specificHeatCapacity

• this is the teacher presentation on heat capacity, notes based on pearson can be found here

mechanical work and thermal energy §

• 1st law of thermodynamics state that energy changes from one form to another and the total energy in an isolated system remains constant.
• internal energy can be changed by heating or cooling or by mechanical work being done on or by the system.
• internal energy (u) total internal kinetic and potential energy of teh system
• heat (Q)
• mechanical wave (w)

calculating the change in internal energy

• e.g. a 1L beaker of water has 25kJ of work done on it and loses 30 kJ of thermal energy to the surroundings. What is the change in energy of the water?
  • heat is removed from system, so Q is neg work is done on the system, so W is pos.
  • $\Delta U=Q+W=(-30)+(+25)$
  • $\Delta U=-5kJ$

changing temperature §

• temp = avg kinetic energy of the particles inside a substance
• ^ temp = ^ kinetic energy of particles
• this occurs when heat is transferred to that substance
• several factors influence temp ^
  • greater the mass, greater energy required to change kinetic energy $\therefore$ heat required to raise temp is proportional to the mass.
  • cooking food somewhere thinking it is an okay temperature and then biting burning your tongue because the inside is hotter than the outside.
    • $\Delta Q\propto m$
  • the more heat that is in transferred, the more temp of that substance increases $\therefore$ amount of energy transferred is proportional to change in T
    • $\Delta \propto \Delta T$
  • heating the same masses of different materials will show that the amount of energy required to heat a given mass of material through temp change depend on the type/nature of material
• the specific heat capacity of a material, c , is the amount of energy that must be transferred to change the temp of 1kg of material by $1°C$ or $1K$.
  • $Q=mc\Delta T$
  • where Q is heat energy transferred (J)
  • m is mass (kg)
  • $\Delta T$ change in temp $°C\text{ or }K$
  • $c$ specific heat capacity $Jk{g}^{-1}{K}^{-1}$
• understand formula -> important for things like engineering.
• must convert units!!!!! teacher notice that rearranging the equation needs to be careful. if you are unsure, do it step by step - probably better so that you know it is right (unless you are certain since it is an easy equation)

practice question §

• a hot water tank contains 135L of water. Initially, the water is at 20deg C. Calculate the amount of energy that must be transferred to the water to raise the temp to 70 deg
  • volume = 135L
  • mass = 135kg
  • delta T. = final - initial = 50deg C
  • heat capacity = 4180J/kg/K
  • put into equation Q = 135 * 4180 * 50 = 28,215,000 J = 2.82 x 10

always use the value from the question for heat capacity - if it is not on there, use the data booklet. do NOT ever use values from the internet in a test....