Heat Energy & Temperature CHAPTER 11

Heat Sources Is fire the only source?

Measuring Temperature Temperature is a measure of how HOT something is and is usually measured using a thermometer. A typical laboratory thermometer is capable of measuring temperatures in DEGREES CELSIUS or o C that lie between 1.0 o C which is the temperature water freezes at. This is the FREEZING POINT of water o C which is the temperature water boils at. This is the BOILING POINT of water.

Here are some typical temperatures

Temperature Measurement After time, temperature is the second most measured physical unit. TEMPERATURE –MEASURING INSTRUMENTS

TEMPERATURE-MEASURING INSTRUMENTS Temperature is the degree of hotness or coldness of a substance measured on a definite scale. Temperature is measured when a measuring instrument, such as a thermometer, is brought into contact with the medium being measured. All temperature-measuring instruments use some change in a material to indicate temperature.

Thermometric properties and substances oAny measurable physical property of a substance that varies with temperature can be used for temperature measurements. Such a property is often called a thermometric property. oA substance with any thermometric property is call a thermometric substance.

Examples of thermometric properties: oThe length of a solid oThe volume of liquid oThe pressure of a gas at constant volume oThe volume of a gas at constant pressure oThe resistance of a conductor or semiconductor oThe induced electromotive force of dissimilar metals in an electrical circuit oThe color of an object oThe color transmitted/reflected by a liquid crystal oThe intensity of thermal radiation

The choice of thermometric property/substance for temperature measurements depends on the range of temperature to be measured and the application. Examples: Mercury is often used in meteorological thermometers, but alcohol is preferred in some cases because it has a much lower freezing point than mercury. For measuring very high temperature, an optical pyrometer (in which the thermometric property employed is the color of an object) is used.

Sensitivity Accuracy Reproducibility Speed Important characteristics of a thermometer :

THERMOMETRY Liquid in glass thermometer: liquid expands faster than glass when thermometer is heated, so level of liquid rises. Calibrate the level variations with temperature to make a thermometer. Thermocouple: wires of dissimilar metal are mechanically joined and form an electric potential (voltage) between one another that depends on temperature. Connect to a voltmeter and calibrate to make a thermometer. Optical pyrometer: measures intensity of infrared radiation from a hot object to measure temperature. Bimetallic strip: dissimilar metal strips are mechanically bonded. Differing rates of thermal expansion causes bending of strip when heated. A strip formed into a coil will rotate with temperature changes and can be calibrated for use as a thermometer o

Temperature Sensor Range Comparison You choose your instrument depending on temperatures involved and accuracy required.

Properties of Temperature and Heat Energy If there is a DIFFERENCE IN TEMPERATURE between any object and its surroundings then this difference results in a FLOW OF HEAT ENERGY from the hotter region to the cooler region. There are two possibilities…

1.If the OBJECT TAKES IN HEAT ENERGY its TEMPERATURE RISES. This is why a can of pop which has been taken from the fridge takes in HEAT energy from its surroundings until its temperature becomes equal to the temperature of its surroundings

2.If the OBJECT GIVES OUT HEAT ENERGY its TEMPERATURE FALLS. This is why a hot cup of tea gives out heat energy to its surroundings until its temperature becomes equal to the temperature of its surroundings.

The difference between temperature and heat energy The temperature of a spark from a sparkler is approximately between 500 o C and 800 o C, and the temperature of the bath water is approximately 50 o C. Although the spark has a much higher temperature the bath contains far more heat energy because the size of the spark is very tiny compared to the volume of the bath water.

HEAT ENERGY TRANSFER Heat energy is transferred from HOTTER PLACES TO COOLER PLACES by four different methods. CONDUCTION CONVECTION RADIATION and EVAPORATION

What conduction is This is the transfer of HEAT ENERGY through a substance from a HOTTER PART to a COOLER PART without any MOVEMENT OF THE SUBSTANCE ITSELF. When a poker is put into a coal fire CONDUCTION OCCURS and HEAT ENERGY passes up the poker until…

Conductors and Insulators Different materials allow heat energy to flow through them differently. A material is a GOOD HEAT CONDUCTOR if heat energy flows through it easily. This is why one material may feel colder to the touch compared to another even though they are both at the same temperature.

Good and bad conductors of heat Most METALS as well as stone are GOOD CONDUCTORS of heat. Materials such as GLASS, WOOD, PLASTIC and AIR are POOR CONDUCTORS and therefore GOOD INSULATORS. Many everyday objects are designed to be either a good conductor or good insulator. Below is one such object that makes use of both properties.

The HANDLE is made of PLASTIC or WOOD. Both are GOOD INSULATORS The PAN is made of ALUMINIUM. A GOOD CONDUCTOR

Are liquids and gases good heat conductors? The simple answer to this is NO! This demonstration shows the poor conductivity of water, a LIQUID. An ICE CUBE is trapped at the bottom of a tube of water using a piece of gauze. The ice at the bottom remains unmelted even when the water at the top is boiling.

Are liquids and gases good heat conductors? GASES are even worse conductors of heat. AIR, which is free and widely available forms the basis of many energy saving devices in the home i.e. LOFT INSULATION, CAVITY WALL INSULATION and DOUBLE GLAZING all have air trapped in them.

Convection, Radiation and Evaporation of heat This is the transfer of HEAT ENERGY through the MOVEMENT OF THE PARTICLES THEMSELVES from a HOTTER to a COOLER region. LIQUIDS and GASES (called fluids) can transfer heat energy by convection because they can move around unlike the particles in a solid. CONVECTION OF HEAT

A simple demonstration to show convection can be done by placing some red dye crystals in water over a source of heat. As the water is heated, it becomes less dense and rises taking some of the dye with it.

This is the transfer of HEAT ENERGY by WAVES. Unlike conduction and convection which both transfer heat energy through particles, radiated heat can travel through a vacuum. i.e. an empty space with no particles in it. RADIATION OF HEAT

This is the transfer of HEAT ENERGY due to the loss of particles from the surface of a liquid. A beaker of water left to stand will evaporate because water particles that have sufficient energy are able to escape form the surface of the liquid. This is because they overcome the forces of attraction leaving behind less energetic particles! EVAPORATION OF HEAT

Whenever energy is transferred from ONE FORM to another, only part of the INPUT ENERGY is USEFULLY TRANSFERRED as OUTPUT ENERGY to where it was wanted and in the form that is wanted. The remainder is transferred in some non- useful way and is therefore ‘WASTED ENERGY’. In most cases the wasted energy is usually heat and sound which just spreads out and can’t be used again. Transfer and Conservation of Energy

However the total amount of energy in JOULES (J) before the transfer is always equal to the total amount of energy after the transfer. In other words energy is always conserved even if some of it is wasted energy. Here are four examples of the intended energy transfer and wastage in everyday devices.