Heat capacity and transfer of heat

HEAT CAPACITY

HEAT

Heat is the transfer of thermal energy from a hotter region to a colder region.

HEAT CAPACITY

It is the amount of heat energy needed to change the temperature of a substance by 1 Kelvin (or 1C).

Formula: unit of heat capacity: J/K Where C = heat capacity (J/K)Q = Amount of heat energy (J)Δθ = change in temperature (K)

SPECIFIC HEAT

CAPACITY

It is the amount heat energy needed to change the temperature of 1 kg of a substance by 1K (or 1C).

Formula:

unit of specific heat capacity: J/kgK

Where c = specific heat capacity (J/kgK)Q = Amount of heat energy (J)Δθ = change in temperature (K)

EXPERIMENT

Apparatus: an aluminum block, heater, thermometer, stopwatch, ammeter, voltmeter. Method: 1. Record the mass of the aluminum block and its initial temperature.2. Start the heater and the stopwatch simultaneously.3. Take the ammeter and voltmeter readings.4. Record the final temperature of the block.5. Use the formula given below to calculate the specificheat capacity of the aluminum.

Formula: C= VIt mΔWhere V = voltage (V)I = current (A) t = time (s)m =mass of the aluminium block (kg)Δ = change in temperature (C)c = specific heat capacity of aluminium (J/kg/K)

MELTING AND BOILING

STATES OF

MATTER

MELTING

SOLIDIFICATION

AND

•On heating, a solid changes to a liquid. We call this change of state ‘Melting’.

•During melting, heat energy is used to break the bonds between the molecules of the solid.

•Melting will occur once these molecular bonds are broken and they move away from their previously fixed positions.

•There is no change in temperature of the body during melting, even when heat is continuously being supplied to the body. Temperature of the body will increase when all the bonds are broken.

•Heat that is absorbed to break these bonds without a change in temperature is called latent heat of fusion.

•Applying pressure on ice will cause its melting point to drop.

From A --- B: Temperature of ice rises from -15C to 0C.

From B --- C: No change in temperature as heat is being absorbed. At this stage, it contains a mixture of ice and water.

From C --- onwards: Temperature starts to increase as now all the ice has melted. Energy is now used to increase the temperature of water.

•The process by which a liquid changes its state to become a solid when heat is removed from a body.

•When a body undergoes solidification, heat is released as intermolecular bonds are formed. This results in a body more rigid as compared to that of the liquid.

•Despite heat being given off during solidification, there is no change in temperature.

•Temperature of the body will only start to drop when it is completely solidified or has the same solid state throughout the entire body.

From A --- B: Temperature of water falls from 100C to 0C.

From B --- C: No change in temperature as heat is being released. At this stage, it contains a mixture of ice and water.

From C --- D: Temperature starts to decrease as now all the water has solidified.

CONDENSATION

AND

SUBLIMATION

•Condensation is the change of gas to liquid.• Heat energy is given out as gas particles slow down and move closer to one another to form liquid.

•Sublimation is the change of solid to gas without melting.•Heat is absorbed.

EVAPORATION

•Evaporation is the change of liquid to gas without boiling, occurs below boiling point on water surface. It gives cooling effect , as heat energy is absorbed from surroundings.

•Boiling and evaporating are not the same.

•Boiling only occurs at at a fixed temperature whereas evaporation takes place at any temperature.

•Boiling takes place within the liquid whereas evaporation only takes place at the surface.

•Evaporation will cause an object to loose heat and cool down whereas temperature remains steady when boiling takes place.

•Boiling produces effervescence whereas evaporation does not.

Factor Its effect on rate of evaporation

1. Temperature The higher the temperature, the faster the rate of evaporation.

2. Humidity The higher the humidity in the surroundings, the slower the rate.

3. Surface area The larger the surface area of the liquid, the faster the rate.

4. Air movement Air blowing will remove the layer of saturated vapour and hence increase the rate.

5. Pressure Reducing pressure increases the rate.

6. Nature of liquid The lower the B.p, the higher the rate.

FACTORS EFFECTING EVAPORATION

TRANSFER OF THERMAL ENERGY

CONDUCTION

This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.

