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Chapter 20
Energy and Heat
Energy has the units of joules, or kJ for food energy values. When energy is trans- ferred from one object to another we say “work has been done”. How fast the energy is transferred (the rate of change of energy) is known as the power which has units of watts. Heat is the name given to the energy that is transferred from one object to another object that is at a lower temperature. The temperature of an object is a mea- sure of the kinetic energy of its particles – roughly speaking, how fast they are mov- ing. The units of temperature are degrees celcius (or more correctly kelvin, where celcius degrees = kelvin + 273). Humans maintain their internal body temperature at 37 °C. A temperature above 38 °C is hyperthermia, while temperatures below 35 °C are hypothermic. Humans can gain or lose heat by the physical processes of convection, conduction and radia- tion. The physiological process of sweating will cool the body, while physical activ- ity increases the metabolic rate which will warm the body. In addition vasoconstriction and vasodilation will redistribute heat as the blood is redistributed. Furthermore ingesting hot or cold food and liquid will alter the body’s heat content as will elimi- nating and urinating. Mitochondria liberate the energy stored in the chemical bonds of the small organic molecules (e.g. glucose) which result from the hydrolysis (within the diges- tive tract) of the large molecules ingested as food. This liberated energy is stored in the chemical bonds of a different molecule – the third phosphate bond of ATP. If we ingest energy that is additional to that required to support our physical and meta- bolic activities, it is stored as fat in adipocytes and our body mass increases.
- When a particular energy value is ascribed to a food what type of energy is being referred to? Its A. translational kinetic energy. B. gravitational potential energy
C. chemical potential energy D. average kinetic energy per molecule Answer is C: The energy value of food is stored in the chemical bonds within the food molecules.
- Energy may be defined from the concepts of
A. force and heat. B. heat and joules. C. force and work. D. work and temperature. Answer is C: When a force causes an object to move, work (in joules) has been done to the object. The work done changes the amount of energy (in joules) stored in an object.
- Which of the following statements about work is NOT correct?
A. More work occurs when a force acts over a small distance ( W = F ÷ s ) B. Simple machines allow us to perform work more easily. C. Work (done on an object) is the amount of energy that transfers to that object. D. It takes energy to perform work. Answer is A: The reverse is true as: W = F × s
- Work is done when an object is moved by a force. What is energy?
A. the capacity to do work. B. the rate at which work is done. C. a force that results in no movement. D. the transformation of work from one form to another. Answer is A: Choice B refers to power. Choice C refers to a balanced force.
- The Système International (Standard International) unit of power is the watt. This unit is the same as which one of the following? A. electronvolt B. joule C. joule per second D. kilojoule Answer is C: Power is the rate of doing work or of converting energy from one form to another. The unit of power = A/s is renamed the watt in honour of James Watt. All the other choices are units of energy.
- Which one of the following statements best describes what is meant by the “principle of conservation of energy”? A. Internal energy is the sum of kinetic energy, thermal energy and potential energy. B. Energy may be transformed from one form into another, but it cannot be created or destroyed.
- When the label on a packaged food item states that 100 g contains 650 kJ of energy, it means that A. the human body is able to extract 650 kJ of heat and useful work by digest- ing the food. B. 650 kJ of energy was consumed in growing or producing or making the food. C. 650 kJ of heat energy is released when 100 g of the food is burned in an atmosphere of pure oxygen. D. ultimately your body will be able to perform 650 kJ of work for every 100 g of the food that you eat. Answer is C: Food energy values are determined by burning a sample of the food in a calorimeter and measuring the heat produced. If the body was 100 % effi cient, choice A would be true.
- In order to lose body fat, diet and exercise must be organized so that the energy value of the food intake is A. less than the energy used daily. B. more than the energy used daily. C. equal to the daily energy use. D. greater than the daily exercise. Answer is A: If more energy is expended than is taken in as food, the body will use some of its energy stored as fat, to make up the difference. Hence the per- centage of body mass that is fat will decrease.
