Heat Production and Transfer

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Semester 1 2012 ENB 130 H9 – Heat Production and Transfer 3rd of May 2012 10-12am Session Group Members: Daniel, Henry and Luke S c i e n c e a n d E n g i n e e r i n g Q U T Aim Part A: To measure the heat output generated by an electrical current. Part B: To rank various materials in decreasing order of thermal conductivity so as to appreciate which materials are good and which are poor conductors. Part C: To examine the rate of cooling of a hot object. Introduction Energy is defined as an objects ability or capacity to do work [1]. Energy exsists as many different forms throughout the universe such as heat, sounds, kinetic, potential, chemical and many more. The law of Conservation of Energy states that energy cannot be created nor destroyed but instead only transformed from one energy type to another [2]. Heat is one form of energy related to the movement of atoms and molecules within an object. It is capable of being transferred through liquids, gases and solids by conduction, convection or radiation. Heat is transferred between objects as a result of temperature differences among them [3]. In part A of the experiment, the conversion of electrical energy into heat is observed through a convection transfer from a heating element in water. Electricity is a form of energy created by the flow of electrical charge or electrons through a conductor. Electrical power (P) is measured in Watts (W) and can be calculated for a circuit with the following formula: P=VI This formula only requires two knows to calculate an unknown in a conductor or circuit. If the voltage (V) and the current (I) are known, then the Electrical power can be calculated. Current, measured in coulombs per second, or ampere (A), is the rate that electrons pass through an electrical conductor or circuit. Voltage is an electrical difference between two points of a circuit. It measures the force required to pass one ampere through a resistance (R) of one ohm (Ω). Every electrical circuit and conductor has a resistance for electrons to pass through. This resistance is a quantity measured in ohms, describing a materials opposing flow of electricity [4]. The relation between voltage, current and resistance can be measured using Ohm’s Law: V=IR The second part of the experiment looks at the thermal conductivity of different substances. The thermal conductivity of an object is its ability to transfer and conduct heat. Materials with higher thermal conductivity are better substances at absorbing and radiating heat energy. The conductivity of four different metals, copper, brass, steel and aluminium, will be analysed by observing the time taken for each material to melt wax off their tips from a Bunsen flame held at a common centre. The time taken for each metal to melt the wax will be relevant to their thermal conductivity. The faster the wax melts, the higher the thermal conductivity will be for that substance. The final part of the experiment looked at the radiation of a cooling substance in relation to Newton’s Law of cooling. This law suggests that if the temperature of an object is doubled, the rate of cooling of that object will also be doubled. While Newton’s Law of Cooling mostly explains the cooling of heated water, it is important to realise that the cooling fins on hot sections of equipment are painted black which also assists the cooling process. This is due to the fact that of all colours, black absorbs and radiates the most heat. Method There were three different experiments that were performed to investigate heat production and transition. The first experiment measured the heat output generated by an electrical current. The equipment used for this experiment were: - 30 Volt AC power unit with stand and support - Digital Multimeter - Beaker - Thermometer - Wire heating element - Stirrer - Clock The procedure for the experiment (Part A) went as follows: 1. Measure the resistance of the heating element and the output voltage of the power unit using the digital multimeter and record. Figure 1: Apparatus for Part A 2. Fill 200ml of water into the beaker 3. Set up the circuit as Figure 2. 4. Record the initial temperature of the water 5. Switch the power unit on and record the temperature of the water at one minute intervals for 10 minutes. 6. Draw a graph of temperature verse time and record its gradient. 7. Calculate the electrical power input and the heat power and record Figure 2: Electrical Circuit for Part A The second experiment (Part B) compared the heat conductivity of four different substances. The time it took for wax to fall off the tips of Copper, Brass, Steel and Aluminium rods was used to determine the thermal conductivity of the four metals. The process was as follows: 1. 1. The 5 different rods were each heated over a small bunsen flame at their common centre and the approximate time for the wax to melt on each different rod was recorded. 2. The order of melting was noted. The last experiment (Part C) looked at radiation and Newton’s law of cooling. This part of the experiment looks at the cooling rate of water from 50° above room temperature. Using the same heated water as the first experiment (Part A) the method for Part C was as follows: 1. The room temperature (T sC) was recorded. 2. The temperature (T 0 of the water was recorded, the power unit was switched off and timing commenced. 3. The temperature of the water was recorded at two-minute intervals. 4. t, T, θ(=T – Ts) and lnθ was tabulated. 5. A graph of lnθ against t was drawn and found to be a straight line of negative slope. Results Part A: Resistance of heating element (R 1 35.4 Ω Output Voltage of Power Unit (V1) 32.5 A Heater Current (I) 0.893 A Table 1. Resistance, Current and Voltage associated with Power Unit Time (min) Temperature (°C) 1 29 2 32 3 35 4 38 5 40 6 41 7 44 8 46 9 48 10 49 Table 2. Time (min) vs Temperature (°C) 60 50 y = 2.2182x + 28 (40 30 20 Temperature Temperature 10 Linear 0 (Temperature) 0 2 4 6 8 10 Time (min) Chart 1. Time (min) vs Temperature (°C) Part B Ordering of Material Approx. Melting Thermal Melting Time (min) Conductivity 1 Copper 2:01 400 2 Aluminium 2:38 250 3 Brass 3:37 109 4 Steel 45:00 18 Table 3. Thermal Conductivity of Different Substances Part C Elapsed Time Temperature, T θ= T- TS (Ts=25°C) ln (θ) (min) (°C) 0 TO= 50 θO= 25 3.21 2 47 22 3.10 4 45 20 2.99 6 43 18 2.89 8 41 16 2.77 10 40 15 2.71 12 38 13 2.56 14 37 12 2.48 16
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