Physics 30 – Unit 2 Forces and Fields – Part 2 To accompany Pearson Physics

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

qq

kF k krE

q

q qE

rq

r

Electric Fields


• 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


• 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


• Do Practice Problem 2, page 566

Electric Potential


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


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 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






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


• Do Check and Reflect, p. 569

• Questions 10a, 11, 12

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



• Concept Check, page 572

initial motion perpendicular to plates

initial motion parallel to plates

or






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



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


• Do Check and Reflect, page 575

• Questions 1, 2, 3, 7• SNAP p. 90 1, 3, 4, 6, 7, 8, 10, 12, 14, 16


Documents

Physics 30 – Unit 2 Forces and Fields – Part 2 To accompany Pearson Physics