Physics 30 – Unit 2 Forces and Fields – Part 2
To accompany Pearson Physics
Electric Fields
• QuickLab 11.1 Shielding Cellular Phones• Watch electric cable inspector video• Ancient Greeks: “violent” and “natural” forces
• Effluvium Theory
• Field Theory developed to explain forces at a distance: gravity, electrostatic, and magnetic forces
• Quantum Theory necessary to understand “how” these forces work
Electric Fields
• Because electric force direction will vary depending on the type of charge on the object, it is necessary to define electric field direction
• It is defined as the direction of force on a positive test charge
• As the diagram shows, field direction is away from the (+) charge and towards the (-) charge
Electric Fields
• The symbol for electric field is
• The symbol for electric field strength is
• Note the difference between this and the symbol for energy which is a scalar! If you get them confused, the consequences can be disastrous
EE
Electric Fields
where q is the charge of the charge in the electric field (the test charge) not the field source
• Review Example 11.1, page 548 and try Practice Problem 1, page 548
eF
Eq
Electric Fields
• Practice Problem 1, page 548
• This question asked for the magnitude of the field therefore the absolute value signs were used
319
3 19 16
1.00 10 /1.60 10
1.00 10 / 1.60 10 1.60 10
e
e
e
Eq
F
F
CF
N CC
N C N
Electric Fields
• If q1 represents the field source and q2 the test charge, the electric field due to q1 is
• Review Example 11.2, page 549• Try Practice Problem 1, page 549
1
1
22
2 22 2
commonly written as e
kF k krE
q
q qE
rq
r
Electric Fields
• Practice Problem 1, page 549
• Because the question states that the field is directed away from the charge, the charge must be (+)
2
29
2
2
2
122
92
8.99 1040.0 /
0.0200
40.0 / 0.02001.78 10
8.99 10
kE
rN mCN Cm
N
q
q
C m
C
q CN m
Electric Fields
• Review Example 11.3, page 550
• Try Practice Problem 1, page 550
Electric Fields
• Practice Problem 1, page 550
• What is the field at point X?
• Find the field due to A and the field due to B and add them together
• They are both in the same direction – to the left
.x 2.10 x 10-2 m
Electric Fields
2
9 62
722
8.99 10 1.50 103.06 10 / left
0.0210A
N mCkq CE N C
r m
29 6
26
22
8.99 10 2.00 106.17 10 / left
0.0210 0.0330B
N mCkq C N C
r m mE
7 6 73.06 10 / 6.17 10 / 3.67 10 / leftat X
N C N C N CE
Electric Fields
• Do Check and Reflect, page 553, questions1, 2, 4, 5 and
• SNAP p. 81, questions 2, 3, 5, 7-9, 13
Electric Field Lines
• Drawing electric field lines:Text gives rules and rationale on page 554
• Light particles sprinkled in oil will line up if an electric field is set up within the oil
• Read pages 555 – 559
• At the simplest level the field lines show the path of movement of a (+) charge if placed along a field line
Electric Fields
• Diagrams of electric fields can be drawn using the principles given
• Example:
• Try the following: a single positive charge, 2 negative charges, a (+)ly charge hollow sphere, a (+)ly charged plate with a (-)ly charged plate
• Discuss
• Diagrams of electric fields can be drawn using the principles given
• Example:
• Try the following: a single positive charge, 2 negative charges, a (+)ly charge hollow sphere, a (+)ly charged plate with a (-)ly charged plate
• Discuss
-+
Electric Potential
• Electrical potential energy is similar to gravitational potential energy
• Gravitational potential energy is easier to understand
• We’ll use a comparison between the two to help you understand electrical
Electric PotentialGravitational Electrical
work done in lifting an object is stored as gravitational potential
energy
