PHY 101

Temperature, Thermometers, and Heat Energy.

By

Mr A.J OMOLIKI

TEMPERATURE

O The temperature of a substance is the degree of hotness or

coldness of the substance. A hot substance is said to have a high

temperature whereas a cold substance is said to have a low

temperature. Therefore, the temperature of a substance is an

indication of the average kinetic energy of the molecules of the

substance.

O Heat always flow from a body at higher temperature to the body at

lower temperature. So, we can also say that temperature of a body

is the property which governs the flow of heat.

O It can easily be demonstrated as follows: When two objects of the

same material are placed together, the object with higher

temperature cools while the cooler object gets warmer until a point

is reached after which no more changes occurs.

THERMAL EQUILIBRIUM

O Heat energy flows from a body at higher temperature to another

body at lower temperature. In other words, heat flows from a

hotter to the colder body.

O The heat energy keeps flowing from the hotter to the cooler

body until the temperature of both the bodies become equal. At

this stage, the two bodies are said to be in thermal equilibrium.

O When two bodies attain the same temperature, they are said to

be in thermal equilibrium with each other because then no heat

flows from one body to another.

O The temperature thus reached is called as equilibrium

temperature. Therefore, now we can say that Temperature is that

quantity which is same for both the system when they are in

thermal equilibrium.

ZEROTH LAW OF THERMODYNAMICS

O ZEROTH law of thermodynamics state that "If two

systems are separately in thermal equilibrium with a third,

then they must also be in thermal equilibrium with each

other, and they all have the same temperature regardless of

the kind of the system they are."

O This law can be restated as follows: If there are three or

more than three systems which when taken together are in

thermal contact and in thermal equilibrium as well, then

any of the two system taken together are in equilibrium

with one another

THERMOMETERS

O A thermometer is a device that measures temperature of

an object or a system

O When a thermometer is in thermal contact with a system,

energy is exchanged until the thermometer and the system

are in thermal equilibrium with each other.

O For accurate readings, the thermometer must be much

smaller than the system, so that the energy the

thermometer gains or loses doesn’t significantly alter the

energy content of the system.

O All thermometers make use of some physical property

that changes with temperature and can be calibrated to

make the temperature measurable.

Thermometric Properties

O Some of the physical properties used are

O the volume of a liquid,

O the length of a solid,

O the pressure of a gas held at constant volume,

O the volume of a gas held at constant pressure,

O the electric resistance of a conductor, and

O the color of a very hot object.

Temperature Scales

O Fahrenheit is a temperature scale used mostly in the

United States.

O Celsius is the temperature scale used mostly in other

countries and in science. Based on 0 C being freezing

point and 100 C being boiling point, the difference

between those two points is divided up into 100 equal

parts.

O Kelvin scale is the temperature scale used by scientists,

where all of the numbers are positive. It is based off the

idea of absolute zero.

What is the Celsius to Fahrenheit conversion?

O F = (1.8 x C) + 32.0

O Convert 25 C to F

O F = 77

O Convert 0 C to F

O F = 32 (The freezing & melting point of

water)

What is the Fahrenheit to Celsius conversion?

O C = (F - 32.0)

1.8

O Convert your body temperature 98.6 F to C .

O C = 37

O Convert a warm Spring day’s temperature of

78 F to C

O C = 26

What is the Celsius to Kelvin conversion?

O K = C + 273

O Convert the temperature of a winter day at the North Pole (- 40.0 C) to both degrees Fahrenheit and Kelvin.

O The temperature gradient between the skin and the air is regulated by cutaneous (skin) blood flow. If the cutaneous blood vessels are constricted, the skin temperature and the temperature of the environment will be about the same. When the vessels are dilated, more blood is brought to the surface. Suppose during dilation the skin warms from 72.0°F to 84.0°F. (a) Convert these temperatures to Celsius and find the difference. (b) Convert the temperatures to Kelvin, again finding the difference

Example

O An extraterrestrial scientist invents a

temperature scale such that water

freezes at 275°E and boils at 325°E,

where E stands for an extraterrestrial

scale. Find an equation that relates

temperature in °E to temperature in °C.

O Answer: 𝑇𝐸 = 4𝑇𝑐 − 75

Absolute zero and Triple point.

O Early gas thermometers made use of ice and steam points for calibration. These points are experimentally difficult to duplicate, however, because they are pressure-sensitive.

