Lecture_4

Lecture_4 - 19.1 Potential Energy The electric force like...

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19.1 Potential Energy The electric force, like gravity, is a conservative force : Recall Conservative Forces 1. The work done on an object by a conservative force depends only on the object’s initial and final position , and not the path taken. 2. The net work done by a conservative force in moving an object around a closed path is zero . Let’s place a positive point charge q in a uniform electric field and let it move from point A to B (no gravity): How much work is done by the field in moving the ++ + + + +q charge from A to B? *Remember, W = F x d, where F is the component of the constant force along the direction of the motion. A E y o Here, , so qE F = y qE y y qE W o f Δ = = ) ( Electrostatic Potential nergy (EPE) -- - - - B y f Energy (EPE) Analogous to gravity, where mgh is the gravitational potential energy.
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Thus, EPE W AB Δ = The work done is equal to the change in electrostatic potential energy! Now let’s divide both sides by the charge, q: EPE W AB Δ The quantity on the right is the potential energy per unit q q = charge. We call this the Electric Potential, V : EPE The electric potential is q V = nergy a scalar! Units? [ ] [ ] V C J = = = Volt Charge Energy Review of Work : 1. Work is not a vector, but it can be either positive or negative: Positive – Force is in the same direction as the motion Negative – Force is in the opposite direction as the motion 2. If positive work is done on an object, the object speeds up . 3. If negative work is done on an object, the object slows down .
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We can talk about the value of the potential at different points in space: For example, what is the difference in electrostatic potential 19.2 Electric Potential Difference between two points, A and B, in an electric field??? ++ + + + EPE EPE V V A B A B = W AB = +q A E q q q W B Why is there a -- - - - B So, q V V V AB A B = = Δ minus sign??? Let’s say the charge we put at point A is positive: If I release it, which way will it move? It moves down toward B!
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Lecture_4 - 19.1 Potential Energy The electric force like...

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