PHYSICS NYB-21E Winter 2012

Chapter three or more: Electric potential energy and electric potential

´ ´ Instructor: Jeremie Vinet Marianopolis College.

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Assessment

Important details from last lectures:

q A point impose q produces an electric ﬁeld E = ke 2 r ˆ r

A point charge q0 placed in a power ﬁeld Elizabeth feels a force Ideologia = q0 E

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NYA ﬂashback: Job

When a net force acts on an thing, it increases it. For the object accelerates, its velocity changes. When the speed associated with an object adjustments, its kinetic energy adjustments. When an object's energy changes, work has been done into it.

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NYA ﬂashback: Work

In less cartoonish terms, The amount of work T done by a force F over a shift ∆r is usually W sama dengan F · ∆r

The amount of work Watts done by a force Farreneheit over two displacements ∆r1 and ∆r2 is W = Farreneheit · (∆r1 + ∆r2 )

Remember the deﬁnition of the appear in product

A · B = |A||B| cos θ = Ax Bx & Ay By + Arizona Bz

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NYA ﬂashback: Function: example

Precisely what is the work done by gravity on a watermelon decreased from the College's roof to the parking lot under?

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NYA ﬂashback: Work: case in point

The power acting on the melon since it falls is Fg sama dengan −mgˆ. The j shift it undergoes is ∆r = −hˆ. We set these t together to ﬁnd the fact that work is usually Wg = Fg · ∆r sama dengan (−mgˆ · (−hˆ sama dengan mgh(ˆ · ˆ sama dengan mgh. And so the j) j) j j) amount of work made by gravity for the melon of mass meters as it lowered a distance h is mgh. (Remember, ˆ · ˆ sama dengan |ˆ ˆ cos θ = 1 × you × cos(0) = 1 . ) j j j||j

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NYA ﬂashback: Potential strength

We only saw that if we drop an object of mass meters from a height h in a gravitational ﬁeld of magnitude g, the work done by the ﬁeld on the subject will be T = mgh. This appearance should remind you of something... Is it doesn't gravitational potential energy associated with an object of mass meters held in a elevation h above the surface of the Earth. The energy is usually equal to the work a ﬁeld can perform on an subject if we relieve the object.

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NYA ﬂashback: Potential energy

Things should bear in mind about potential energy: The particular difference inside the potential energy between two points matters, not really the actual ideals. We can for that reason set U to no wherever we all please. The effort done by the force is definitely equal to without the change in the energy via point A to point B; Wohngemeinschaft = −∆Ug. For a conservative force (such the gravitational for or maybe the electric force), the path obtained from point A to stage B makes no difference!

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NYA ﬂashback: Potential strength

What is the work done by the law of gravity on a melon dropped on a parabolic course from the College's roof to the parking lot below?

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NYA ﬂashback: Potential energy

The only thing that concerns here is the preliminary and ﬁnal heights, considering that the potential energy is Ug = mgh. So the work done by the law of gravity on the object is again Wg sama dengan mgh. Remember that this is minus the change in the potential energy ∆Ug = Uf − User interface = 0 − mgh = −mgh = −Wg, as it should. Notice that seeing that we are liberated to set Ug = 0 wherever we want, we could have stated that Ug = zero at the college's roof. In this instance, however , the potential energy on the ground is usually Ug = −mgh, and that we still ﬁnd ∆Ug = Uf − Ui sama dengan −mgh − 0 sama dengan −mgh sama dengan −Wg, which conﬁrms the choice of in which Ug = 0 will not change the answer to the problem.

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NYA ﬂashback: Potential energy

Remember that the work done doesn't be based upon the path obtained from the initial towards the ﬁnal point, so we're able to ﬁnd the work by looking with the path demonstrated here,

in which clearly the effort done around the horizontal expand by the law of gravity is no since the angle between force and displacement is 90o, and the work done on the vertical stretch is usually mgh.

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Work and the electric ﬁeld

If we place a charge q0 in an electrical ﬁeld Electronic, the electrical ﬁeld can do work on the charge. Without a doubt, in this case, the force around the charge can be F sama dengan q0 Electronic, so that the work done on q0 by the electric power ﬁeld is We = Fe · ∆r = q0 At the · ∆r

and the change in the potential strength of the charge-ﬁeld system is ∆Ue = −We...