AP_Physics_B_-_Electric_potential

# AP_Physics_B_-_Electric_potential -...

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Electrical Energy, Potential  and Capacitance AP Physics B

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Electric Fields and WORK In order to bring two like charges near each other work must be done. In order to separate two opposite charges, work must be done. Remember that whenever work gets done, energy changes form. As the monkey does work on the positive charge, he increases the energy of that charge. The closer he brings it, the more electrical potential energy it has. When he releases the charge, work gets done on the charge which changes its energy from electrical potential energy to kinetic energy. Every time he brings the charge back, he does work on the charge. If he brought the charge closer to the other object, it would have more electrical potential energy. If he brought 2 or 3 charges instead of one, then he would have had to do more work so he would have created more electrical potential energy. Electrical potential energy could be measured in Joules just like any other form of energy.
Electric Fields and WORK Consider a negative charge moving in between 2 oppositely charged parallel plates initial KE=0 Final KE= 0, therefore in this case Work = PE We call this ELECTRICAL potential energy, U E , and it is equal to the amount of work done by the ELECTRIC FORCE, caused by the ELECTRIC FIELD over distance, d, which in this case is the plate separation distance. Is there a symbolic relationship with the FORMULA for gravitational potential energy?

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Electric Potential Ed q W qEd W U d x h E g q m W or U U mgh U E E g g = = = ) ( ) ( Here we see the equation for gravitational potential energy. Instead of gravitational potential energy we are talking about ELECTRIC POTENTIAL ENERGY A charge will be in the field instead of a mass The field will be an ELECTRIC FIELD instead of a gravitational field The displacement is the same in any reference frame and use various symbols Putting it all together! Question: What does the LEFT side of the equation mean in words? The amount of Energy per charge!
Energy per charge The amount of energy per charge has a specific name and it is called, VOLTAGE or ELECTRIC POTENTIAL (difference). Why the “ difference ”? q mv q K q W V 2 2 1 = = =

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Understanding “Difference” Let’s say we have a proton placed between a set of charged plates. If the proton is held fixed at the positive plate, the ELECTRIC FIELD will apply a FORCE on the proton (charge). Since like charges repel, the proton is considered to have a high potential (voltage) similar to being above the ground. It moves towards the negative plate or low potential (voltage). The plates are charged using a battery source where one side is positive and the other is negative. The positive side is at 9V, for example, and the negative side is at 0V. So basically the charge travels through a “change in voltage” much like a falling mass experiences a “change in height. (Note: The electron does the opposite)
BEWARE!!!!!!

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