HEAT ENERGY
Heat Energy is an energy that is transferred from an object that is hot to a cooler object as a result of their difference in temperature. Heat energy is a form of energy called Thermal energy.
Temperature is the degree of hotness or coldness of a body. It is a scalar quantity. The temperature of a body is measured with an instrument called thermometer.
The following are the effects of heat on a body;
- Heat will cause a body to rise.
- Heat can bring about a change of state to that body.
- Heat leads to expansion of a body.
- Addition of heat to a body may cause changes in the physical properties of the body.
- Addition of heat to a body may cause changes in the chemical properties of the body.
- When heat is added to a gas, the pressure of the gas increases.
- When Heat is added to a metal, it leads to a process known as Thermionic emission(the emission of electrons from the surface of a hot metal).
This states that
- Matter is made up of molecules
- These molecules are always in a constant state of motion.
- The molecules possess kinetic energy because they are in a state of motion.
- There is always an attractive force between the molecules.
Thermal expansion can be defined as the tendency of a body to change in shape and volume in response to change in temperature. Expansion means an increase in the size of an object.
Both solid, liquid and gas expand when they are heated, but the rate at which they will expand differs, this is as a result of the strength of the intermolecular forces binding the molecules. The intermolecular forces in solid is stronger than that of liquid and weakest in gases, as a result, gases will expand more than liquids, and liquids will expand more than solids.
The linear expansivity of a solid is defined as the increase in length per unit length per degree rise in temperature.
α = L2 = L1[1 + α(θ1 - θ2)]
Where α =linear expansivity
L1 = Length of metal at temperature θ1
L2 = Length of metal at temperature θ2
This can be defined as the increase in area per unit area per degree Kelvin rise in temperature.
β = Area expansivity,
A2 =A1(1 + βθ)
The increase in area = A2 – A1
The increase in area = A2 – A1
This is the increase in volume of a substance per unit volume per Kelvin rise in temperature.
V2 =V1(1 + ϓ θ)
NOTE: Area expansivity is twice linear expansivity (β=2α) while cubic expansivity is thrice linear expansivity (ϓ=3α)
Questions
- A glass bottle of initial volume 2×10⁴cm³ is heated from 20°C to 50°C. If the linear expansivity of glass is 9× 10⁻⁴K⁻¹ The volume of the bottle at 50°C is
- A metal sheet of area 100cm² was heated through 70°C. Calculate its new area if the linear expansivity of the metal is 0.000017K⁻¹ ).
- A metal rod of length 50cm is heated from 40°C to 80°C. If the linear expansivity of the metal is α. calculate the increase in length of the rod in terms of α.
Solution
Take note of the following
- A man standing 3m from a charcoal fire is warmed mainly by radiation.
- The mechanism of heat transfer from one point to another through the vibration of the molecules of the medium is conduction.
- In a good thermos flask, the main cause of heat loss is conduction through the cork.
- During summer, the balance wheel of a clock expands. What effect does this have on the accuracy of the clock? The clock loses time
- Tea pots are often silver-coated to prevent heat loss by radiation.
- The main reason for making the cover of a vacuum flask air tight is to prevent heat loss by convection.
- The vacuum in a thermos flask helps to reduce heat transfer by convection and conduction.
- Thermal equilibrium between two objects exists when the temperatures of both objects are equal
- When heat is applied to one end of a metal rod, the molecules at the other end soon begin to vibrate with greater amplitude than before because heat has been transferred by conduction
- The melting point of a substance is equivalent to its solidification temperature
- What will happen to the boiling point of pure water when it is heated in a place 30m below the sea level? ANS: it will be more than 100°C
- Steam from boiling water causes more damage on the skin than does boiling water because. AnS: steam brings heat more easily by convection
- Foods cook quicker in salt water than in pure water because of the effect of dissolved substances on the boiling point
- The ground is always cold at night because earth radiates heat to the atmosphere at night
- The phenomenon that shows that increase in pressure lowers the melting point can be observed in regelation
- When cold water is poured on a can containing hot water, the can collapses because the External pressure becomes greater than the pressure within the can
- When left in a freezer, a bottle full of water cracks on freezing into ice because of the increase in the volume of water
- A quantity of water at 0°C is heated to about 30°C. At each degree rise in Temperature, its density will rise then fall.
The three states of matter are solid, liquid and gas. The processes involved in the change of state are
Melting: this is the process whereby a substance changes from solid to liquid when heat is applied to it.
Freezing: this is the process whereby a substance changes from liquid to solid.
Vaporization: this is the process whereby a substance changes from liquid to gas when heat is applied to it.
NB: The process whereby a substance changes from liquid to vapour before its boiling point is called evaporation.
Sublimation: the process whereby a substance changes from solid to gas.
Condensation: this is the process whereby a substance changes from gas to liquid.
Desublimation: this is the process whereby a substance changes from gas to solid.
- Expansion in bridges
- Expansion in railway lines
- Balance wheel of clocks and watches
The transfer of heat can happen in three ways
Conduction, convection and radiation
Conduction: this is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules.
NB: the ability of a metal to conduct heat is known as thermal conductivity
Convection: this is the process by which heat is transferred in a liquid or gas by the actual movement of the heated fluid from the hot part to the colder parts.
During the day, the land heats up faster than the sea (because the land has a lower specific heat capacity). As a result, the air near the land warms up and rises. Cooler air from the sea moves in to replace the risen air. The breeze that blows from the sea to land is known as sea breeze.
At night the reverse takes place. The sea retains much of its heat in the night while the land temperature drops considerably (the sea is warmer than the land at night). The breeze that blows from the land to the sea is known as land breeze.
Radiation :This is the process by which heat is transferred from a hotter to a cooler place without heating of the intervening medium. Heat from the sun reaches us through radiation.
NB: radiation do not require material medium .
- A vacuum between the walls.
- The two interior facing walls Coated with silver.
- Insulating cork support at the bottom.
- An insulating plastic or cork stopper.
The vacuum prevents heat losses by conduction and convection. The silvered walls prevent loss of heat by radiation (silver is a poor radiator), the insulating stopper reduces loss of heat from hot liquid, upwards to the outside by convection, the insulating cork supports prevent heat loss from the glass to the surrounding flask by conduction.
Exercises
- A wire of length 100m at 30°c has a linear expansivity of 2× 10⁻⁵K⁻¹. Calculate the length of the wire at a temperature of -10°C
- Two metals P and Q are heated through the same temperature difference. If the ratio of the linear expansivity of P and Q is 2:3 and the ratio of their length is 3:4 respectively. The ratio of the increase in lengths of P and Q is
- A metal of volume 40cm³ is heated from 30°C to 90°C. The increase in volume is? (Linear expansivity of the metal= 2× 10³ K⁻¹).
- A blacksmith heated a metal whose cubic expansivity is 6.3× 10⁻⁶K⁻¹. The area expansivity is?