How does a ____________ (or a collection of ____________)exert a force on another _____________ from a distance?

charge chargescharge

chargedobject A

chargedobject BFe

Answer: By means of their electric ________________ .

(Note: The letter E can also represent ______________ .)

fields, E

III. ____________ FieldsElectric

energy

An electric field E exists around ___________________.

Its strength is found by placing a positive "__________________" (any charge) q in the field and measuring the electric force Fe that ________________.

E =

E is a _______________ because it has ______________ .

Its direction is given by the direction of the _____________

____________ that acts on the __________________________

that is placed in the field

units of E: E = Fe / q [ E ] = [ ]/[ ] = /

every charge

Fe/q,

vector

electricalforce positive test charge

test chargeacts on q

N CFe q

direction

where q is the _______ charge test

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Ex 1: A positive charge q = 5.0 mC experiences an electrical force of 10.0 x 103 N when placed at the position shown.

Determine the strength and direction of the electric field at the point shown.

strength: E =

==

Fe

q

direction: (__________ direction as ____)

Fe / q

10.0 x 103 N / 5.0 x 10-3 C 2.0 x 106 N/C

same Fe

Ex 2: What electrical force will a proton experiencewhen placed in the electric field at point P shown below?

magnitude: =

=

= Fe

Ex 3: What electrical force will an electron experiencewhen placed in the same point as the previous example?

PE = ____________

direction: ______________ direction as _____

magnitude: _________________

direction: __________________

Fe / 1.6 x 10-19 C

5.9 x 10-13 N

same E

same

opposite

Fe / qE

3.7x106 N/C

3.7 x 106 N/C

Ex 4: The E field around a ______________ point charge qp

+Fe

positive

pos. test qqp

2 force lines stronger

1 forceline weaker

Note:

1. The arrows represent “_________________” that act

on a ______________ test charge q.

2. E is directed _____________________ away from “____”

3. The closeness of the lines show the field ___________.

• The arrows show ________________ because

they point ______________________. 5. Field lines _________________, because if that weretrue, the field would have ______________________________

6. E is _________________________. It is NOT constant.

lines of force

positive

radially outward +

inverse square in r

repulsion

outward

never cross2 directions at one place

strength

For a point charge:

Let q1 = the __________ charge qp,

and q2 = the __________ charge q,

Then, _________________ Law: Fe = kq1q2/r2

becomes Fe =

and E = Fe/q

becomes E =

E E

qp r

point

test

Coulomb's

kqpq/r2

kqpq/r2

For a point charge:

q= kqp/r2

Ex 5: The E field around a ____________ point charge.

Note:1. The arrows represent “__________________________ .” 2. E is directed radially __________ towards the ___ .3. E is __________________________. It is NOT constant.4. The _________________ of the lines represents the field strength.5. The arrows show ________________because they point __________________.6. This E resembles Earth’s _____________________ field as itwould be seen _____________________

-

pos. test q

Fe

negative

inwardinverse square in rcloseness

attractioninward

lines of force"-"

from far awaygravitational

Ex 6: The E field around 2 __________________ charges.

Start by placing a ____ test charge q at one point. Find resultant of the forces from both charges on q.

+ -

Then repeat for every other possible point.

where Fe+ = Fe due to the ___charge

and Fe- = Fe due to the ___charge

R

The result will be…

q

Fe+

Fe-

pos.

opposite

+

-

R

Note: 1. Near each charge, E is like that of a ___________ charge.2. The direction of the force at any point is ______________to the field lines.

3. Lines are always: ____________ the “+

and ___________ the “-”4. E lines ______________ cross b/c the net force at any

point ____________________________________.

singletangent

out of

into

never

cannot have two directions

Ex. 8: The E look like around two ______________ charges will look _______________________ but with______________________________________the arrows pointing in.

Ex. 7: For two _________________ charges, E will look like:

R

positive

q

negativethe same

Ex. 8: The electric field around two charged ___________________________

Notes:1. Lines are ____________ “+" and _________________ “-”

2. Within the plates, E is _______________.

_______________ spaced lines ____________ field

3. Same idea as the constant ___________________ field

near Earth’s ________________.

4. In reality, the lines near the edges ___________________

positive

negative

oppositely charged plates

out of the into the

constantEqually constant

gravitational

surface

bend a little

Suppose ___________________ is placed between the plates and released from rest. The strength of the electric field is __________________.

What direction will the electrostatic force on it be?

After released, it will fall _____________ . Its accelerationwill ____________________ because the ___________________________________________ in the ______________ E field.

Compare the electrostatic force to the gravitationalforce.

Fg = w = mg =

Fe = qE =

(9.11 x 10-31 kg)

(1.6 x 10-19 C)

7.6 x102 N/C

an electron

upwardsremain constant F acting on it

remains constant constant

up

(9.8 m/s2)

(7.6 x102 N/C)

In region A, it follows a ____________________ because the net force acting on it is _________________ . Within the plates, it follows a ____________________ because the net force acting on it is _________________ In region B, it follows a ____________________ because the net force acting on it is _________________ .

Suppose an e- is fired ___________________ between the plates. Draw its trajectory. Ignore gravity.

horizontally

vi

straight line

zero

A B

initialdirection

straight line

zero

parabola

constant

Ex. 9: Two interesting facts:

1. Any charge placed on a conductor _______________________________________________.

2. The E field inside any conductor is ____________ . This is called ______________________________ .

remains on thesurface of the conductor

zeroelectrical shielding

Why are you safe within your car during a lightning storm?

Any charge resulting from a strike remains on the outside, and the E field within the car will be zero.

You are a __________________ . E field lines "end on"the surface of a conductor and___________________ .

Painting rooms with a _______________ paint prevents(electromagnetic) radio signals from entering.

cannot get inconductor

conducting

Tesla invented:modern generators andmotors (Niagara Falls)radioelectromagnetic inductiontesla coilsx-ray tubesarc lightingetc…and died penniless…