- A block of wood (a poor conductor of heat) whose temperature is 15 °C is placed in contact with a block of steel (a good conductor of heat) of the same size but whose temperature is 20 °C. Both are touched with a hand whose skin temperature is 28 °C. Which of the following is true? A. The steel block will feel colder than the wood. B. The wood block will withdraw more heat from the hand than will the steel block. C. Heat will flow from the steel block to the wood block. D. Heat will flow from the steel block to the hand. Answer is A: “Feeling colder” is somewhat subjective. Placing a warm hand against a steel block at a lower temperature will make the hand feel cold as heat fl ows from the hand to all parts the steel block. Wood does not produce this feeling as it conducts heat poorly so the hand will soon warm up the wood in contact with it. Choice C will happen only slightly as the wood block is a poor conductor of heat.
- What is heat? It is a
A. measure of the temperature of an object. B. transfer of energy by convection currents.
C. measure of the average translational kinetic energy of the particles. D. form of energy transfer along a temperature gradient. Answer is D: Heat is the energy that is transferred between 2 objects due to their difference in temperature. Choice C refers to the temperature of an object.
- Which of the statements about heat is true? Heat is
A. one of the forms of infra-red radiation. B. transmitted through a solid object by convection currents. C. a measure of the temperature of an object. D. the fl ow of energy from one body to another at a lower temperature. Answer is D: Heat is the energy that is transferred between 2 objects due to their difference in temperature. Infra-red radiation is one form of heat (but not the reverse).
- What does kinetic theory allows us to understand that the temperature of an object measures? The A. heat that it contains. B. average kinetic energy of its particles. C. hotness or coldness of it. D. number of degrees kelvin it is. Answer is B: All particles of an object move or vibrate. Temperature is the aver- age kinetic energy of the particles of an object.
- What may the ‘thermal energy’ of the particles in an object be defined as?
A. the amount of heat that is contained in the object. B. sum of the random translational, rotational and vibrational kinetic energies. C. sum of the random translational, rotational and vibrational kinetic energies and the work done to overcome the intermolecular forces. D. average random kinetic energy of the particles. Answer is B: In gases, thermal energy is energy stored as kinetic energy, rota- tional motion, vibrational motion and associated potential energies In other substances, in cases where some of the thermal energy is stored in atomic vibra- tion or by increased separation of particles having mutual forces of attraction, the thermal energy is partitioned between potential energy and kinetic energy.
- Which of the following factors contributes LEAST to the human sensation of ‘hot’ or ‘cold’ when an object is touched? A. The amount of heat in the object. B. The temperature of the object being touched. C. The thermal conductivity of the object. D. The local skin temperature. Answer is A: The amount of heat in an object depends on the size of the object. The sensation of hot or cold depends on the other three things.
C. radiation. D. evaporation. Answer is C: Heat loss through radiation (of infra-red radiation) occurs con- tinuously and despite being clothed. Heat loss by conduction is low as clothes are insulators and also decrease convection losses. Sweating will be low on a cool winter day so evaporation will not be a major heat loss avenue.
- What factor does NOT affect the amount of heat lost from the human body by radiation? A. The area of skin facing the external environment. B. The temperature difference between the skin and the surroundings. C. The surface area of the body. D. The mass of the body. Answer is D: Body mass produces heat, but radiative losses are affected by the other three factors.
- By what means does a person sitting in the shade of a tree on a very hot dry summer day lose most of their heat? By A. radiation. B. conduction. C. convection. D. evaporation. Answer is D: On a hot day sweating will be occurring and its evaporation into a dry atmosphere would produce cooling. As the surroundings and air are hot, not much heat will be lost by radiation, convection or conduction.
- Why is heat loss from a hot object prevented when a material that is a poor conductor of heat, is wrapped around a hot object? Because A. the particles of the poor conductor easily transfer the kinetic energy of their vibrations to their neighbors. B. water vapour is prevented from escaping to the air. C. it refl ects radiated heat back into the hot object. D. air trapped within the poor conductor prevents convection currents from occurring. Answer is D: In this case, “clothing” the object in an insulator, will prevent air convection from carrying away the warm air in contact with the hot object. Hence cool air is prevented from replacing the heated air.
- Why does the evaporation of perspiration from our bodies cool our skin? Because the evaporated water carries with it the A. latent heat of vaporisation. B. heat lost by radiation.
