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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
  • 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

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
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)

19
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

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!!)
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