•It occurs mostly in solids, for example in metals like iron and copper.

•Conduction is the process of heat transfer that occurs between atoms or particles that are vibrating in their fixed positions.

•For example, when one end of a metal rod is heated, the particles on the heated end gain energy and start to vibrate faster. They then collide with the less energetic neighboring particles, causing them to vibrate together.

•Through vibrations of the particles, heat energy is transferred from the hotter end of the rod to the colder end until an equilibrium temperature is attained.

•Conduction also occurs in non-metals, such as wood.

This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.

•Whether a material is a good conductor or poor conductor of heat depends on the thermal conductivity of that material.

•A material with a high value of conductivity will be a better conductor of heat.

•For example, metals are better conductors of heat compared to wood and air because metals have higher thermal conductivity.

•Solids like metals are good conductors of electricity and heat because they have free electrons that move relatively easily in the metal itself.

•These electrons move in a very high speed, and being energized, they carry kinetic energy and will collide with neighboring atoms or molecules.

•This explains why metals are better conductors compared to non-metals.

GOOD AND POOR CONDUCTORS

This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.This is the method of heat transfer in solids.

•Trapping a layer of air to reduce the amount of heat lost to the cold environment to keep our body warm.

•Use of metals such as aluminum and stainless steel as cooking utensils and sauce pans.

•Use of insulating materials such as wood for handles of cooking utensils to reduce amount of heat flow.

APPLICATIONS

CONVECTION

•It occurs mostly in fluids such as air or water.

•When a glass of liquid, such as water, is heated from the bottom, the layer of water closer to the heat source expands and hence becomes less dense compared to the water layer above it.

•As we know that a less dense object will float above a denser object.

•Hence the warmer water layer will rise above the colder layer of water and cause a current to be generated in the glass of water.

•The process of heat transfer from one region to another through the movement of heated particles is known as convection.

•Convection cannot occur in solids because the particles are in fixed positions and are unable to move freely.


APPLICATIONS

•Heating coil of an electric kettle is placed at the bottom to allow convection currents to form and heat up the water.

•Air conditioners are always fixed close to the ceiling as cold air sinks.

RADIATION

•Unlike conduction and convection which require a medium to travel, radiation can transfer heat through a vacuum.

•It is the only way heat can be transferred in a through a vacuum.

•It is the process whereby heat energy is transmitted in the form of electromagnetic waves or Infrared radiation.

•One common example is the heat from the sun which travels through a vacuum before arriving on earth’s surface.

•Factors that affect the rate of emitting and absorbing radiation are the colour, shininess and area of the material.

•A dull and dark coloured object is a better emitter and absorber of radiation compared to a shiny and bright coloured object.


APPLICATIONS•On a hot day, we should try to wear clothes of brighter colours instead of dull and dark colours to reflect most of the radiant heat.

•Exposing our skin for very long to harmful UV radiation from the Sun may cause skin cancer.

•Plants are kept in a greenhouse which traps heat. the glass panels will allow shorter wavelengths of radiation and will trap the reflected radiation with longer wavelengths.

THE VACUUM FLASK

A vacuum flask is able to reduce heat loss (to the surrounding) by minimizing heat loss in 4 possible ways; conduction, convection, radiation and evaporation.1. Conduction and convection through

the sides of the flask are prevented by the vacuum between the double-glass walls of the flask.

2. Conduction through the trapped air above the liquid is minimal since air is a very poor conductor of heat and the stopper too is made up of plastic.

3. Convection and evaporation can only occur when the stopper is removed during use.

4. Heat loss by radiation is harder to stop as radiant heat can pass through a vacuum. to minimize heat loss through it , the walls of the glass are silvered so as to reflect the radiant heat back into the liquid.

THE END

Heat capacity and transfer of heat

Education