C. latent heat of fusion. D. heat lost by convection. Answer is A: Radiation and convection are not involved. Latent heat of fusion refers to the heat required to melt a solid, latent heat of vaporization refers to the heat that leaves the body with the evaporated sweat.
- What is the reason that the temperature of a substance does not rise while it is in the act of melting, even if heat is added to it? A. Melting will not occur until the latent heat of vaporisation has been sup- plied, this prevents a rise in temperature until all the solid has melted. B. The added heat energy is used to increase the vibrational and rotational kinetic energies rather than the temperature (i.e. translational kinetic energy) of the particles C. The presence of colder unmelted particles alongside particles that are in liquid form causes their temperature to remain low. D. Any energy added to the substance is used to break the bonds that hold the particles into the solid form so does not contribute to a temperature rise. Answer is D: Once the bonds between the adjacent particles of a solid have been broken, they are free to move about and the particles can increase their kinetic energy, that is, rise in temperature.
- Which of the following forms of heat therapy relies mainly on conduction?
A. heat lamp. B. microwave diathermy. C. hot water bottle. D. ultrasound waves. Answer is C: A hot water bottle will transfer heat to objects in contact with it. The other three rely on a form of radiation.
- What is the most effective way to deposit heat in bones and joints?
A. ultrasound. B. infrared radiation. C. conductive heating (heat packs). D. diathermy. Answer is A: Ultrasound is the most effective form of diathermy as bones absorb ultrasound more effectively than soft tissue.
- Which of the following is a biological mechanism of preventing heat loss?
A. the production and evaporation of sweat B. increasing muscular activity C. vasoconstriction of superficial blood vessels D. seeking a warm environment Answer is C: Vasoconstricting blood vessel close to the body surface will with- draw blood from the surface and allow the skin temperature to cool.
- What does “hypothermia” refer to?
A. a body temperature below 41 °C B. a body temperature below 38 °C C. a body temperature below 35 °C D. a body temperature above 38 °C Answer is C: Hypothermia is a body temperature that is below the healthy range. That is, below 35 °C
- In what situation will the ability to lose heat by evaporation of sweat be diminished? A. When the body is dehydrated. B. When the ambient temperature of the surrounding environment is signifi- cantly higher than body temperature. C. When the relative humidity of the surrounding air is very low. D. When very little bare skin is exposed. Answer is A: If the body is dehydrated, its ability to secrete sweat is dimin- ished. The amount of bare skin does not affect sweating.
- The amount of heat lost as radiation depends on the following things except one. Which one? Answer is A: Radiation will occur whether the skin is covered by clothing or not.
- What is a definition of the temperature of an object?
A. Temperature is a measure of the average kinetic energy of the particles of the object B. Temperature is the amount of heat energy contained within the object. C. Temperature is the amount of energy that flows from the object being mea- sured, to the thermometer D. Temperature is a measure of the hotness (or coldness) of the object. Answer is A: The faster the particles of an object are vibrating, the higher is its temperature. Temperature is the average kinetic energy of its particles.
- Which of the listed factors does NOT affect the amount of heat that the human body loses by radiation? A. the temperature difference between the skin and the surroundings B. an individual’s behaviour C. being wrapped in a “space blanket” with a silver foil lining. D. the area of uncovered skin Answer is D: Heat will be lost by radiation even through clothing. A person may seek a cool environment to maximize their heat loss to the environment, so their behavior may influence heat loss. A silver foil lining will reflect infra-red radiation back towards the body.
- In which situation will the evaporation of sweat from the skin be ineffective as a heat loss method? A. the air temperature is greater than human body temperature. B. the air temperature is less than human body temperature. C. the surrounding air is saturated with water vapor. D. the human body is immersed in the water of a swimming pool. Answer is C: Sweat will evaporate only if the surrounding air is not already saturated with water vapor. High humidity decreases the effectiveness of sweat- ing as a heat loss mechanism.
- Which mechanism of heat loss from the human body is minimised by wearing clothes? A. convection of air B. radiation to the environment C. warming of inhaled air to body temperature before exhaling D. evaporation of sweat Answer is A: Clothes trap a layer of air close to the skin. This air is warmed to skin temperature and is prevented from blowing away by being contained within the clothing.
