Energy and Work
Dec 13, 2015
Energy and Work
Energy In physics, an object has energy if it has
the ability to act on another object (i.e. apply a force)
Energy can be stored in many different forms and can be transferred in three ways
Energy is an abstract idea. We can never measure energy, only calculate it
Types of Energy
Kinetic Energy (Ek) or (KE)energy associated with
the movement of an object
Types of Energy (cont.)
Gravitational Potential Energy (Eg) or (P.E.)
the energy associated with an objects distance from the earth. (If this object were to fall on my foot, it would transfer its energy to my foot by means of a force)
Types of Energy (cont.)
Elastic Potential Energy (Eelas) or (Elastic PE)
The energy associated with the compression or extension of a spring or other elastic material
Types of Energy (cont.) Dissipated Energy (Ediss) Energy that has
been degraded into a more random form such as sound, vibrations, internal and thermal energy
Internal Energy (Einter) the energy associated with the molecules and atoms in a substance. The energy from their movements and the bonds between them.
Thermal Energy (Etherm) the energy associated with just the random movement of molecules and atoms in a substance. This gives rise to temperature.
Energy Transfers
Energy can be transferred from one object to another in 3 different ways.
Work!
Work (W) - an applied force over a given distance with transfer energy
Mathematically: Work = W = ΔE = F║Δ x Words: Work = change in energy = (force parallel
to displacement)(displacement) Work done BY an object is negative work
(energy loss; -ΔE) Work done ON an object is positive work (energy
gain; +ΔE)
Work is equal to the area under a force vs. distance graph
Heat it!
Heat (Q) - a means of transferring energy due to a difference in temperatures
Energy will always flow from a hotter object to a colder object, Always!! (2nd law of Thermodynamics)
Conduction and convection are two similar processes that heat objects
Will learn more in thermo unit.
The Conservation of Energy
The amount of energy in the universe is finite. No matter the hopes that Nature goes through, or processes she undertakes, if we calculate the energy before an event, it will be equal to the energy after the event.
The Conservation of Energy (cont.)
If we look at one object, we will see that the amount of energy it has changed by will be equal to the energy transferred through work, heat and radiation.
ET = K.E. + P.E. + ΔW. For our class we will ignore other losses.
Mathematics of Energy
All energies, work, and heat are measured in joules (j)
One joule is equal to a (Newton)(meter)
Equations Kinetic Energy
Ek = ½ m v2 Work =F*d W= ΔP.E.= mgΔh
Gravitational Potential Energy Eg = mgΔy (near earth; Δy= height above reference height)
Elastic Potential Energy (Will not use in this class) Felas = - kx (restoring force of a spring)
Eelas = ½ k x2
Internal and Thermal Energy ( Will use later in Thermo Unit)
Ether = mcΔt (energy dealing with temperature change)
Einter = mcsolidΔt + mHf + mcliquidΔt + mHv + mcgasΔt
Conservative forces
Conservative forces do work on an object so that no energy becomes dissipated energy. (i.e. no energy is lost to sound, heating or vibrations)
Conservative forces allow all the energy to be regained in a useful manner
Non-conservative forces
Non-conservative forces do work that degrades energy and allows it to be dissipated into non-useful forms, such as thermal energy, vibrations or sound.
Non-conservative forces lead to the problem of an energy crisis. Energy is always conserved, but may end up in a form that is useless to do actual work.
Friction is a non conservative force meaning it matters the path that the object takes
Power
Power is the rate at which work is done (measured in watts)
Work / time = Power
Power (cont.)
Power companies (i.e. PG&E) sell ENERGY in the unit of kilowatt-hours, kWh, which is really a unit of energy.
kW x hour = Power x time = Joules/sec x 3600 sec = Joules, which is energy
If two objects do the same work, the one that does it faster is more powerful
Horsepower is another unit of power (1 hp = 746 watts)