heat energy

the sensation of hotness or coldness.

factor for heat transfer

The bodies must have different temperatures.

temperature

measure of the degree of hotness or coldness.

thermal equilibrium

when 2 bodies attain the same temperature. *Note: temperature flows from the body of a higher temperature to the body of a lower temperature *

SI unit

joules

1 calorie

defined as the heat needed to raise 1 gram of water by one degree.

1 kilocalorie

defined as the heat needed to raise 1 kilogram of water by one degree.

1 cal - joule

1 cal = 4.2 joules

1 kcal - joule

1 kcal = 4200 joules

1 joule - cal

1 joule = 0.24 cal

motion of the molecules of a substance at a high temperature

its molecules vibrate very vigorously

parts of a thermometer

it consists of a long, graduated glass tube, a capillary tube, and a bulb at one end which contains thermometric fluid.

qualities of a good thermometer

* The bulb is thin, so it can attain temperature fast. * The thermometric fluid should expand uniformly, be opaque, not stick to the sides of the tube, and have a large range of temperatures. * The capillary tube should be thin so that a small temperature change is easily visible. * the glass tube should be thick so the capillary tube can be protected.

why is mercury a good thermometric fluid?

It is non-volatile, expands evenly, has a low freezing point and high boiling point, and is opaque and clearly visible.

celsius scale

0 degrees to 100 degrees.

fahrenheit scale

32 degrees to 212 degrees

kelvin scale

273 K to 373 K

lowest temperature of the universe

0 K

change in state

when heat is supplied to matter, the energy increases and they vibrate more vigorously, causing change in state. the same goes for when heat is liberated from a body, as the energy decreases and they come closer. hence, they change state.

states of matter

solid, liquid, gas, plasma, Bose - Einstein condensate

applications of thermal expansion in solids (1. glass jar)

When a glass jar with a metal lid is tightly sealed and cannot be opened, we pour hot water over the lid so it expands and opens easily.

applications of thermal expansion in solids (2. telephone wires)

telephone wires are placed a little loose as the wires expand in summers and contract in winters. if there was no space, then with the contraction in winter, it would fall.

applications of thermal expansion in solids (3. metal bridge)

When metal bridges are constructed, one end is placed on rollers so it can expand and contract easily.

applications of thermal expansion in solids (4. concrete path)

when we construct concrete footpaths or roads, we leave a gap for expansion, so the concrete does not crack.

applications of thermal expansion in solids (5. train tracks)

when we construct meatal train tracks, we leave a gap for expansion, so they do not bend outwards after expansion.

applications of thermal expansion in solids (6. bimetallic strip)

Ex 1: Brass expands 1.5 times more than steel, so when they are welded together and heated together with brass on top, it will bend downwards as brass expands more. Ex 2: aluminium expands 2 times more than steel.

thermal expansion of water

contracts on heating from 0 to 4 degrees C and expands on further heating, hence, ice is less dense than water

applications of thermal expansion in liquids (thermometers)

the thermometric fluid in thermometers expands on heating and contract on cooling.

applications of thermal expansion in liquids (water bottle)

We should never fill bottles to the top as the liquid can expand on heating, causing the bottle to explode.

state of matter with most expansion

gas

state of matter with least expansion

solid

conduction

method of heat transfer that requires direct contact of the substances. The molecules vibrate about their mean position.

good conductors of heat

substances that allow heat to flow through them easily. All metals are good conductors, but silver is the best.

insulators

substances that do not allow heat to pass through them. Cotton, glass, plastic and wood are insulators.

uses of conductors

* kitchen utensils like pans are made with metals to pass heat easily. * mercury is used as thermometric fluid. * cooling coils in fridges and ACs are made of copper to get rid of the heat.

uses of insulators

* snow is a bad conductor of heat so eskimos use snow to build igloos. * we cover cooking utensils' handles with plastic or wood to avoid getting burnt. * the fur of animals acts as an insulator.

convection

method of heat transfer with actual movement of molecules. it mainly takes place in liquids and gases.

examples of convection in nature

sea breeze and land breeze

uses of convection

* AC is placed on the ceiling * The heater is placed on the floor * Firemen crawl on the floor to avoid inhaling smoke as the smoke rises upwards

radiation

method of heat transfer without affecting the intermediate space. the heat emitted is called radiant heat. heat is transferred in a straight path.

factors for radiation

* surface area * color and texture of the surface[dark and dull surfaces absorb the most] * surface temperature

uses of radiation

we wear light colours in summer and dark colours in winter, the inner surface of solar cookers are painted black.

thermos flask's prevention of convection

it has a cork to prevent heat loss. [thermos flask]

thermos flask's prevention of conduction

it is double walled with a vacuum between the walls and its walls are made of glass. [thermos flask]

thermos flask's prevention of radiation

the insides are silvered, so the heat bounces and reflects against the wall. [thermos flask]