- Evaporation of sweat cools our body because the evaporating water molecules
A. have a high heat capacity. B. transfer kinetic energy away from us. C. radiate heat away from us. D. remove heat by conduction. Answer is B: The water molecules in sweat that have the highest kinetic energy are the ones that evaporate first. This leaves the slower moving molecules behind and they have a lower temperature. These slower molecules will gain energy from the body heat and eventually have enough kinetic energy to evapo- rate, again cooling the body as more kinetic energy is transferred away from the body.
- Human body core temperature is usually maintained within which of the fol- lowing ranges? A. 35.0 to 37.5 °C B. 36.5 to 38.5 °C C. 36.5 to 37.5 °C D. 35.0 to 38.5 °C Answer is C: This is considered the usual healthy range. Above 38 is a fever (except if as a result of vigorous exercise), while below 35 is hypothermia.
C. evaporation and convection. D. radiation and conduction. Answer is B: Clothes trap air between the clothes and skin, minimising convec- tion. Clothes lying between skin and a contacting surface insulate against con- ducting losses. Radiation is not prevented by clothing.
- What may be said about the amount of energy in the form of infra-red electro- magnetic waves that is radiated from our bodies? A. It depends on our body’s temperature. B. It would be greater if our layer of subcutaneous fat was thicker. C. It may be increased by contact between the body and an object with tem- perature lower than the body’s temperature D. It may be increased by exposing more bare skin. Answer is A: The amount of energy radiated as infra-red depends on the body temperature. (E ∝ T^4 )
- Except for one situation described below, water molecules changing state from liquid to gas are involved in the cooling effect. Which one? A. a cold wind blowing against your skin B. drying off after a swim. C. the evaporation of sweat D. the exhalation of breath from the lungs Answer is A: A cold wind cools us by convection. It continually removes the air that has been warmed by contact with the skin.
- What is the healthy human core body temperature?
A. Below 38 °C except during fever B. It lies between 36.5 and 37.5 °C C. It lies between 36 and 37 °C D. It lies between 37 and 38 °C Answer is B: Healthy human body temperature is close to 37. It can be elevated to 38 (and more) by vigorous activity.
- How does subcutaneous adipose tissue assist the body to regulate its temperature? A. It conducts heat more readily than lean tissue so promotes heat loss. B. It stores heat energy so acts like a “heat sink”. C. It produces sweat for secretion via sweat glands. D. It conducts heat less readily than lean tissue so insulates the body. Answer is D: Adipose tissue (fat) conducts heat less readily than does lean tis- sue. Hence a layer of fat insulates the body against heat loss.
- When is radiation an effective form of heat loss from the body?
A. When we expose a greater amount of bare skin. B. When our body temperature is greater than that of our surroundings. C. When blood vessels close to the body surface are vasoconstricted. D. When our body temperature is less than that of our surroundings. Answer is B: All objects radiate IR rays. If our body temp is greater than that of our surroundings, we will radiate more than we absorb from the surround- ings. It is body surface area, not amount of bare skin, that determines the amount of energy radiated.
- Which choice explains how the evaporation of sweat “cools” our body?
A. Evaporating water molecules carry with them more than the average amount of kinetic energy which leaves the remaining molecules with a lower aver- age kinetic energy. B. The body loses more heat through the infrared radiation emitted by sweat than it gains from the infrared radiation emitted by the surroundings. C. Sweat is at a lower temperature than our core body temperature so sweat on our skin cools us by conduction. D. The water molecules in sweat are at a higher temperature than our core body temperature so losing sweat leaves us cooler due to the mass of water lost. Answer is A: Water molecules have a range of KE. The water molecules with the most KE are those that vaporise first. Sweat will be at the same temperature as the skin it sits on.
- Why does an ice pack applied to a bruise reduce swelling?
A. Less fluid leaks from the bruise due to the diminished nerve impulses. B. It causes vasoconstriction. C. It reduces the metabolic rate at the local site. D. It increases the viscosity of blood below the ice pack. Answer is B: If blood vessels constrict, less blood can flow out of them. Choices C and D are true statements but are beside the point.
