Unit 2 Heat And Temperature

Unit 2 Heat and temperature • • • •

2.1 Definition of Heat and temperature 2.2 Temperature scale 2.3 Body heat and body temperature 2.4 Conduction, convection, radiation and evaporation • 2.5 Temperature measurement • 2.6 Regulation of temperature

2.1 Definition of Heat and temperature • Heat is a form of energy. It is produced by burning fuels, by human bodies, and by friction when two surfaces are rubbed together. • Temperature is a measure of the tendency of a body to transfer heat from or to other bodies.

2.2 Temperature scale T(°F) = 9/5 T(°C) + 32 T(°C) = 5/9[T (°F)-32] T(K) = T(°C) + 273.15

Fahrenheit

Celsius

Kelvin

2.3 Body heat and body temperature • A balance between heat production and heat loss, which occurs through conduction, convection, radiation and evaporation. • Balancing heat production within the body against heat loss to the surroundings determines the body temperature. • Body fat is energy, and it is part of the heat storage process in our bodies.

Body temperature

2.4 Conduction, convection, radiation and evaporation Conduction • Heat passes from one molecule to the next • Occurs most rapidly in solids. Metals are good thermal conductors

• Does not easily pass through thermal insulators such as glass, wool, plastic, etc. • Heat produced by friction passes by conduction through the skin to underlying muscle during massage

Convection • Heat passes by the movement of heated molecules • Occurs in liquids and gases • Expansion of the heated gas or liquid increases its volume and reduces its density. The hot gas or liquid therefore rises. A stream of heated molecules forms a convection current • The movement of hot water in a hot water system and movement of warm air in a sauna are due to convection • Natural convection and forced convection: – Natural convection has fluid flow due to the density difference between the hot and cold parts of the whole system. – Forced convection has fluid flow caused by external driving forces, e.g. a fan.

Radiation • Heat travels in straight line as heat rays. All hot objects give out heat rays • There must be a temperature difference between the radiator and its surrounding • Energy is transferred by electromagnetic waves • Occurs in gases and space • Dull, dark surfaces in the path of heat rays absorb them and become hotter. Shiny and light surfaces reflect heat rays and remain cool • Nichrome heating element and infra-red heaters give out radiant heat. heat from the sun reaches the earth’s atmosphere by radiation.

• the intensity of radiation emission increases dramatically with temperature, and the spectrum shifts toward the visible and ultraviolet parts of the spectrum as temperature rises.

•All objects absorb radiation as well as radiate it. •It is the color of the object that affects its rate of absorption.

Greenhouse effect

Cooling by evaporation • a liquid changes to a vapour it requires an additional quantity of heat energy (latent heat ) • causes a cooling effect on the surface • Volatile: for a liquid that evaporates quickly, it is easy for it to take away energy from surface and causes cooling effect. • perspiration, when the evaporating sweat helps to remove heat from the body, thus assisting in the control of body temperature • Factors affect the rate of evaporation include: temperature, wind, surface area, and humidity.

• Humidity is measured by relative humidity (RH) %RH =

actural amount of water vapour in the air ×100% amount of water vapour saturating the air at the same temperature

• For comfort, air should have a %RH between 40 and 50. If it reaches 70, sweat will not evaporate to cool the body adequately and heat fatigue will cause headache, tiredness and irritability. High humidity results in a lowering of oxygen level in the blood. Yawning then occurs to inject more oxygen into the blood stream.

example Calculate the percent relative humidity on a day when the temperature is 25o C, and the air contains 9.40 g of water vapor per m3. Given that the saturating water vapor density at 25o C is 23.0 g/m3. Solution:

9.40 g/m 3 %RH = ×100% = 40.9% 3 23.0 g/m

2.5 Temperature measurement • Core temperature (body temperature): the temperature in the interior. It is maintained constant. The core temperature is an indicator of the health of a person. • Surface temperature: the temperature of the skin or subcutaneous tissues, varies with the temperature of the surroundings. – important factor when discussing heat loss through sweating (evaporation) from the skin.

Clinical temperature measurement • Thermometer - a device that detects a change in temperature. by detecting a change in the physical property of a sensor • Measuring sites: an oral cavity, rectum, or under the armpit by contact measurement or tympanic membrane (ear drum)

Surface temperature measurement • indicator for bone fractures and inflammation • Thermography - to diagnosis tumors and breast cancers • Liquid crystal thermometer

Core temperature measurement • Mercury thermometer – Infection and cross-contamination are its main disadvantages

• Thermistors - temperature-dependent electrical resistance • Thermocouple dissimilar materials join to form junctions. Temperature difference at junction causes a voltage difference.

• Infrared thermometer – The main advantages of the infrared thermometer are that it only contacts the ear canal and not the eardrum (i.e. less infection), ease of use, and fast response. – But inaccurate

2.6 Regulation of Temperature Thermostat

Regulation of body temperature • If the body temperature rises above normal, the blood and temperature sensors in the skin feed back information to a part of the brain which responds in two ways. • First it increases the rate of flow of blood to skin capillaries, so increasing the rate at which the blood is cooled. • Second, sweating starts, producing cooling of the body by evaporation. • If the temperature falls below normal, feedback reduces the blood flow to the skin and no sweating occurs, and shivering is activated to generate heat in the muscles.

Feedback and control • In many kinds of system, all or part of the output (or information about it) is fed back to the input and affects the output. • Feedback is positive if it acts in the same direction as the input and increases the output. • Feedback is negative if it acts in the opposite direction to the input and reduces the output.

Positive feedback

Negative feedback