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 = Fnetd or __Ek = Ek2 - Ek1

• 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 = Fgh = magh

• When work is done on an object to overcome friction the thermal energy (heat energy) of the object increases and can be calculated by:

Wf = Ffd or __TE = Ffd

• (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 outwork in

• Efficiency = power outpower 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: __Eh 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/kgoC• Different substances have different

capacities to hold heat and therefore different specific heat capacities

• Water: c = 4184 J/kgoC• Copper: c = 390 J/kgoC• Aluminumc = 190 J/kgoC

• 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:solid liquid gas

• (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)