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PHYSICS NYB-21E Winter 2012
Chapter 3: Electric potential energy and electric potential
´ ´ Instructor: Jeremie Vinet Marianopolis College.
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Review
Important points from last lectures: q A point charge q creates an electric field E = ke 2 r ˆ r
A point charge q0 placed in an electric field E feels a force Fe = q0 E
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NYA flashback: Work
When a net force acts on an object, it accelerates it. When an object accelerates, its velocity changes. When the speed of an object changes, its kinetic energy changes. When an object’s energy changes, work has been done on it.
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NYA flashback: Work
In less cartoonish terms, The amount of work W done by a force F over a displacement ∆r is
W = F · ∆r
The amount of work W done by a force F over two displacements ∆r1 and ∆r2 is
W = F · (∆r1 + ∆r2 )
Remember the definition of the dot product
A · B = |A||B| cos θ = Ax Bx + Ay By + Az Bz
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NYA flashback: Work: example
What is the work done by gravity on a watermelon dropped from the College’s roof to the parking lot below?
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NYA flashback: Work: example
The force acting on the melon as it falls is Fg = −mgˆ. The j displacement it undergoes is ∆r = −hˆ. We put these j together to find that the work is Wg = Fg · ∆r = (−mgˆ · (−hˆ = mgh(ˆ · ˆ = mgh. So the j) j) j j) amount of work done by gravity on the melon of mass m as it dropped a distance h is mgh. (Remember, ˆ · ˆ = |ˆ ˆ cos θ = 1 × 1 × cos(0) = 1.) j j j||j|
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NYA flashback: Potential energy
We just saw that if we drop an object of mass m from a height h in a gravitational field of magnitude g , the work done by the field on the object will be W = mgh. This expression should remind you of something... It is the gravitational potential energy of an object of mass m held at a height h above the surface of the Earth. The potential energy is equal to the work a field can do on an object if we release the object.
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NYA flashback: Potential energy
Things you
Chapter 3: Electric potential energy and electric potential
´ ´ Instructor: Jeremie Vinet Marianopolis College.
– p.
Review
Important points from last lectures: q A point charge q creates an electric field E = ke 2 r ˆ r
A point charge q0 placed in an electric field E feels a force Fe = q0 E
– p.
NYA flashback: Work
When a net force acts on an object, it accelerates it. When an object accelerates, its velocity changes. When the speed of an object changes, its kinetic energy changes. When an object’s energy changes, work has been done on it.
– p.
NYA flashback: Work
In less cartoonish terms, The amount of work W done by a force F over a displacement ∆r is
W = F · ∆r
The amount of work W done by a force F over two displacements ∆r1 and ∆r2 is
W = F · (∆r1 + ∆r2 )
Remember the definition of the dot product
A · B = |A||B| cos θ = Ax Bx + Ay By + Az Bz
– p.
NYA flashback: Work: example
What is the work done by gravity on a watermelon dropped from the College’s roof to the parking lot below?
– p.
NYA flashback: Work: example
The force acting on the melon as it falls is Fg = −mgˆ. The j displacement it undergoes is ∆r = −hˆ. We put these j together to find that the work is Wg = Fg · ∆r = (−mgˆ · (−hˆ = mgh(ˆ · ˆ = mgh. So the j) j) j j) amount of work done by gravity on the melon of mass m as it dropped a distance h is mgh. (Remember, ˆ · ˆ = |ˆ ˆ cos θ = 1 × 1 × cos(0) = 1.) j j j||j|
– p.
NYA flashback: Potential energy
We just saw that if we drop an object of mass m from a height h in a gravitational field of magnitude g , the work done by the field on the object will be W = mgh. This expression should remind you of something... It is the gravitational potential energy of an object of mass m held at a height h above the surface of the Earth. The potential energy is equal to the work a field can do on an object if we release the object.
– p.
NYA flashback: Potential energy
Things you