Lesson_2.5_Printable_PPT

Lesson_2.5_Printable_PPT - Electric Potential Comparing...

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Unformatted text preview: Electric Potential Comparing gravitational and electric fields. You are familiar with gravitational potential energy. Gravity does egative Gravity does positive An external force does work against gravity to raise an object. The work done by gravity on the object is negative negative work work This work done by the external force gives the object potential energy ( U ) When the object is allowed to fall, gravity does positive work on the object The net work on the object in the Potential energy is a measure of the negative work done by gravity closed path is zero. A force field where the net work done in a closed path is zero is called a conservative force field . Potential energy in a conservative force field is the negative work done by the conservative force. U F ds = - Here we study the motion of electric charges in an electric field. Since there are two types of electric charges, we will take the positive charge as the standard test charge . positive st charge Like gravitational field, electric field is a conservative force field. A positive test charge is placed in a field produced by a positive charge. An external force has to do work to move the test charge from position A to position B increasing its potential energy. The electric field does negative work on the test harge increasing its When the test charge is allowed to move, it falls to low potential in the direction of the electric field when the electric field does positive work on it. charge increasing its U The potential decreases in the direction of the field. The potential energy of the test charge at infinity is zero. When the test charge is moved opposite to the direction of the field, the gain in potential energy is a measure of the negative work done by the filed. U F ds = - Consider an electric field E generated by a positive charge . Since E varies with the distance from the charge, the electric field has different values at different points. + + + q o F = E q o E F = -E q o A test charge q o placed in the field experiences a force F = E q o . To move the test charge in the direction of increasing potential, an equal opposite external force is needed. F = - Eq o + + + q o F = E q o E F = -E q o When the test charge is moved a distance dr opposite to the direction of the field, its increase in potential energy is: dr dU = - Eq o dr F = - Eq o The increase in potential nergy per unit charge is: This is a measure of the potential difference between the two points and it is the work done on a unit test charge to move it through a distance dr . energy per unit charge is: o dU q = - E dr + + + q o F = E q o E F = -E q o dr o dU q = - E dr We will use V to represent potential at a point and V the potential difference between two points separated by small distance r ....
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Lesson_2.5_Printable_PPT - Electric Potential Comparing...

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