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## Electric and Magnetic Fields

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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 ... – PowerPoint PPT presentation

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Title: Electric and Magnetic Fields

1
Electric and Magnetic Fields
• Chapters 17 21

2
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

3
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

4
Electric Field Lines
• The lines for a positive charge point away from
the charge
• The lines for a negative charge point towards the
charge

5
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

6
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

7
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

8
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

9
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!!

10
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?

11
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
• The electric field is pointing along the positive
x axis (to the right) which means theres a
positive charge to the left

12
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)

13
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)

14
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

15
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

16
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

17
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

18
• 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)

19
• B. What is the acceleration?
• 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

20
Magnetism!
21
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

22
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)

23
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

24
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

25
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

26
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)

27
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

28
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
29
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
30
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?

31
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!

32
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.

33
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!!)