- The skin is usually at a lower temperature than the body’s core temperature ( °C). What is the reason for this? A. The layer of adipose tissue in the hypodermis insulates the skin from the core temperature. B. The sweating mechanism is able to lower the skin’s temperature. C. Vasoconstriction restricts the amount of blood that is brought close to the skin surface. D. Heat loss through conduction, convection and radiation keeps the skin at a lower temperature. Answer is C: Vasoconstriction limits the amount of blood (at 37) that flows near the skin which allows the skin temperature to approach that of the surroundings.
- Which of the following is the most acceptable definition of the term “power”?
A. the rate of doing work. B. that which is stored and can be fully recovered and converted to kinetic energy. C. the concept applied to that which gives an object the ability to do work. D. the sum of an object’s potential and kinetic energies. Answer is A: Power refers to how fast work is done. That is, the time taken for the energy in joules to be used. The unit of power is the “watt” which is the same as joules per second.
- Which of the following correctly states the principle of conservation of energy in terms of the human body? (assume no foodstuffs are consumed and no urine or faeces are excreted). A. Q = s × m × ΔT (Q = energy, s = specific heat of tissue, m = body mass, T = body temperature) B. The energy stored in the human body is equal to the energy lost from the body plus the work done by the body. C. The energy value of the food we eat must exceed the energy value of the muscular activity we perform. D. The change in the energy stored in the body is equal to the heat lost from the body plus the work done by the body. Answer is D: Energy that leaves the body (without mass) must be in the form of heat loss or work done by body movements. The formula in A may be used to determine the energy value of a food burned in a calorimeter.
- Which of the statements is a description of basal metabolic rate in a human?
A. The sum total of the energy released per minute by all of the chemical reac- tions that occur in the body. B. the rate of energy utilisation during “absolute rest”. C. The power generated by the body’s activities. D. The oxygen consumption (in l/min) of an individual. Answer is B: “Basal” metabolic rate refers to the minimum amount. This occurs when there is no movement (apart from breathing and cardiovascular move- ments) and little mental activity.
- Which explanation of the cooling effect produced by an ice-pack when applied to the skin is the best one? A. Cold applied to the skin causes vasodilation thus allowing more blood to pass through the tissue adjacent to the ice pack and be cooled. B. Heat withdrawn from the body is used to provide the ice with the latent heat of vaporisation it requires to melt, until melting is complete, the tempera- ture of the icepack remains constant.
C. Cold transfers along its temperature gradient – from ice pack to skin – the melting ice & water mixture ensures good contact with the skin. D. Heat withdrawn from the body (at 37 °C) is used to provide the ice (at 0 °C) with the latent heat of fusion it requires to melt, until melting is complete, the temperature of the icepack doesn’t rise appreciably. Answer is D: Cold cause vasoconstriction no vasodilation. Body heat provides the icepack with latent heat of fusion (melting) not vaporisation. Heat transfers, not “cold”.
- Which is the correct distinction between temperature and heat?
A. Temperature is a measure of the amount of heat in an object and heat is the energy that flows between objects which have different temperatures. B. Heat is a measure of the energy contained within an object and temperature is the objective measurement of heat. C. Temperature is a measure of the average kinetic energy of particles and heat is the energy that flows between objects as a result of a temperature difference. D. Heat is the energy that flows from a cold object to a hotter object and tem- perature measures the average random kinetic energy of the particles. Answer is C: Heat flows from the object at higher temperature to the object at lower temperature. Choices A and B have nonsense as their reference to temperature.
- A cold pack, applied to reduce swelling, is more effective if it contains melting ice at 0 °C rather than water at 0 °C. Why is this? A. Because ice cools by conduction whereas water cools by convection. B. Because initially the melting ice is colder than the cold water. C. Because ice has a higher latent heat of vaporisation than water. D. Because melting ice remains at 0 °C until it has all melted. Answer is D: The heat that transfers from the bruised part is used to break the bonds between adjacent water molecules to melt the ice, rather than to increase the kinetic energy of the molecules. Temp measures the average kinetic energy of the molecules. Hence temperature does not rise until all the ice has melted.
