Ele

ctric

vs. G

ravita

tion

al

Pote

ntia

l Energ

y

• Work is needed to push an object against gravity and give it Potential Energy (PEg)

Gravitational

Potential

Energy

Increasesg

• Work is needed to

push a charge

against a field and

give it Electric

Potential Energy

Electric

Potential

Energy

Increases

E

+

-

Un

iform

Ele

ctric

Fie

lds

Allesandro Volta(Italian: 1745 – 1827)

Parallel Plates

Point charge

Uniform electric field

Note the field lines

Pote

ntia

l Energ

y &

Po

ten

tial D

iffere

nce

–2–

Low

Potential

Energy

- - - - - - - - -

+++++++++E

High

Potential

Energy

Low

Potential

Energy

Higher

Potential

Energy

Why does this one

have more potential

energy?

Pote

ntia

l Energ

y &

Po

ten

tial D

iffere

nce

2+

- - - - - - - - -

+++++++++E

High or

Low PE?

High or

Low PE?A change in

PE means …

Some stuff you should remember:

dΔPEelectric q E=

W ΔPE=

Fon test charge

q E=

W Fd=

W F d=

Some new stuff

W ΔPE=

Fon test charge

q E=

Only for uniform electric field

Why?

Po

ten

tial E

ne

rgy &

Pote

ntia

l Diffe

ren

ce

fyi: k = 1/(4πεo) εo=8.85 x 10-12 C2/Nm2

Pote

ntia

l Energ

y &

Po

ten

tial D

iffere

nce

Potential

Difference

(V)

=ΔElectric Potential Energy (J)

Charge of particle (C)

Potential Difference Voltage (scalar or vector)

ΔPEelectric=qΔV

=ΔV = dqEdq

=kQr2

kQr

r

What does voltage actually measure?

– Measures how much energy a charge will gain/lose

– Measures how much work that can be done by a charge

Pote

ntia

l Energ

y &

Po

ten

tial D

iffere

nce

–2–

- - - - - - - - -

+++++++++E

Which of these has

greater electrical potential

energy per charge?

more ELECTRIC POTENTIAL

ENERGY

Same ELECTRIC POTENTIAL

Pote

ntia

l Energ

y &

Po

ten

tial D

iffere

nce

–2–

- - - - - - - - -

+++++++++E

Which of these has

experienced a greater

change in electrical

potential energy?

Which of these has experienced

a greater change in electrical

potential energy per charge?

Analo

gy

12 V0 V

Battery gives the

electron 12 J of

Potential energy

e “loses” energy as it

travels through the

circuit.

Battery “gives” energy back

to electron to repeat cycle

Pote

ntia

l Energ

y &

Po

ten

tial D

iffere

nce

• Sample Problem

The electric field is 625 N/C between 2 large charged parallel plates are 4 cm apart.a. What is the electric potential difference

between the plates?

b. What work will you do to move a proton from the – to the + plate?

+ + +

–––

D = 0.04m

p+

a) V = E d

= (625N/C)(0.04m)

= 25 V

E = 625 N/C

d = 0.04 m

q = 1.6 x 10-19 C

b) W = PEe = q ∆V

= (25 V)(1.6 x 10-19C)

PEe = 4.0 x 10-18 J

Key Id

ea

s

Electric

Potential

Energy

Electric

Potential,

Potential,

Voltage

Electric

Potential

Difference

Defin-

itionTotal

Potential

Energy

Energy per

charge

Change in

voltage

Unit Joules Volts(J/C)

Volts

(J/C)

Equation PEe= kQq/r

= qEd

= qV

V = PE/q ΔV = ΔPE/q

Hid

den E

qu

atio

ns

W = Fd F = kQq/r2

W = kQq/r2 r

W = kQq/r = PEelectric

if PEelectric = W = qV V = kQ/r

k = 1/(4πεo)

εo=8.85 x 10-12 C2/Nm2