HEAT ENERGY

Behavioral Objectives: at the end of the lesson, you should be able to: explain the concept of heat and temperature (ii) State the differences between heat and temperature state the kinetic theory of matter list the effects of heat on substances ( expansion, vaporization) (iv) define linear, area , and volume expansivity Solve questions relating to linear, area, and volume expansivity. (vi) State the advantages and disadvantages of expansion.

Differences between heat and temperature Concept of heat and temperature Heat is a form of energy that moves from one point to the other due to temperature difference. When you dip one end of an iron rod into fire and hold the other end with your hand, this other end soon becomes hot because energy has flowed from the point dipped into the fire to this other end. This energy flow is what is known as heat. Temperature is a measure of how cold or hot a body is. Differences between heat and temperature Heat Temperature It is a form of energy It is the degree of hotness or coldness of a body It is measured in joules It is measured in Kelvin it is not determined directly by an instrument it is directly determined by a thermometer it is a derived quantity It is a fundamental quantity

The kinetic theory of matter The kinetic theory of matter states that: (i) Matter is made up of atoms and molecules (ii) The molecules are in a state of constant random motion (iii) They possess kinetic energy because of their motion (iv) The kinetic energy of the molecules is directly proportional to the temperature of the body.

Effects of heat on substances When heat is applied to a substance, it can lead to the following changes: (i) Chemical changes (ii) Temperature changes (iii) Expansion/contraction (iv) Change of state (melting, vaporization, sublimation) (v) Change in pressure (vi) Thermionic emission

Thermal Expansion in Solids Most solid substances expand when heated. The rate of expansion varies from solid to solid. Expansion is more pronounced in gases followed by liquids and least in solids. A substance whether solid, liquid or gaseous consists of molecules. When the substance is heated, the molecules gain kinetic energy and move faster and hence the molecules take up more space in the substance. This leads to expansion.

Ball and ring experiment An experiment can be carried out to demonstrate expansion of a solid. Apparatus: Bunsen burner, ball and ring apparatus

Procedure: Allow the metal ball to pass through the ring. Heat the metal ball for some time in the Bunsen burner and make it pass through the same ring. The metal ball will no longer pass through the same ring it passed through earlier as a result of expansion. When allowed to cool down for some time and allowed to pass through the ring once more, it will pass through because it has contracted and regained its original size.

Using kinetic theory to explain temperature of a body According to the kinetic theory of matter, the average kinetic energy of the molecules is directly proportional to the temperature. This means that as the kinetic energy of the molecules increases, the temperature also increases. When a body is subjected to heat, the velocities of the molecules increases and hence they gain more kinetic energy this of course will lead to increase in the temperature of the body. On the other hand, if we reduce or lower the heat, the velocities of the molecules will decrease leading to a decrease in the kinetic energy of the molecules. Hence the temperature falls or reduces.

THERMAL EXPANSIVITY TYPES OF EXPANSION: I. Linear expansion   THERMAL EXPANSIVITY TYPES OF EXPANSION: I. Linear expansion 2. Area or Superficial Expansion 3. Volume or cubic Expansion LINEAR EXPANSIVITY (α) It is defined as increase in length per unit length per degree rise in temperature. The unit is per Kelvin or 1/k or k – 1 α = L2 – L1 L1(θ1 – θ2 ) θ2 is final temperature, θ1 is initial temperature, L1 is the initial length, L2 is the new length

Area or Superficial expansivity(β) It is defined as the increase in area per unit area per degree rise in temperature β = A2 – A1 A1(θ2 – θ1)   θ1 is initial temperature, θ2 is final temperature A2 is new area, A1 is original area Relationship between Linear expansivity and Area expansivity: Β = 2α i.e. Area expansivity = 2 x linear expansivity.

Relationship between Linear expansivity and Cubic expansivity It is defined as the increase in volume per unit volume per degree rise in temperature γ = V2 – V1 V1(θ2 – θ1) V2 is final volume. V1 is the initial volume, θ2 is final temperature, θ1 is initial temperature   Relationship between Linear expansivity and Cubic expansivity Cubic Expansivity = 3 x linear expansivity i.e. γ = 3α

Applications of Expansion (A) ADVANTAGES OF EXPANSION: 1. The use of the bimetallic strip in: a. Electric fire alarm b. Bi-metallic thermometer c. Electric pressing iron 2. Red hot rivet used in ship 3. Removal of tight glass stopper 4. Fitting of wheels in rims

Bi-metallic strip: It consists of two different metals joined together. They expand at different rates when heated e.g. brass and iron.

ELECTRIC FIRE ALARM: When a fire breaks out in a building, the resulting heat causes the bi-metallic strip to bend towards the contact, thus completing the circuit. This causes the bell to ring out a fire alarm.  

BI-METALLIC THERMOMETER It consists of a coiled bi-metallic strip which expands outwards when heated. As this happens, the pointer moves along the scale and the reading on the scale is taken as the temperature.

ELECTRIC PRESSING IRON: It has a device known as Thermostat, it is made of bi-metallic strip and it is used to regulate the temperature of the pressing iron, Gas cooker, Refrigerator. MODE OF OPERATION OF PRESSING IRON When the current is switched on, current flows through the circuit and the bi-metallic strip expands and bends away from the contact point thereby switching off the flow of current. The pressing iron cools down and contact is re-made and current flows again and the process continues. This make-and-break device regulates the temperature of a pressing iron.  

2. RED-HOT RIVET USED IN SHIP: Steel plates and girders which are used in ship building and other constructional works are usually riveted together. 3. FITTING OF WHEELS IN RIMS: The large driving wheels of locomotive are fixed with steel tyre which are renewed from time to time as they wear out. In order to ensure a tight fitting, the tyre is made slightly smaller in diameter than the wheel. The tyre contracts on cooling thus ensuring tight fitting. 4. REMOVAL OF TIGHT GLASS STOPPER: A tight glass stopper can be removed by standing the bottle in hot water. The glass bottle expands and the stopper becomes loose.  

(B) DISADVANTAGES OF EXPANSION: Expansion of metal on steel bridges/galvanized iron sheets: Cracking sounds are heard when galvanized iron sheets used in the roof of buildings are being heated. This is due to the expansion of sheet when heated. Bridges made of steel equally expand during hot weather. Cracking of glass cup when hot water is poured into it: When hot water is poured into the glass tumbler, it often cracks due to uneven expansion of the interior walls and exterior walls of the glass cup. 3. Expansion of balance wheel of a wrist watch. This makes the watch to give wrong reading Sagging of overhead wires: Telegraph wires when laid in hot weather are allowed to sag so that in cold weather they can contract without snapping. 5. Expansion of railway lines: Gaps are left between rails in railway lines to allow for free expansion and contraction of rails, without the gaps, there would be buckling of rails.  

Evaluation. (i) Define temperature and state its unit (ii) State three assumptions of the kinetic theory of matter (iii) Give three differences between heat and temperature (iv) Explain the phenomenon of expansion using the kinetic theory of matter (v) Give four effects of heat on a substance (vi) Explain four advantages of expansion in solids. (vii) Explain three disadvantages of expansion in solids