sph4uc_unit_3_lesson_11.pdf - SPH4U-C 11 Lesson Electric...

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SPH4U-C Electric Potential Energy and Electric Potential 11 Lesson
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Introduction Many people today have cell phones with touch-screen technology. When your finger makes contact with one of these screens, it can somehow detect where the contact is made and use this information to run programs. This allows you to operate the electronic device without needing a keyboard or mouse. For devices like a cell phone, this type of technology is a tremendous advantage because the device needs to be small and easy to use. In this lesson, you will learn about electric potential energy and electric potential difference, two key concepts involved with some types of touch-screen technology. Planning Your Study You may find this time grid helpful in planning when and how you will work through this lesson. Suggested Timing for This Lesson (hours) Electric Potential Energy 1 Electric Potential 1 The Millikan Oil Drop Experiment 1 Applications of Electric Potential 1 Key Questions 1 What You Will Learn After completing this lesson, you will be able to analyze and solve problems related to electric potential energy analyze and solve problems related to electric potential conduct a computer simulation of the Millikan oil drop experiment analyze technology that uses concepts related to electric potential Lesson 11 Physics SPH4U-C 1
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Electric Potential Energy In an earlier unit, you learned that the gravitational force could do work on an object that moves through a change in height. This work is actually the change in gravitational potential energy of the object. This implies that action-at-a-distance forces can be used to store potential energy. Now think of two charges separated by a distance, r . If the charges are similar in sign (that is, both are positive or both are negative), they will repel each other and move further apart, gaining kinetic energy. Where does this new kinetic energy come from? Recall that the law of conservation of energy tells you that energy cannot be created or destroyed, but only transferred from one form to another. If the charges are gaining kinetic energy, then they must be losing electric potential energy. Electric potential energy is the energy stored in a system of charges that are separated by a distance. Keep in mind that energy is a scalar quantity, so electric potential energy has no direction and the unit of measure is the joule (J). The equation for the electric potential energy stored by two point charges separated by a distance, r , is: E E = kq 1 q 2 r This equation looks a lot like Coulomb’s law. In fact, Coulomb’s law is used to derive the equation for electric potential energy, although the derivation is beyond the scope of this course. (It involves finding the area under a force-distance graph using integral calculus.
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