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# How does a ____________ (or a collection of ____________) - PowerPoint PPT Presentation

Electric. III. ____________ Fields. charge. charges. How does a ____________ (or a collection of ____________) exert a force on another _____________ from a distance ?. charge. charged object B. F e. charged object A. fields, E.

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III. ____________ Fields

charge

charges

How does a ____________ (or a collection of ____________)

exert a force on another _____________ from a distance?

charge

charged

object B

Fe

charged

object A

fields, E

Answer: By means of their electric ________________ .

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

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 ________________.

test charge

acts on q

test

Fe/q,

E =

where q is the _______ charge

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

direction

vector

electrical

force

positive test charge

units of E: E = Fe / q

[ E ] = [ ]/[ ]

= /

Fe

q

C

N

an electrical force of 10.0 x 103 N when placed at the

position shown.

q

Fe

Determine the strength and direction of the

electric field at the point shown.

strength: E =

=

=

Fe / q

10.0 x 103 N / 5.0 x 10-3 C

2.0 x 106 N/C

Fe

same

(__________ direction as ____)

direction:

when placed in the electric field at point P shown below?

3.7 x 106 N/C

E = ____________

P

Fe / q

magnitude: =

=

=

E

Fe / 1.6 x 10-19C

3.7x106 N/C

5.9 x 10-13 N

Fe

E

same

direction: ______________ direction as _____

Ex 3: What electrical force will an electron experience

when placed in the same point as the previous example?

same

_________________

magnitude:

opposite

__________________

direction:

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

pos. test q

qp

+

Fe

1 force

line  weaker

2 force

lines stronger

• Note:

• The arrows represent “_________________” that act

• on a ______________ test charge q.

• 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 were

• true, the field would have ______________________________

• 6. E is _________________________. It is NOT constant.

lines of force

positive

+

strength

repulsion

outward

never cross

2 directions at one place

inverse square in r

Let q1 = the __________ charge qp,

and q2 = the __________ charge q,

Then, _________________ Law: Fe = kq1q2/r2

becomes Fe =

and E = Fe/q

becomes E =

point

test

Coulomb's

kqpq/r2

kqpq/r2

= kqp/r2

q

E

E

For a

point

charge:

qp

r

E field

around a

____________

point charge.

pos.

test q

Fe

negative

-

• Note:

• The arrows represent

• “__________________________ .”

• 2. E is directed radially __________ towards the ___ .

• 3. E is __________________________. It is NOT constant.

• The _________________ of the lines represents the field

• strength.

• The arrows show ________________because they point

• __________________.

• This E resembles Earth’s _____________________ field as it

• would be seen _____________________

lines of force

"-"

inward

inverse square in r

closeness

attraction

inward

gravitational

from far away

opposite

pos.

Start by placing a ____ test charge q at one point.

Find resultant of the forces from both charges on q.

Fe+

R

q

Fe-

-

+

+

where Fe+ = Fe due to the ___charge

and Fe- = Fe due to the ___charge

-

Then repeat for every other possible point.

The result will be…

• Note:

• 1. Near each charge, E is like that of a ___________ charge.

• The direction of the force at any point is ______________

• to the field lines.

• Lines are always: ____________ the “+

• and ___________ the “-”

• E lines ______________ cross b/c the net force at any

• point ____________________________________.

single

tangent

out of

into

never

cannot have two directions

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

R

q

negative

Ex. 8: The E look like around two ______________

charges will look _______________________ but with

______________________________________

the same

the arrows pointing in.

___________________________

oppositely charged plates

positive

negative

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 ___________________

out of the

into the

constant

Equally

constant

gravitational

surface

bend a little

Suppose ___________________ is placed between the

plates and released from rest. The strength of the

electric field is __________________.

7.6 x102 N/C

What direction will the

electrostatic force on it be?

up

Compare the electrostatic force to the gravitational

force.

(9.8 m/s2)

(9.11 x 10-31 kg)

Fg = w = mg =

(1.6 x 10-19 C)

Fe = qE =

(7.6 x102 N/C)

upwards

After released, it will fall _____________ . Its acceleration

will ____________________ because the _________________

__________________________ in the ______________ E field.

F acting on it

remain constant

constant

remains constant

Suppose an e- is fired ___________________ between

the plates. Draw its trajectory. Ignore gravity.

vi

initial

direction

A

B

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 _________________ .

straight line

zero

parabola

constant

straight line

zero

• Ex. 9: Two interesting facts:

• Any charge placed on a conductor ____________________

• ___________________________.

• 2. The E field inside any conductor is ____________ .

• This is called ______________________________ .

remains on the

surface of the conductor

zero

electrical shielding

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___________________ .

cannot get in

conducting

Painting rooms with a _______________ paint prevents

modern generators and

motors (Niagara Falls)