PHYS 219 Recitation

1. PHYS 219 Recitation Kevin Ralphs 31 Aug 2013

2. Overview • HW Questions • Potential Energy • Potential • Capacitors

3. HW Questions Ask Away….

4. Potential Energy • In a closed system with no dissipative forces • The work done is due to the electric force so This formula assume the following: • is constant in both magnitude and direction • The displacement is parallel to • WARNING: Since charge can be negative, and might point in opposite directions (this is called antiparallel) which would change the sign of W • This can be combined with the work-energy theorem to obtain the velocity a charged particle has after moving through an electric field

5. Potential • What does it tell me? • The change in potential energy per unit charge an object has when moved between two points • Why do I care? • The energy in a system is preserved unless there is some kind of dissipative force • So the potential allows you to use all the conservation of energy tools from previous courses (i.e. quick path to getting the velocity of a particle after it has moved through a potential difference)

6. Potential • Word of caution: • Potential is not the same as potential energy, but they are intimately related • Electrostatic potential energy is not the same as potential energy of a particle. The former is the work to construct the entire configuration, while the later is the work required to bring that one particle in from infinity • There is no physical meaning to a potential, only difference in potential matter. This means that you can assign any point as a reference point for the potential. When possible, we usually choose the potential to be zero very far away • The potential must be continuous

7. Tying it Together Multiply by q Electric Field Electric Force Vectors Multiply by -Δx Multiply by -Δx Change in Potential Change in PE Scalars Multiply by q

8. Analogies with Gravity • Electricity and magnetism can feel very abstract because we don’t usually recognize how much we interact with these forces • There are many similarities between gravitational and electric forces • The major difference is that the electric force can be repulsive • Gravity even has a version of Gauss’s law

9. Capacitance • What does it tell me? • The charge that accumulates on two conductors is proportional to the voltage between them • Why do I care? • Capacitors are vital components in electronics • They can be used to temporarily store charge and energy, and play an even more important role when we move to alternating current systems • Camera flashes, touch screen devices, modern keyboards all exploit capacitance

10. Capacitance • What does capacitance depend on? • Geometry of the plates • For parallel plates: • Material between the plates • Unit of capacitance is the Farad • To demystify this, units are (meters*permittivity)

11. Capacitance • The permittivity of free space has no physical meaning • It merely changes physical quantities into their appropriate SI units

12. Capacitance • A charged capacitor has potential energy

13. Capacitance • In circuits • In well-behaved configurations, capacitors may be combined into a single equivalent capacitor • Parallel * This is like increasing the area of the plates * • Series * This is like increasing the separation distance *

14. Capacitance • Capacitors are in equilibrium • Series: when they have the same charge • Parallel: when they have the same voltage