Heat

SI unit of temperature

K

What is temperature?

Measurement of how hot an object is

What is the measuring instrument?

Thermometer, thermocouple

Why cant we use our hands to measure temperature?

Inaccurate

when pouring a liquid of different temperature into another liquid, what affects the temperature?

Volume of water

Temperature of water

How is temperature affected by pressure

Higher pressure = higher boiling point

lower pressure = lower boiling point

Why do we need to install expansion gaps on floor tiles, railway tracks, bridges (rollers) etc?

To provide space for the track to expand without damage (crack) on hot days.

What do expansion joints do in concrete bridge?

Expansion joints provide space for expansion and contraction during hot and cold weather conditions to prevent the bridge from damaging

How does climate change affect the sea level? As a result what happens?

Oceans absorb more heat

Water expands, polar ice caps and glaciers melt, thus sea level rises

As a result, more coastal land washed away

How do solids expand? (particles)

When a substance is heated, their particles gain kinetic energy and vibrate more vigourously about their fixed positions due to the increase in kinetic energy. The distance between the particles increases. This causes the volume of substances to increase as a result the substance expands

How do solids contract? (particles)

When the substance loses heat, the particles lose kinetic energy, vibrate less vigorously about their fixed positions and the distance between particles decreases. This causes the volume to decrease and the substance contracts.

How do liquids expand?

When it is heated, its particles gain energy and move much faster gaining kinetic energy. The distance between the particles increases this causes the volume of the substance to increase as a result the substance expands

How do liquids contract?

When the substance loses energy, the particles move slower and the distance between the particles decreases, this causes the volume to decrease and the substance contracts.

Metal pipes what?

expand and contract depending on the surrounding temperature and the substances they carry like steam or hot water, oil and gases, have bends which enables the pipes to expand or contract safely without breaking.

How do gases expand

When a gas gains heat, their particles gain kinetic energy and zoom past each other much faster. The distance between the particles increases, this causes the volume of the substance to increase as a result the substance expands

How does gases contract

When the substance loses energy, the particles move less slowly and the distance between particles decrease. This causes the volume to decrease and the substance contracts

What happens to particles during contraction and expansion?

Number of particles remains the same so the mass remains the same

The size of the particles does not change

Upon heating the particles does not change

Upon heating the particles gain heat energy and vibrate more vigorously. The distance between the particles increases as a result the substance expands and the volume increases

Upon cooling the particles lose heat energy and vibrates less vigorously about their fixed positions. The distance between the particles decreases as a result the substances contracts and the volume decreases.

Rate of expansion of matter from fastest to slowest

Gas

Liquid

Solid

What is weird about water?

Maximum density at 4dc

from 0dc to 4dc, it contracts when heated

from 4dc and above, it expands when heated.

What are the factors that affect heat transfer for conduction?

Rate of conductivity

State of matter

What mechanism does the bimetallic strip and bimetallic thermometers work based on?

Expansion rate

How does it work?

Made up of two metals that expand or contracts at different rates upon the same change in temperature. One mental in the bimetallic strip expands or contracts more than the other. This makes it curve.

e.g. Brass expands more than steel.

Bimetallic strip in thermometer how does it work?

As the temperature changes, the different expansion rates of the metals causes the coil to unwind when the temperature increases and it tightens when the temperature decreases. Thus the pointer moves over the scale.

How does a hot air balloon work?

When air is heated, it volume increases while its mass stays constant, hence the density decreases. Since the air in the balloon is less dense than the surrounding air, it rises and pushes the balloon up.

What are the modes of heat transfer?

Conduction

Convection

Radiation

How is heat transferred?

Net transfer of heat from a hotter region to a colder region,

How does conduction work?

Particles contact to transfer heat

What affects conduction?

Type of material and state of matter

What state of matter conducts heat faster? From descending order

Solids, liquids, gas

Why does solids generally conduct heat better than fluids?

Solid particles are very closely packed and collide more frequently.

Why are metals the best heat conductor

Metals are the best heat conductors as they also have mobile valence electrons to transfer heat through the metal.

Fluids are?

Poor heat conductors

Why does paper over metal burn slower?

Metal is a good conductor of heat, so more heat is transferred to the metal, slowing down the amount of heat the paper has.

How does convection work

Fluid that is heated/cooled will expand/contract, causing density to increase/decrease and resulting in hotter fluid rising/colder fluid sinking to take up the space of the hot/cold air. This sets up convection currents for heat transfer.

When can fluids transfer heat via convection?

If heat source is at the bottom or the cooler is at the top.

How is heat transferred via radiation?

Electromagnetic waves emitted from surroundings, no need for medium

What affects emission and absorption of radiation?

Surface area

Texture

Colour

Elab

Black rough surfaces are good absorbers and emitters of radiation

White shiny smooth surfaces are poor emitters and absorbers of radiation but are good reflectors of radiation

Why should we have air between surfaces?

heat cannot transfer through conduction QUICKLY. This reduces heat transfer to the surroundings.

Why should we have a cover covering a thermoflask?

Cover must be poor conductor of heat

Prevent convection with the surroundings - reduces heat loss to the surroundings

Movement of land and sea breezes

What doesnt occur and occurs in vacuums?

Not in vacuum

• Conduction

• Convection

Vacuum

• Radiation

Air spaces in objects are?

Poor conductors of heat - this prevents quick heat loss via conduction.

small beaker containing a coloured liquid was placed into a large beaker containing water at room temperature. The other was placed in a beaker at 80 dc. The smaller beakers are covered with aluminium foil secured with a rubber band and a small hole is made. How will the water move in the beaker filled with room temperature water?

Diffusion

Movement:

The coloured liquid will escape from the hole in the aluminium foil covering the small beaker and spreads out

How about the water at 80dc?

Convection

Movement:

Coloured liquid escapes the hole in the aluminium foil covering the beaker, and moves upwards in a stream before spreading out, with a little liquid spreading out near the hole.

where is the heat source in order for radiation to occur?

 - Radiation occurs in all directions. 

 - If the heat source is near/beside the object, then there is only radiation. 

 - If the heat source is touching the object, then there is conduction. 

 - If the heat source is below the object, there is convection and radiation.

 - If the heat source is above the object, there is only radiation.