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Electric and Magnetic Fields

- Chapters 17 21

Electric Field

- Electric force, like gravitational force, is a

field force - Remember Field forces can act through space even

when there is no physical contact between the

objects involved - A charged object has an electric field in the

space around it

Electric Field Lines

- Electric Field Lines point in the direction of

the electric field - The number and spacing of field lines is

proportional to the electric field strength - The electric field is strong where the field

lines are close together and weaker when they are

far apart

Electric Field Lines

- The lines for a positive charge point away from

the charge - The lines for a negative charge point towards the

charge

Electric Field Lines

- This diagram shows the electric field lines for

two equal and opposite point charges - Notice that the lines begin on the positive

charge and end on the negative charge

Electric Field Lines

- This diagram shows the electric field lines for

two positive point charges - Notice that the same number of lines emerges from

each charge because they are equal in magnitude

Electric Field Lines

- If the charges are unequal, then the number of

lines emerging from them will be different - Notice that the positive charge has twice as many

lines

Calculating Electric Field Strength

- The equation for the electric field produced by a

point charge is - Kc9x109 Nm2/C2 ,r is the distance from the

charge and q is the charge producing the field - The unit for E is N/C
- Electric field strength is a vector!!
- If q is positive, then E is directed away from q
- If q is negative, then E is directed toward q

Calculating the force from an electric field

- If a charged object is placed in an electric

field, we can calculate the force acting on it

from the electric field - Remember that F is a vector!!

Sample Problem p. 647 3

- An electric field of 2.0 x 104 N/C is directed

along the positive x-axis - a. What is the electric force on an electron in

this field? - b. What is the electric force on a proton in

this field?

Sample Problem p. 647 3

- E 2.0 x 104 N/C , q 1.6 x 10-19 C
- FqE 3.2 x 10-15 N for both the electron and the

proton - What about the direction?
- The electric field is pointing along the positive

x axis (to the right) which means theres a

positive charge to the left

For the proton

- Since the electric field is pointing to the

right, if you put a proton in it, the proton will

want to move away towards the right and the

direction of the force on it will be to the right - Answer 3.2 x 10-15 N along the positive x axis

(to the right)

For the electron

- Since theres a positive charge causing the

electric field to point towards the right, an

electron would feel attracted to the positive

charge. Therefore, the force acting on it is

toward the left - Answer 3.2 x 10-15 N along the negative x axis

(to the left)

Sample Problem p. 656 38

- Find the electric field at a point midway between

two charges of 30 nC and 60 nC separated by a

distance of 30.0 cm

Sample Problem p. 656 38

- For the 30 nC charge
- Direction of the E-field for both charges is

away since theyre both positive

- For the 60 nC charge

Which one will win?

- At the midway point, the 30nC charges field

strength is 12000 N/C toward the 60 nC charge and

the 60 nC charges field strength is 24,000 N/C

toward the 30 nC charge. - The 60 nC charge will win. Since the fields

point in opposite directions, you have to

subtract - Answer 12,000 N/C toward the 30 nC charge

Sample Problem (p.659 66)

- A constant electric field directed along the

positive x-axis has a strength of 2.0 x 103 N/C. - Find the electric force exerted on a proton by

the field - Find the acceleration of the proton

Answer ?

- FqE(1.6x10-19 C)(2.0 x 103 N/C)
- 3.2 x 10-16 N
- Direction?
- Answer 3.2 x 10-16 N along the positive x-axis

(to the right)

Answer ?

- B. What is the acceleration?
- Ask Newton!
- Fma
- a F/m 3.2 x 10-16 N/1.6x10-27 kg
- a 2 x 1011 m/s2 along the positive x axis

Magnetism!

Magnets

- The ends of a bar magnet are called poles
- Like poles repel and unlike poles attract
- Regardless of their shape, all magnets have a

north and south pole

Magnetic Fields

- Magnetic Field lines point from the north pole to

the south pole of the magnet - The north pole of a compass needle always points

in the direction of the field (from North to

South)

Magnetic Field of the Earth

- The Earths geographic North pole is actually the

magnetic south pole - The north pole of a compass points towards

geographic north and since opposites attract, we

know that the Earths geographic pole is magnetic

south

Magnetic Field of a wire

- Moving charges produce magnetic fields
- If there is a current moving through a wire, a

magnetic field is produced around the wire

Magnetic Field of a wire

- The Right Hand Rule for the magnetic field
- Point your thumb in the direction of the current

and curl your fingers in the direction of the

field

Magnetic Force

- A charge moving through a magnetic field

experiences a force - q magnitude of charge
- v speed of charge
- B Strength of the magnetic field (measured in

Tesla, T)

A second Right-Hand Rule

- Of course, force is a vector!
- To find the direction of the magnetic force use

another right hand rule - Fingers point in direction of the field
- Thumb points in direction of v
- Palm points in direction of magnetic force

Conventions for direction of field

WARNING The right hand rule is for the direction

of the force acting on a POSITIVE CHARGE. To

find the direction of the force acting on a

negative charge, youll have to use the rule and

change the sign!

Direction of Field Symbol

Into the page X

Out of the page

Examples

Direction of F Direction of v Direction of B Sign of Charge

Out of the page East North

Into the page East North -

Out of the page West South

Into the page West South -

South West Into the page

South West Out of the page -

East North Out of the page

South Out of the page East -

Out of the page South West -

Into the page west North

Sample Problem p. 775 2 (edited)

- A proton traveling to the right along the x-axis

enters a region where there is a magnetic field

of 2.5 T directed north. If the proton

experiences a force of 3.2 x 10-12 N, find the

speed of the proton. What is the direction of the

force exerted on the proton?

The speed of the proton

- Whats the direction of F? Use the RHR!!
- v is east, B is northF is.
- Out of the page!
- If it was an electron, the force would be into

the page!

Sample Problem (not in book)

- An electron is moving with a velocity of 6 x 106

m/s westward in a 3.0 T magnetic field that is

pointed out of the page. - Find the magnitude and direction of the force

acting on the electron.

Sample Problem (not in book)

- F 2.88 x 10-12 N
- Direction? Use the RHR
- V points west, B points out of the page
- F points SOUTH (remember its an electron!!)