Capacitors. Lecture 10. Chapter 26. My Capacitance is limited. PHYS.1440 Lecture 10 Danylov. Department of Physics and Applied Physics

Transcription

1 Lecture 10 Chapter 26 Capacitors My Capacitance is limited Course website:

2 Today we are going to discuss: Chapter 26: Section 26.2 The Geometry of Potential and Field Section 26.3 A Conductor in Electrostatic equilibrium Section 26.5 Capacitance and Capacitors Class Average 51.8/80pts which is 64.7%

3 Connecting Potential and Field (Review) We can find the potential difference between two points if we know the electric field. V V f V i f i E ds E x dv dx E y dv dy E z dv dz

4 Example Example 26.1

5 ConcepTest An electron is released from rest at x = 2 m in the potential shown. What does the electron do right after being released? My Favorite question A) Stay at x 2 m B) Move to the right ( x) at steady speed C) Move to the right with increasing speed D) Move to the left (x) at steady speed E) Move to the left with increasing speed E x dv dx 1) Slope of V negative => E x is positive (field to the right). 2) Electron is negative => force to the left. 3) Force to the left => acceleration to the left. F x qe F x ma F x a q E x

6 The Geometry of Potential and Field E s dv ds Let s be along an equipotential line, where V=const. So dv=0. So, it is impossible to have an electric field along equipotential lines, E E s equipotent 0 dv ds ial 0 lines

7 ConcepTest Which set of equipotential surfaces matches this electric field? E Geometry E and V A) B) C) D) E) F)

8 Potential of a Conductor V const f i f V V V E ds V 0 f i i V V f i A conductor in electrostatic equilibrium is at the same potential.

9 The Electric Battery A battery transforms chemical energy into electrical energy. Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them. Two electrons get attracted by the ion of Zn leaving behind positively charged electrode Atom of Zn gets dissolved leaving two electrons behind

10 Capacitor

11 Capacitor stores energy A capacitor is a system that stores potential energy in a form of an electric field. Ugh! It s just energy

12 Capacitance (definition) V C The ratio of the charge Q to the potential difference V C is called the capacitance C: The SI unit of capacitance is the farad:

13 Parallel-plate capacitor In its simplest form, a capacitor consists of a pair of parallel metal plates separated by air/insulating material.

14 Q Parallel-plate capacitor Let s find capacitance of a parallel-plate capacitor E +Q area A We need to find Q and ΔV: The electric field between the plates is surface charge density d The potential difference between plates: (Eq.25.26) This gives the capacitance: Capacitance is a purely geometric property of two electrodes because it depends only on their surface area and spacing. =

15 Parallel-plate capacitor We can increase capacitance by increasing area A by making a roll of metal and insulator

16 Parallel-plate capacitor/keyboard The keys on most computer keyboards are capacitor switches. Pressing the key pushes two capacitor plates closer together, increasing their capacitance. Capacitors are important elements in electric circuits. They come in a variety of sizes and shapes.

17 Thank you