work done in moving a charge with respect to another charge is stored
as electrical potential energy
Reference point → surface of earth Reference point → charges in contact
∆Ep between surface of earth and final point
∆Ep between 2 charges in contact and in the final position
∆Ep for macroscopic objects easy to measure and sensible
∆Ep for sub-microscopic objects easy to measure but not sensible for individual
particles
No analogous concept for gravitational Electric potential (voltage) defined as
1 Volt = 1 J/C
pW E F d pW E F d
pE
Vq
Electric Potential
• Electric Potential Difference (commonly called voltage) is the difference in electric potential between any two points
• For example the potential difference between the (-) and (+) electrodes of an alkaline dry cell is1.5 V
final initialV V V
Electric Potential
can be rewritten as
• Basis for a non-SI unit used for energy of subatomic particles
• If one electron was accelerated across a potential difference of 1 V it would have:
• 1 eV = 1.60 x 10-19 J (page 2 of Data Sheets)
pE
Vq
pE qV
1 1 1 energypE qV e V eV
e-
1 V
Electric Potential
• Review Example 11.8, page 566
• Do Practice Problem 2, page 566
Electric Potential
• Practice Problem 2, page 566
4 41 4.00 10 4.00 10pE qV e V eV
4 19 154.00 10 1.60 10 / 6.40 10eV J eV J
Electric Field between Parallel Plates
• is not valid for electric field between parallel plates
• Recall that
for both electric and gravitational fields
• Also recall that
• Put the 2 together and you get a new formula for electric field between parallel plates:
pW E F d
eF q E
2
kqE
r
Electric Field between Parallel Plates
p
p
E q E d
EE d
q
V E d
VE
d
Field between parallel plates is constant everywhere between the plates
Electric Field between Parallel Plates
• Electric field between parallel plates has units V/m
• Earlier you used N/C for electric field units
• Your book shows on page 568, that these are really the same units
• You should be capable of showing this
Electric Field between Parallel Plates
• Review Example 11.9, page 568
• Do Practice Problem 2, page 568
Electric Field between Parallel Plates
• Practice Problem 2, page 568
63
36 4
3.00 10 /5.00 10
3.00 10 / 5.00 10 1.50 10
Ed
V
V mm
V m
V
V m V
Electric Field between Parallel Plates
• Do Check and Reflect, p. 569
• Questions 10a, 11, 12
Conservation of Energy and Electric Charges
• Review Example 11.10, page 571
• Do Practice Problem 2, page 571
Conservation of Energy and Electric Charges
23 412
6
0 0 1.70 10 5.20 10 /
2.30 10
ki ppi
p
ff
i
pi
kEE
E
E
E E
m s
J
smaller charge initially at rest
q = -2.00 μC
m = 1.70 x 10-3 kg
larger charge
• Practice Problem 2, page 571
Conservation of Energy and Electric Charges
• Concept Check, page 572
initial motion perpendicular to plates
initial motion parallel to plates
or
Conservation of Energy and Electric Charges
• Review Example 11.11, page 572
• Do Practice Problem 1, page 573
Conservation of Energy and Electric Charges
• Practice Problem 1, page 573
212
19 4 27 212
19 42 12 2 2
27
212 6
2
0 0
3.20 10 4.00 10 0 0 6.65 10
3.20 10 4.00 10 23.85 10 /
6.65 10
3.85 10 1.96 10 /
kfpi ki p f E
v
v
v
v
E E E
qV m
C V kg
C Vm s
kg
mm s
s
Conservation of Energy and Electric Charges
• Review Example 11.12, page 574
Note: at this acceleration, it would be travelling at half the speed of light in 1 s! Why is this not possible?
12 5 7
78 2
15
2.6 10 1.7 10 / 4.4 10
4.4 101.5 10 /
3.0 10
q E
C V m N
aN
m sm kg
F
F
F
212
28 2 612
3
0 1.5 10 / 6.0 10
2.6 10
iv t at
m
d
s s
d
d
m
Conservation of Energy and Electric Charges
• Do Check and Reflect, page 575
• Questions 1, 2, 3, 7• SNAP p. 90 1, 3, 4, 6, 7, 8, 10, 12, 14, 16
Conservation of Energy and Electric Charges
Top Related