Monday 7 December 2009

Heat and internal energy

Definition: Heat is the energy transferred from one body to another as a result of temperature difference between two bodies. The unit is Joule (J). Note that “heat” is not energy, it’s just a transferring process of energy. Internal energy : energy stored in the object, equal to the sum of kinetic energy due to random motion and potential energy of all its molecules. (Σ(K.E)+Total P.E.) The unit of internal energy (energy) is Joule. Higher temperature means higher average kinetic energy, then the formula of internal energy tells us the internal energy will be higher with the temperature as well. Power : the rate of transfer (energy). The unit is Watt (W). We have: Power = energy/time P=E/t (E=Pt or t=E/P) Definition: 1W=1J/1s=Js^-1 1W power means transferring 1 Joule of energy in 1 second. Another unit of energy if kilowatt-hour. It’s the amount of energy transferred by 1000W (kilowatt) in 1 hour. Its unit is kWh. 1kWh= 3,600,000J = 3.6*10^6J = 3.6 MJ. Heat capacity Definition: Heat capacity of an object is the energy required to transfer by heating to the object to raise the temperature of the object through 1˚C. It’s symbol is C. C=Q/ΔT where Q (energy transfer) and ΔT (change in temperature) have unit of J and ˚C respectively, therefore the unit of C is J˚C^-1 Specific heat capacity Definition: Specific heat capacity of a substance is the energy transferred by heating required to raise the temperature of a unit mass (kg) of the substance through 1˚C. It’s symbol is c. c=Q/mΔT=C/m (or mc=C), where m is the mass. *experiment about finding specific heat capacity of water* Put m kg of water into a polystyrene cup, then insert an immersion heater, stirrer and thermometer into the cup. The power supply of immersion heater passes through a Joulemeter.Assume the initial readings of temperature and energy is E and T and the final readings is E’ and T’ respectively. c=Q/mΔT=(E'-E)/m(T'-T) Note that immersion heater should be fully immersed into the water to ensure that all energy from the heater is transferred to the water. Assumption: 1) No energy lost from the water to the surrounding air/cup. 2) No energy lost from the heater to the surrounding air. 3) The temperature within water is unique such that the readings of thermometer is accurate (we use stirrer to make this more accurate) 4) All energy from the heater is transferred to the water (we record the highest temperature reading after the heater is off) Another experiment about specific heat capacity of aluminium has similar procedure. But stirrer is not needed. Instead, a polystyrene tile is under the aluminium block and cotton wool surrounding the block. Also we put some oil into the immersing holds to keep a well thermal contact between the block, heater and thermometer. Law of conservation of energy states that the total amount of energy in a system must remain constant. Then energy lost of an object = energy gain of another object. (mass of a)(spec. heat cap. of a)(temp. diff. of a)=(mass ofb)(spec. heat cap. of b)(temp. diff. of b) Or (heat cap. of a)(temp. diff. of a)=(heat cap. of b)(temp. diff. of b) If the object contains the same substance, then (mass of a)(temp. diff. of a)=(mass of b)(temp. diff. of b) Importance of high specific heat capacity of water (4200Jkg^-1˚C^-1) 1) As a coolants for motors, CPUs… 2) Reduce daily temperature range for coastal area. (Coastal area have more water stored with the area. In day, water absorb energy in a more effective way than soil. In night they release energy.) 3) Monitoring body temperature: we release energy through sweating. Moreover 70% of our body is water. We can gain or loss a large amount of energy without a big change in body temperature. Notes available!

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