Computations
Equation (9): Q = mwcw(Tf-Ti) + mcalccal(Tf-Ti) + mhch(Tf-Ti)
20{(167.2(1) + 47.4(.217) + 15(.217) =3615 cal
Equation (13): J = W/Q
% difference:
Table 5. Dependence of the temperature on time
Initial temperature, Ti = 21.9ºC
Final temperature, Tf = 41.9ºC
Voltage = 9.4V
Amperage = 2A
Slope of the graph of the difference of temperature, (T-Ti) versus time, t:
Slope = 0.0222ºC/s (see graph)
Resistance of heating coil from the slope of the graph, R,
Equation (14): R =
% difference:
Graph
Questions
1. Why are we using the calorimeter for this experiment?
We used the calorimeter because we need the vessel containing the cold water to be a good insulator so that no heat is gained from or lost to the surroundings.
2. Is energy conserved in this experiment? What are the major sources in the experiment?
Yes, according to the first law of thermodynamics, energy is conserved. Energy can be transferred from the system to its surroundings or vice versa, but it can neither be created nor destroyed.
Some major sources of errors could be a defective thermometer or incorrect reading of the thermometer (human error), uneven distribution of heat and the surroundings.
Conclusion: Joule Heating
Joule heating is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature rise due to a known current flowing through the wire for a 30 minute period. By varying the current and the length of the wire he deduced that the heat produced was proportional to the electrical resistance of the wire multiplied by the square of the current. This relationship is known as Joule's First Law. The SI unit of energy was subsequently named the joule and given the symbol J. The commonly known unit of power, the watt, is equivalent to one joule per second.
It is now known that Joule heating is caused by interactions between the moving particles that form the current (usually, but not always, electrons) and the atomic ions that make up the body of the conductor. Charged particles in an electric circuit are accelerated by an electric field but give up some of their kinetic energy each time they collide with an ion. The increase in the kinetic or vibrational energy of the ions manifests itself as heat and a rise in the temperature of the conductor. Hence energy is transferred from the electrical power supply to the conductor and any materials with which it is in thermal contact.
Joule heating is also referred to as ohmic heating or resistive heating because of its relationship to Ohm's Law. It forms the basis for the myriad of practical applications involving electric heating.
