Chapter 19. Electric Potential Energy and the Electric Potential

Transcription

1 Chapter 19 Electric Ptential Energy and the Electric Ptential

2 19.1 Ptential Energy W mgh mgh GPE GPE

3 19.1 Ptential Energy

4 19.1 Ptential Energy W EPE EPE

5 19. The Electric Ptential Difference W q EPE q EPE q The ptential energy per unit charge is called the electric ptential.

6 19. The Electric Ptential Difference DEFINITION OF ELECTRIC POTENTIL The electric ptential at a given pint is the electric ptential energy f a small test charge divided by the charge itself: V EPE q SI Unit f Electric Ptential: jule/culmb = vlt (V) V V EPE q EPE q W q V EPE q W q

7 19. The Electric Ptential Difference Example 1 Wrk, Ptential Energy, and Electric Ptential The wrk dne by the electric frce as the test charge (+.0x10-6 C) mves frm t is +5.0x10-5 J. (a) Find the difference in EPE between these pints. (b) Determine the ptential difference between these pints. W EPE EPE V V EPE q EPE q W q

8 19. The Electric Ptential Difference (a) W EPE EPE EPE EPE W J (b) V V W q J C 5 V

9 19. The Electric Ptential Difference Cnceptual Example The cceleratins f Psitive and Negative Charges psitive test charge is released frm and accelerates twards. Upn reaching, the test charge cntinues t accelerate tward C. ssuming that nly mtin alng the line is pssible, what will a negative test charge d when released frm rest at?

10 19. The Electric Ptential Difference psitive charge accelerates frm a regin f higher electric ptential tward a regin f lwer electric ptential. negative charge accelerates frm a regin f lwer ptential tward a regin f higher ptential.

11 19. The Electric Ptential Difference We nw include electric ptential energy EPE as part f the ttal energy that an bject can have: E 1 1 mv I mgh 1 kx EPE One electrn vlt is the magnitude f the amunt by which the ptential energy f an electrn changes when the electrn mves thrugh a ptential difference f ne vlt. 1eV V

12 19. The Electric Ptential Difference Example 4 The Cnservatin f Energy particle has a mass f 1.8x10-5 kg and a charge f +3.0x10-5 C. It is released frm pint and accelerates hrizntally until it reaches pint. The nly frce acting n the particle is the electric frce, and the electric ptential at is 5V greater than at C. (a) What is the speed f the particle at pint? (b) If the same particle had a negative charge and were released frm pint, what wuld be its speed at?

13 19. The Electric Ptential Difference EPE mv EPE mv 1 1 EPE EPE mv mv 1 1 V V q mv mv 1 1

14 19. The Electric Ptential Difference (a) 1 mv q V V v q V V m C5 V kg 9.1m s 3.0 (a) v q V V m C5 V kg 9.1m s

15 19.3 The Electric Ptential Difference Created by Pint Charges W kqq r kqq r V V W q kq r kq r Ptential f a pint charge V kq r

16 19.3 The Electric Ptential Difference Created by Pint Charges Example 5 The Ptential f a Pint Charge Using a zer reference ptential at infinity, determine the amunt by which a pint charge f 4.0x10-8 C alters the electric ptential at a spt 1.m away when the charge is (a) psitive and (b) negative.

17 19.3 The Electric Ptential Difference Created by Pint Charges (a) V kq r N m C C 300 V 1. m (b) V 300 V

18 19.3 The Electric Ptential Difference Created by Pint Charges Example 6 The Ttal Electric Ptential t lcatins and, find the ttal electric ptential.

19 19.3 The Electric Ptential Difference Created by Pint Charges V Nm C C Nm C C 40 V 0.0 m 0.60 m V Nm C C Nm C C 0 V 0.40 m 0.40 m

20 19.3 The Electric Ptential Difference Created by Pint Charges Cnceptual Example 7 Where is the Ptential Zer? Tw pint charges are fixed in place. The psitive charge is +q and the negative charge is q. On the line that passes thrugh the charges, hw many places are there at which the ttal ptential is zer?

