Study about the nature electricmagnetic waves

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:
 Kc=9x109 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
2
r
q
k
E c


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!!
F qE


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
 F=qE= 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 there’s a positive
charge to the left E field
+

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)
+ +
F

13. For the electron
 Since there’s 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)
+ -
F

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
+30 nC +60 nC

15. Sample Problem p. 656 #38
 For the 30 nC charge:
 Direction of the E-field for both charges is “away”
since they’re both positive
 For the 60 nC charge:
+30 nC +60 nC
 
  C
N
m
x
x
r
q
k
E c
000
,
12
15
.
0
)
10
30
(
10
9
2
9
9
2




 
  C
N
m
x
x
r
q
k
E c
000
,
24
15
.
0
)
10
60
(
10
9
2
9
9
2





16. Which one will win?
 At the midway point, the 30nC charge’s field
strength is 12000 N/C toward the 60 nC charge
and the 60 nC charge’s field strength is 24,000
N/C toward the 30 nC charge.
 The 60 nC charge will win. Since the field’s
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. Answer 
 F=qE=(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)
E field
+ +
F

19. Answer 
 B. What is the acceleration?
 Ask Newton!
 F=ma
 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 Earth’s 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 Earth’s 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)
qvB
Fmagnetic 

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
Direction
of Field
Symbol
Into the
page
X
Out of the
page
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, you’ll
have to use the rule and
change the sign!

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
 What’s the direction of F? Use the RHR!!
 v is east, B is north…F is….
 Out of the page!
 If it was an electron, the force would be into the
page!
s
m
x
T
C
x
N
x
qB
F
v
magnetic 6
19
12
10
0
.
8
)
5
.
2
)(
10
6
.
1
(
10
2
.
3


 


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 it’s an electron!!)
)
0
.
3
)(
10
6
)(
10
6
.
1
( 6
19
T
s
m
x
C
x
qvB
Fmagnetic