O Consequently, a procedure based on two new points was adopted in 1954 by the International Committee on Weights and Measures.

O This procedure are the Absolute Zero and Triple point of water

O Absolute zero is the lowest possible temperature . An object’s energy is zero. There is no possible transfer of energy.

O Absolute zero is used as the basis for the Kelvin temperature scale, which sets -273.15°C as its zero point (0 K)

O the triple point of water, which is the single temperature and pressure at which water, water vapor, and ice can coexist in equilibrium.

O The temperature at the triple point of water on the Kelvin scale occurs at 273.16 K.

O Therefore, the SI unit of temperature, the kelvin, is defined as 1/273.16 of the temperature of the triple point of water.

DEALING WITH TEMPERATURE WHEN DOES AN

ENERGY TRANSFER OCCUR?

O The feeling associated with temperature difference

results from energy transfer.

O Energy is transferred from a hotter object to a cooler

object.

O There no transfer of energy in regards to temperature

O If both objects are the same temperature there is no

transfer of energy AND

O when the temperature is at absolute zero.

O Example –Holding a piece of ice.

O The ice is at a lower temperature than your hand,

so the molecules of ice move very slowly. Your

hand’s molecules are moving much faster than

the ice because it is at a higher temperature. As a

result, the molecules of your hand collide with the

ice molecules and energy is transferred so the ice

molecules start to move faster causing the ice to

melt.

• Heat

• is the transfer of energy from the

particles of one object due to a

temperature difference between

the two objects.

Molecules begin to move faster as thermal

energy is added.

More

Thermal

Energy

Molecules begin to move slower as thermal

energy is taken away.

Less

Thermal

Energy

• Three methods of energy transfer …

• Conduction

• Convection

• Radiation

What is conduction?

• It is the transfer of energy as heat

between particles as they collide

within a substance or between two

objects in contact.

• Two factors involved in conduction

• Has the direct contact of objects or

atoms.

• Usually is an energy transfer

between solids

• Example:

• Leaving a metal spoon in a pot of

soup cooking on the stove.

• When you heat a metal strip at one end,

the heat travels to the other end.

• What is convection?

• It is the transfer of energy by the

movement of fluids with different

temperature.

• How does convection move?

• It is a result

from the

movement of

hotter fluids to

colder fluids.

• Two Types of Fluids • Gases and

liquids.

Convection current.

• is the flow of a fluid due to the heated expansion followed by cooling and contraction

• Examples:

• Glowing embers caught in the warmed air above a campfire, or the movement inside the earth for the plate tectonic movement.

• Water movement

Hot water

rises

Cooler

water sinks

Convection

current

Cools at the

surface

What is radiation?

• Radiation is the transfer of energy

by electromagnetic waves.

• Examples:

• When you stand by fire, your

skin absorbs the energy

radiated by the fire.

How does heat energy get

from the Sun to the Earth? There are no particles

between the Sun and the

Earth so it CANNOT travel

by conduction or by

convection.

? RADIATION

How does radiation differ from conduction

and convection?

• It does not involve or the movement of

matter (or physical contact between

objects). So it can travel through a

vacuum like space.

• How is radiation like convection?

• Radiation is like convection in that it

can travel through fluids.

• Conductors

• are materials through which

energy can easily be transferred as

heat.

• Examples - Some cooking pan,

copper, and silver.

• In general metals are better than

nonmetals.

Insulators

• are materials that are poor energy

conductors.

• Examples - Some insulators are

wood, foam, rubber, and

polystyrene

Exercise

O For each of the following temperatures, find the equivalent temperature on the indicated scale: (a) -273.15°C on the Fahrenheit scale, (b) 98.6°F on the Celsius scale, and (c) 100 K on the Fahrenheit scale.

O The pressure in a constant-volume gas thermometer is 0.700 atm at 100°C and 0.512 atm at 0°C. (a) What is the temperature when the pressure is 0.040 0 atm? (b) What is the pressure at 450°C?

O Show that if the temperature on the Celsius

scale changes by ∆𝑇𝑐, the Fahrenheit

temperature changes by ∆𝑇𝐹=2

5∆𝑇𝐶.

O The temperature difference between the inside

and the outside of a home on a cold winter day

is 57.0°F. Express this difference on (a) the

Celsius scale and (b) the Kelvin scale