Conservation of Energy

Law of Conservation of Energy (p222) The law of conservation of energy states that energy cannot be created nor destroyed, but it can be changed from one form to another Energy is changed from one form to another when work is done on the object When work is done on an object to accelerate the object, the kinetic energy of the object changes & can be calculated by: W = Fd or __E = F net d or __E k = E k2 - E k1

When work is done on an object to change the vertical position the gravitational potential energy of the object changes and can be calculated by: W = Fd or __Ep = F g h = ma g h When work is done on an object to overcome friction the thermal energy (heat energy) of the object increases and can be calculated by: W f = F f d or __TE = F f d

(p 223) The law of conservation of mechanical energy states that in a frictionless system mechanical energy is conserved: (add)

If there is friction, then some mechanical energy is converted to thermal energy: (add)

Modify example problems p 223: 1.

2.

(add example 3 to bottom of p 224): 3. A 55 kg fat toddler slides down a slide that is 5.0m high. If his/her speed at the bottom is 2.9m/s how much heat was generated?

Power (p231) Power is the rate of doing work Formulae: P = W/t or P = _E/t Units: J/s or Watts (W) “W” (Watts) should not be confused with “W” (work) Power is scalar another useful equation: P = Fv from: See example problems 1 and 2 p 232

Efficiency Efficiency = work out work in Efficiency = power out power in

Thermal Energy (p 216) When work is done to accelerate an object the kinetic energy increases When work is done on an object to change the vertical position the gravitational potential energy changes When work is done on an object to overcome friction the thermal energy of the object increases

When the thermal energy of an object changes the temperature of the object increases Thermal energy is kinetic energy at the molecular level Heat is the energy that is transferred from a warm object to a cooler one Symbols: __E h or Q

Specific Heat Capacity (p 217) Specific heat capacity is defined as the amount of heat that a unit mass of substance can gain or lose in order to change its temperature by one degree Symbol: c Units:J/kg o C Different substances have different capacities to hold heat and therefore different specific heat capacities

Water: c = 4184 J/kg o C Copper:c = 390 J/kg o C Aluminumc = 190 J/kg o C The heat gained or lost by a substance depends on: mass of the substance (m) temperature of a substance (__t) specific heat capacity (c) Formula:

Specific Heat Capacity

Example problems (p217) 1.

2.

3.

(add to margin of p 217) When a substance reaches its boiling point or melting point, heat energy must be added or removed for a phase change to occur This energy does not change the temperature of the substance but it does do work to change the distance (potential energy) between the particles: solidliquidgas

(add in the margin of p 218) During a phase change there is no temperature change so a new formula is required: __E = __H m Vaporization involves liquid gas transitions Fusion involves solid liquid transitions See p 254 table 12.2 See example 1 p 254 of Merrill text

Multi-Step problems Sometimes an object undergoes both a temperature change and a phase change: (see example 2 p 255 of Merrill text)