Electric Fields

Electric Fields

• Electric fields are similar to gravitational fields.

• The only difference is that two objects with mass will always attract each other.

• Charges can either repel or attract when held some distance apart.

Here are the gravitational field lines around the Earth. Notice how they

point toward the center and are perpendicular at the surface of the

Earth.

Charges produce similar fields…

• A charged object creates an electric field - an alteration of the space in the region which surrounds it.

• Other charges in that field would feel the unusual alteration of the space.

-

+

Electric field between two opposite charges…

Electric field between two like charges…

5 Rules for Electric Fields:1. Electric field lines always go from positive to negative.

This is the path that a positive “test charge”

would follow.

5 Rules for Electric Fields:2. Electric field lines always enter and leave the charge perpendicularly.

5 Rules for Electric Fields:3. Electric field lines never cross.

This is just like contour lines.

5 Rules for Electric Fields:4. Where the electric field lines are closer the electric field is stronger.

Again remember when the contour lines were closer together the slope was

steeper.

5 Rules for Electric Fields:5. Make sure the electric field lines are in contact with charged objects.

+++++++

-------

Charged Parallel Plates

What would the electric field look like around these charges?

- -

What would the electric field look like around these charges?

+ +

-

Analyzing the path of a test charge:

A test charge is a positively charged object that is used to test the electric field around other charged objects.

-+

+

+

+

Electric Field Strength (E)

• This is also similar to the gravitational field strength.

• The gravitational field strength of the Earth is 10 m/s2.

g = w m

= Fg

m= GmM r2

m

g = GM r2

E = FE

q= kqQ r2

q

E = kQ r2

What is the magnitude of the electric field strength at a point in a field

where a positive test charge of 8.00 x 10-2 C experiences a 2 N force?

+

E = FE

qE = 2 N 8.0 x 10-2 C

E = 2 N 8.0 x 10-2 C

E = 25 N/C

Electric Potential Difference- VOLTAGE

Let’s first revisit gravitational potential energy…

PE = mgh

What is gravitational potential energy dependent on?

2h

h

m

2mWork (energy) is required to lift these rocks against the force of gravity.

Work is also required to move charges in an electric field. If the direction of an electric field is such that it opposes the motion of a charged particle, work must be done to move

the particle in that direction.

+

+++

+

+

+

++

W = Fd+The potential difference between

two points in an electric field is the work done per unit charge.

V = W q

1 J/C = 1 volt

For each situation below determine if work is done on the test charge to move it from point A

to point B.+ + + + + + +

- - - - - - -

+ + + + + + +

- - - - - - -

+

+

B

B

A

A

NO work is done. The + charge is moving with nature; work is not required when it moves

with the electric field.

YES work is done. The + charge is moving against nature; work is required when it moves against the electric field.

If there is no difference in an electric field than no work needs to be done and there is no

voltage.

50e 50e

metal spheres

Both have the same charge- no potential difference- no voltage

0100e

Here there is a potential difference- work is done to move a charge of 50e

010e

Here there less of a potential difference- work is done to move a charge of 5e

THINK OF VOLTAGE AS ELECTRIC PRESSURE.

If 8 Joules of work are required to move 2 Coulombs of charge through a 3-ohm

resistor, what is the potential difference across the resistor?

V = W q

V = 8J 2C

V = 4V

Units- Joules vs. eV

How much work is done to move an elementary charge (+/- e) against an electric field through a potential difference of 1 volt?

V = W q

qV = Wq q

W = Vq

W = (1V)(1.6 x 10-19 C)

= 1.6 x 10-19 J

1.6 x 10-19 J = 1 eV (electronvolt)

Electronvolts are like inches and Joules are like miles.It’s like saying 1.894 x 10-4 miles = 1 foot

How much energy in eV is needed to move one electron through a potential difference

of 1.0 x 102V?

W = energy

W = Vq

W = (1.0 x 102V)(-1.6 x 10-19C)

W = 1.6 x 10-19J

1 eV = 1.6 x 10-17 J

= 1 eV

How many electronvolts are in 320 x 10-19J of energy?

1 eV = 1.6 x 10-19 J

200 eV