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

Physics 30 – Unit 2 Forces and Fields

To accompany Pearson Physics

Electrical Interactions

• The van de Graff Generator• Quick Lab 10.1, page 511

Electrical Interactions

• Law of Electrical Charges:• There appeared to be 2 different

types of charge because sometimes charged objects attracted, sometimes they repelled

• Law of Electric Charges: Like charges repel; unlike charges attract

Electrical Interactions

• Differences between magnetism and electrostatic forces:• magnets don’t need to be rubbed to show magnetic effects • magnets only attract a few things –

electrified, many • magnets attract strongly at 2 poles, electrified objects towards central region

Electrical Interactions

• Study of electric charges at rest – electrostatics

• Today’s theory: protons +, electrons -, equal number of each in neutral objects

Electrical Interactions

• Electrical Conductivity

Insulators:Electrons tightly bound to nucleus

Semi-Conductors:Good conductors and good insulators depending upon conditions

Conductors:Electrons in outermost part of the atom; free to travel

Electrical Interactions

• Superconductors – no noticeable resistance to current flow

• First superconductors discovered at liquid helium temperature, -269°C

• 1986 – first superconductor at significantly higher temperatures

• Today highest temperature superconductor: -78°C (July 2008)

Electrical Interactions

• Charging Objects• All methods based on the Law of

Conservation of Electric Charge: electric charge can’t be created or destroyed – just rearranged

Electrical Interactions

• Charged objects can attract neutral ones a charged balloon will stick to a wall. This happens by electrostatic induction

Electrical Interactions3 methods of charging

Friction: objects are rubbed together and (-) charge is transferred between them so that 1 ends up with a shortage of electrons (+) and the other has excess electrons (-)

Conduction or Contact:

(-) charge flows from an object with excess electrons

amount of charge transferred depends on size and shape of the objects

charge distributes over the surface of a conductor and remains at point of contact on an insulator

Induction:

Process of charging an object 1st by polarizing it by induction and then retaining the charge by grounding

charge is opposite to that of the charging object

Electrical Interactions

• Lightning• Read page 522

Electrical Interactions

• Do Check and Reflect page 523,questions 2, 3, 4, 5, 6, 8

Electrical Interactions

• Coulomb’s Law – very similar to Newton’s gravitational force law studied in Physics 20

1 22g

Gm mF

r

1 2gF m m

2

1gF

r

Coulomb suspected that like gravity, the electrostatic force would be inversely proportional to the square of the distance between charges, and similar to gravity it would be directly proportional to the product of the charges

Electrical Interactions• In 1777 using his torsional balance

Coulomb did indeed discover that

• exactly symmetrical to the gravitational force law

• k is the Coulomb’s Law constant - 2

928.99 10

N mC

r is distance not radius

1 22e

kq qF

r

1 2eF q q

2

1eF

r

Electrical Interactions

• How did Coulomb determine relative sizes of charge for his experiment – there wasn’t even a unit to measure charge in???

Electrical Interactions

• 1 Coulomb (1 C) is a gigantic amount of charge

• As your text states, a lightning stroke might transfer 1 C of charge between the cloud and the Earth

• 1 C is the charge of 6.25 x 1018

electrons

Electrical Interactions

• Review example 10.1, page 530• Try Practice Problem 1, page 530

• Review example 10.2, page 531• Try Practice Problem 2, page 531

Electrical Interactions

• Practice Problem 1, page 530

Do NOT use negatives and positives in the formula; they will only confuse you

Use common sense: 2 negatives or 2 positives will repel; a negative and a positive will attract

29 19 192

1 222 11

8

8.99 10 1.60 10 1.60 10

5.29 10

8.22 10 (attraction)

e

N m C Ckq q CFr m

N

Electrical Interactions

• Practice Problem 1, page 531• Charge on each sphere:

• Repulsion, since each charge is negative

73.00 2.00

0.500 5.00 102

C CC C

2 29 72

1 222

8.99 10 5.00 100.156

0.120e

N m Ckqr

Fq C N

m

Electrical Interactions

• 1d interactions with more than 2 charges:

• Review examples 10.3 and 10.4, pages 532 and 533

• Try Practice Problem 1, page 532 and Practice Problem 1, page 533

Electrical Interactions

• Practice Problem 1, page 532

2 29 62

1 222

8.99 10 2.00 10360 left

0.010e AB

N m CqF

kq C Nr m

2 29 62

1 222

8.99 10 2.00 1090.0 right

0.020e CB

N m Ckq qN

r mF C

ABF

CBF

Electrical Interactions

• Practice Problem 1, page 533

-2.50 x 10-9 C

+1.50 x 10-9

C

-1.00 x 10-9 C

2.00 cm

1.50 cm

AB

C

29 9 92

41 222

8.99 10 2.50 10 1.50 101.50 10 left

0.0150e AB

N m C Ckq q C Nr m

F

29 9 92

51 222

8.99 10 1.00 10 1.50 103.37 10 right

0.0200e CB

N m C Ck qNF

q Cr m

4 5 41.50 10 3.37 10 1.16 10 lefte netNF N N

Electrical Interactions

• 2d interactions with more than 2 charges

• Review examples 10.5 and 10.6, pages 534 and 535

• Try Practice Problem 1, page 534 and Practice Problem 1, page 535

Electrical Interactions

• Practice Problem 1, page 534

29

2141 2

22

8.99 10 2.50 3.004.68 10

0.0120lefte XY

N m C Ckq q C Nr m

F

X Y

Z

-2.50 C+3.00 C

+4.00 C

1.20 cm

1.2

0

cm

29

2141 2

22

8.99 10 4.00 3.007.49 10

0.0120upe ZY

N m C Ckq q C Nr m

F

Note: these are gigantic charges and could never exist in nature spaced this close together

I think the book meant μC, not C

Electrical Interactions

• Since force is up and to the left it is in the 2nd quadrant – in standard position, 180°- 58.0°= 122°

141 1

14

7.49 10tan tan 58.0

4.68 10y

x

R NR N

2 214 14 144.68 10 7.49 10 8.83 10on yF N N N

Electrical Interactions

• Do Check and Reflect, page 538• Questions 1, 5, 6, 7, 8