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hEAT. Section C Parts: 2.1, 2.2 4.1,4.2. heat. Heat is a form of energy . It is the total energy of all moving molecules in a substance. It is measured in joules (J). Heat energy would flow between any two points that are in thermal contact .
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hEAT Section CParts:2.1, 2.2 4.1,4.2
heat • Heat is a form of energy. It is the total energy of all moving molecules in a substance. It is measured in joules (J). • Heat energy would flow between any two points that are in thermal contact. • Heat energy flows from a body of high temperature of low temperature. • Temperature is the degree of hotness in a body. • Temperature and heat is not the same where heat is the energy given to an object to raise it’s temperature. • Heat theories were developed in the eighteenth century. Two main theories were the Caloric Heat Theory and the Kinetic Heat Theory.
Caloric Heat theory This theory suggested that heat was an invisible, weightless, self-repellent fluid called Caloric and was present in all matter. Temperature changes were explained in terms of the addition or removal of caloric to or from a body and the body’s temperature depended on the amount of caloric it contained. A hot body was thought to have more caloric that a cold body. The caloric theory suggested that caloric was lost by one system and gained by the other system. This theory was supported by experts involving the mixing of systems at different temperature initially and which eventually attains the same temperature. According to the Caloric Theory: • Caloric flows more easily through metals than non-metals. • Since caloric repels itself, a body with a lot of caloric expands, ie: a heated body expands. • When a nail is heated, caloric squeezes to the surface and the nail becomes hotter. • The smaller the object the less caloric it can hold. • Caloric being a fluid would have a mass and therefore a body, when hot would have a bigger mass than when cold. • Latent heat combines with particles in matter causing it to melt or boil. • Heat is weightless, which is the only valid part of the theory.
Caloric theory Weaknesses of the theory included: • No mass change. If caloric was a material substance (fluid) then it’s gain or lost should result in a change in mass. • The theory could not amounted for the fact that if two systems initially at the same temperature could gain or lose the same amount of caloric and yet not attain the same temperature . • The theory could not also account for the heat generated through friction. Such heat appeared to be limitless and this should not be if caloric was a substance. Therefore the Caloric Heat Theory was incorrect. Therefore the Caloric Heat Theory was incorrect.
Kinetic theory of heat Kinetic theory states that all matter is made up of particles that are in constant motion and the amount of motion depends on the amount of energy present. These particles also possessed kinetic energy due to there movement and potential energy due to their inter-particle forces and separation and their order. The sum of these forces is the internal energy of the system. The temperature of a body is a measure of the average kinetic energy of the particles that make up that body. Heat is that thermal energy which is transferred from particles of a body at a higher temperature to a particle at a lower system through the process of conduction, convection and radiation. According to the kinetic theory of heat: • Heat is a form of energy. • molecules, since they are in constant motion must possess kinetic energy.
Kinetic theory • The molecules of a hot body have more kinetic energy than that of a cold body. • If a body is heated, the heat energy is converted to kinetic energy of the molecules and since they have more kinetic energy the body becomes hot. • When a hot body comes in contact with a cold body, since it has more kinetic energy it is converted into heat energy and flows through the cold body and is converted back into kinetic energy therefore only when the energy is in transit, it is called heat energy. • Molecules of a cold body, when heated, vibrates faster and with bigger amplitude. Each molecule takes up more space and this results in overall expansion. Therefore this theory is proven true. Therefore this theory is proven true.
Joule’s experiment James Joule (1818-1889) conducted many experiments to determine the amount of work require to raise the temperature of a fixed mass of water by 10 C. His experiments linked mechanical work, electrical energy , chemical energy and heat. From these experiments concluded that energy could exist in many forms, including heat and that these forms could be interconnected. However no energy could be created or destroyed, thus establishing the Principle of Conservation of Energy: Energy can neither be created nor destroyed but converted from one form to another. In one of his experiments to determine this was by attaching weights to paddles in a container of water. When the weights fell, the paddles turned and heat was generated. The amount of heat generated was measured by observing the rise in temperature and the loss in mechanical energy was calculated through knowing the mass and the distance it fell true.
CONDUCTION Heat can be transferred from one point to another by this process. It is the transfer of thermal energy though a substance by the vibration of its molecules but without the movement of the matter as a hold. Heat is transferred by conduction when adjacent atoms vibrates against one another, or as electrons move from atom to atom. A conductor is material that allows current (heat) to flow through them with minimum opposition. Conduction is the greatest in solids(a metal) where the atoms are in constant contact and have one or more loosely held electrons in their outer shell. These atoms easily move from atom to atom in the structure. In liquids and gases the atoms are further apart giving a lower chance of atoms or molecules colliding and passing on thermal energy therefore having less conductivity. Air is the poorest of conductors as the atoms have the greatest distance between them.
Conduction Expanded materials such as polystyrene and hollow blocks have low conductivity but make get insulators (an insulator is a poor conductor of heat) as the materials are expanded and the atoms are apart from each other. Their electrons do not move from atoms to atoms under normal conditions. Large amounts of energy is needed to free the electrons and for heat to be conducted. The rate of conduction depends on : • The nature of the material. • The thickness of the material. • The area of the cross-section. Metals are great conductors of heat. Non-metals are poor conductors due to the lack of “free” electrons.
convection This is the process of heat transfer through a fluid (a liquid or a gas). Convection is heat transfer by bulk movement in fluids as a result of differences in density. The fluid close to the heat source becomes hot and expands. The increase in volume for the same mass causes a decrease in density(D=M/V) and the hot fluid rises. The cold fluid takes it’s place and the process is repeated. Convection only takes place in liquids and gases as the particles must be free to move around. Convection can be clearly seen in the formation of land and sea breeze.
sea breeze Land and sea breeze are caused by convection. This can be explained as the specific heat capacity of water is five times that of land(soil). The temperature over the land is higher than that of the sea during the day. As a result the hotter, less dense air over the land rises and the cooler, denser air from the sea takes its place. This is called Sea Breeze.
Land breeze Water cools slower than land, therefore at night the air over the sea is hotter than the air over the land. The warmer, less dense air over the sea rises and the cooler, denser air from the sea takes its place causing Land Breeze.
Credits and bibliography • Physics for You – Keith Johnson • Practical Physics for CXC – Gillian B- Herbert • Heinemann Physics for CXC –Norman Lambert • *** Mr. Gobinnotes • http://en.wikipedia.org/wiki/Conservation_of_energy • http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html#c1 Done By:Fadilah Khan 5C
