so-called absolute zero (273o.16 C) and 1 K = 1oC.The Fahrenheit scale zero-point is attained by mixing equal parts of water, ice, and salt (=32oC). The water boiling point is 212oF. 1oC=1.8oF.

Relations between the Temperature Scales

Celsius to Kelvin: K = °C + 273Celsius to Fahrenheit: °F = 1.8(°C) + 32Fahrenheit to Celsius: °C = 5/9 (°F 32o)

The normal body temperature is 37oC = 98o.6 F = 310 KThe coldest temperature ever registered on Earth: 89oC = 128oF = 245 K

Heat

The heat in a material body is a sum of the kinetic energies of all its separate particles.

The more heat the body contains, the higher its temperature.

Joule is the unit for heat

Substances differ by the amount of heat needed to raise their temperature by 1o.

It takes 4.2 kJ to raise the temperature of 1 kg of water by 1oC.

Heat Transfer

Conduction (transfer through a solid body)Convection (transfer through a fluid or gas)Radiation (by electromagnetic waves)

Metabolic Energy

The rest of the energy is converted into heat.

The biochemical processes that extract energy from food are called metabolism.

Only 1020% of metabolic energy can be converted into mechanical work.Measured in kilocalories: 1kcal = 4.2 kJ.

The maximum power output of an animal depends on the surface area and mass.Metabolic rate of a person varies from 80 to ~1200 W.

FluidsParticles vibrate around fixed positions in a solid, move around them in a liquid, and move freely in a gas.

Liquids and gases are called fluids (because they flow).

Some properties of materials:

Density is mass per unit volume

d = density m = mass V = volume

d = m/V

Density

Pressure

Pressure is a force per unit area

p = pressure F = force A = area

p = F/A

Unit of pressure = 1 pascal (Pa) = 1 N/m2

1 kPa = 1000 Pa

BarometersPressure

Buoyancy

Buoyancy is an ability of a solid body to float in a fluid

If an object is immersed in a fluid, an upward force acts upon it.

The force on the bottom is higher than that on the top.

The difference between the 2 forces is the buoyant force.It enables balloons to float in the air and ships to float in the sea.

Archimedes’ Principle

Buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object.

Fd = dVgFd = buoyant force, d = density, V = volume, g = 9.8 m/s2 = acceleration of gravity

Demonstration1 Demonstration2

Summary

• Temperature is a measure of the average kinetic energy of all particles in an object

• Pressure is a force applied to a unit area

• Density describes mass of a unit volume and is different for different materials

• Buoyancy is a property of objects to float in a fluid.Weight of an object has to be less than the buoyant force.