21 19.4 Equiptential Surfaces and Their Relatin t the Electric Field n equiptential surface is a surface n which the electric ptential is the same everywhere. V kq r The net electric frce des n wrk n a charge as it mves n an equiptential surface.

22 19.4 Equiptential Surfaces and Their Relatin t the Electric Field The electric field created by any charge r grup f charges is everywhere perpendicular t the assciated equiptential surfaces and pints in the directin f decreasing ptential.

23 19.4 Equiptential Surfaces and Their Relatin t the Electric Field

24 19.4 Equiptential Surfaces and Their Relatin t the Electric Field V E s

25 19.4 Equiptential Surfaces and Their Relatin t the Electric Field Example 9 The Electric Field and Ptential re Related The plates f the capacitr are separated by a distance f 0.03 m, and the ptential difference between them is V -V =-64V. etween the tw equiptential surfaces shwn in clr, there is a ptential difference f -3.0V. Find the spacing between the tw clred surfaces.

26 19.4 Equiptential Surfaces and Their Relatin t the Electric Field V E s 64 V m V m s V E.0.0 V 3 10 V 3 3 m m

27 19.5 Capacitrs and Dielectrics parallel plate capacitr cnsists f tw metal plates, ne carrying charge +q and the ther carrying charge q. It is cmmn t fill the regin between the plates with an electrically insulating substance called a dielectric.

28 19.5 Capacitrs and Dielectrics THE RELTION ETWEEN CHRGE ND POTENTIL DIFFERENCE FOR CPCITOR The magnitude f the charge n each plate f the capacitr is directly prprtinal t the magnitude f the ptential difference between the plates. q CV The capacitance C is the prprtinality cnstant. SI Unit f Capacitance: culmb/vlt = farad (F)

29 19.5 Capacitrs and Dielectrics THE DIELECTRIC CONSTNT If a dielectric is inserted between the plates f a capacitr, the capacitance can increase markedly. Dielectric cnstant E E

30 19.5 Capacitrs and Dielectrics

31 19.5 Capacitrs and Dielectrics THE CPCITNCE OF PRLLEL PLTE CPCITOR E q E E V d q V d Parallel plate capacitr filled with a dielectric C d

32 19.5 Capacitrs and Dielectrics Cnceptual Example 11 The Effect f a Dielectric When a Capacitr Has a Cnstant Charge n empty capacitr is cnnected t a battery and charged up. The capacitr is then discnnected frm the battery, and a slab f dielectric material is inserted between the plates. Des the vltage acrss the plates increase, remain the same, r decrease?

33 19.5 Capacitrs and Dielectrics Example 1 Cmputer Keybard One cmmn kind f cmputer keybard is based n the idea f capacitance. Each key is munted n ne end f a plunger, the ther end being attached t a mvable metal plate. The mvable plate and the fixed plate frm a capacitr. When the key is pressed, the capacitance increases. The change in capacitance is detected, thereby recgnizing the key which has been pressed. The separatin between the plates is 5.00 mm, but is reduced t mm when a key is pressed. The plate area is 9.50x10-5 m and the capacitr is filled with a material whse dielectric cnstant is Determine the change in capacitance detected by the cmputer.

34 19.5 Capacitrs and Dielectrics C d C Nm m F m C d C Nm m F m C F

35 19.5 Capacitrs and Dielectrics ENERGY STORGE IN CPCITOR Energy 1 CV Vlume d Energy 1 d Ed Energy density Energy Vlume 1 E

36 19.6 imedical pplicatins f Electrical Ptential Differences

37 19.6 imedical pplicatins f Electrical Ptential Differences

38 19.6 imedical pplicatins f Electrical Ptential Differences

39 19.6 imedical pplicatins f Electrical Ptential Differences

40 19.6 imedical pplicatins f Electrical Ptential Differences

41 19.6 imedical pplicatins f Electrical Ptential Differences