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 (E k ) or (KE)  energy associated with the movement of an object

Types of Energy (cont.) Gravitational Potential Energy (E g ) 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 (E elas ) or (Elastic PE)  The energy associated with the compression or extension of a spring or other elastic material

Types of Energy (cont.)  Dissipated Energy (E diss ) Energy that has been degraded into a more random form such as sound, vibrations, internal and thermal energy Internal Energy (E inter ) the energy associated with the molecules and atoms in a substance. The energy from their movements and the bonds between them. Thermal Energy (E therm ) 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. E T = 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 E k = ½ m v 2 Work =F*d W= ΔP.E.= mgΔh  Gravitational Potential Energy E g = mgΔy (near earth; Δy= height above reference height)  Elastic Potential Energy (Will not use in this class) F elas = - kx (restoring force of a spring) E elas = ½ k x 2  Internal and Thermal Energy ( Will use later in Thermo Unit) E ther = mcΔt (energy dealing with temperature change) E inter = mc solid Δt + mH f + mc liquid Δt + mH v + mc gas Δt

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)