Chapter 16 Electrical Energy Capacitance. HW: 1, 2, 3, 5, 7, 12, 13, 17, 21, 25, 27 33, 35, 37a, 43, 45, 49, 51

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1 Chapter 16 Electrical Energy Capacitance HW: 1, 2, 3, 5, 7, 12, 13, 17, 21, 25, 27 33, 35, 37a, 43, 45, 49, 51

2 Electrical Potential Reminder from physics 1: Work done by a conservative force, depends only on the initial and final positions. Work is path independent. Produces Potential energy Examples: gravitational potential energy elastic potential energy

3 Both the coulomb force and the gravitational force are proportional to 1/r 2. Both are examples of a central force. Both of these forces are conservative forces.

4 When you do work against gravity, you changed the gravitational potential energy. Doing work against the Coulomb force(electric force) changes the electrical potential energy.

5 Work and potential energy Work done by a force to move an object is the product of the component of the force parallel to the displacement and the displacement. W = F d cos θ in figure 16.1 W ab = F x x= qe x (x f x i ) This is the work that moves a charge from A to B.

6 Work and Energy Reminder from physics 1: Work-Energy Theorem: W net = KE From ch 5, the work done by a conservative force is equal to the negative of the change in potential energy associated with the force.

7 For a charge in an electric field: PE = -W AB = -qe x x In fig. 16.1, as the positive charge moves from left to right, positive work is done on the charge, the charge loses some electrical potential energy. (Note that this equation is valid when the field is constant.) See also figure 16.2

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9 Can repeat the situation with a negative charge. If released from rest the negative charge will move in the opposite direction that the positive charge traveled. A positive charge loses potential energy as it travels along the electric field. Opposite for a negative charge. work example 16.1 a

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14 TV tube example A proton is injected between two parallel plates with a speed of 1x10 6 m/s. The plates are 5 cm apart. a) what must be the potential difference if the proton is to exit with a speed of 3x10 6 m/s? b) What is the magnitude of the electric field between the two plates. Page 564

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16 Fig 16.5 Electrical Potential for Point Charges

17 What happens when we introduce a proton and an electron and let them go?

18 If you have two or more charges, you can find the electric potential via the superposition principle. Find the potential from each charge and add up the values. Potential is a scalar, so no need to worry about vectors and direction this time. Fig shows the potential of an electric dipole.

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22 No work is needed to move a charge around inside a conductor.

23 Electron Volt We want to have a conveniently sized unit of energy. An electron volt is the kinetic energy that an electron gains when accelerated through a potential difference of 1V. 1V = 1 J/C Magnitude of charge of electron = 1.6x10-19 C 1eV = 1.6x10-19 C V = 1.6x10-19 J This is convenient because 1 J is a lot of energy to give an electron.

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26 Application Van de graaf generator/accelerator

27 Van de graaf accelerator Uses principles from chapters 15, 16 to accelerate charged particles.

28 Capacitance capacitor electrical device used in many circuits that is used to store electrical energy to be used later. Consists of two conductors separated from each other. Example: parallel plate capacitor. Two parallel metal plates separated by distance, d, and connected to positive and negative terminals of a battery. One plate loses electrons and receives a charge of +Q. The electrons are transferred through the battery to the other plate which obtains a charge of -Q.

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32 See fig for electric field of parallel plate capacitor. Applications: Camera flash Keyboard Timing devices

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36 For two capacitors, we found: C eq = C 1 + C 2 We can extend this to more capacitors. C eq = C 1 + C 2 + C 3 + C 4 + The total capacitance of capacitors in parallel is the sum of the capacitances. Thus the equivalent capacitance is larger than any of the individual capacitances. (Electrical devices in parallel have the same potential difference across each device.)

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40 Energy stored in a capacitor Capacitors store electrical energy That amount of energy is the same as the work required to move charge, Q, onto the plates of the capacitor. When a capacitor discharges, it releases the energy (sparks). Find out how much work is required to charge a capacitor.

41 As more and more charge is place on a capacitor, the voltage between the capacitor s plates increases. It requires more and more work to add each sequential charge. See figure Total work to fully charge the capacitor is the area under the graph. In this case, the area of the triangle whose base and height are Q and V. Substituting Q =C V, yields Energy stored = ½ C( V) 2 = ½ Q 2 /C

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45 Atomic description of a dielectric, fig 16.33

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47 Capacitors with multiple dielectrics See examples on page 588.

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