ch20.1 – static electricity like charges repel, opposites charges attract protons can’t be...

Ch20.1 – Static Electricity

• Like charges repel, opposites charges attract

• Protons can’t be touched

(except in nuclear rxns) only electrons can be transferred between substances.

• Electricity is the flow of electrons.• Static electricity is a short flow of

electricity. Exs: a shock, the Van de graaf, a lighting bolt, static, etc.

• Neutral objects have an equal # of protons and electrons.

• (-) charge = excess electrons.• (+) charge = lacking electrons • Some objects gain electrons

easier than others

Protons

Triboelectric Sequence

Rubber

Plastic

Copper

Amber

Wood

Cotton

Aluminum

Human skin

Silk

Lead

Cat fur

Quartz

Wool

Glass

Rabbit fur

Asbestos

Ea

sie

r to

lose

ele

ctro

ns

Ea

sie

r to

gai

n e

lect

rons

When 2 materials are rubbed together, one gains electrons, becomes (–) charged.Other loses electrons, becomes (+) charged.Charge is always conserved.

Ex: PVC plastic and rabbit fur:

+ =

+-+-+-+-

+ - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + -

+

+

+-+

+ - + - - + - + - + - + -

- + - + - + - + -

- + - + - + - + - + - + -

Insulators and Conductors

Insulators - have the ability to hold charge and confine it to a region for a longer period of time.

Conductors - allow charge to flow freely until charge is equally spread out.

Copper wire:

All metals are good conductors.

• Water is a good insulator (doesn’t allow charge to flow very well.)

- add salt (puts ions in there), becomes an excellent conductor.

• Air is a better insulator. (down jackets)

-must place enough charge near by, ions in air will align,

charge will flow.Charge a conductor, charge seek making separation

221

r

qqkFE

Ch20 HW#1 1 – 5

Ch20 HW#1 1 – 5

1) a.) static electricity is a ______ of electricity.

b.) When atoms become charge they gain/lose _____.

c.) Two materials touch, electrons transfer. Overall, charge is____.

d.) Materials that allow charge to flow freely are_____.

e.) Materials that hold charge to region are____.

f.) Distilled water is a good _____.

g.) Adding salt to water makes it conduct because salt adds _____.

h.) Air is a good ______.

2. PVC rubbed with wool. What charge does each get?

3. Glass & humans skin?

4. 2 charges of + 6.0C are separated by 0.50m. Find force.

Ch20 HW#1 1 – 5












221

r

qqkFE

Ch20 HW#1 1 – 5












(repels)

Nxm

CCC

mNx

r

qqkFE

122

2

29

221 103.1

)5.0(

)0.6()0.6()109(

5. A + 2C & + 3C charge are separated by 4.5 m. Force?

5. A + 2C & + 3C charge are separated by 4.5 m. Force?

(repel)

Nxm

CCC

mNx

r

qqkFE

92

2

29

221 107.2

)5.4(

)0.3()0.2()109(

Electroscopes – Charge Detectors

-- - - - - -- - - - - - -


-- - - - - -- - - - - - -- -- -

- -- -

+ + + ++ + + +

+ ++ +

+ ++ +



Don’t indicate if the charge is + or –. Leaves always spread when either charge is present.

+ + + ++ + + +

+ ++ +

+ ++ +


Induction – a neutral object feels the presence of a charge - caused by the electric field. (Ch 16)

As the charged object is brought near the neutral object, it induces a charge in the neutral object.

-- - - - - -- - - - - - -+

+ ++ +- -

- -

Ex1) Two charges are separated by 3.0 cm. Object A has a charge of

+6.0μC, while object B has a charge of +3.0μC. What is the force on object A?

Ex2) A sphere with a charge of +6.0μC is located near 2 other charge

spheres. A -3.0μC sphere is located 4.00 cm to the right and

a +1.5μC sphere is located 3.00 cm directly underneath.

Determine the net force on the 6.0μC sphere.

Ch20 HW#2 6 – 10

Lab20.1 Statics

- due tomorrow

- Ch20 HW#2 due at beginning of period

Ch20 HW#2 6 – 10

6. A -2x10-4C charge and a +8x10-4C charge separated by 0.30m. Find force.

7. A -3mC charge and a -6.2mC charge are placed 1.5m apart.

Find force.

8. A +10μC charge & -10μC charge 1m apart. Force?

Ch20 HW#2 6 – 10

6. A -2x10-4C charge and a +8x10-4C charge separated by 0.30m. Find force.

(attract)

7. A -3mC charge and a -6.2mC charge are placed 1.5m apart.

Find force.

(repel)

8. A +10μC charge & -10μC charge 1m apart. Force?

(attract)

Nxm

CxCxC

mNx

r

qqkFE

42

442

29

221 106.1

)3.0(

)108()102()109(

Nm

CxCxC

mNx

r

qqkFE 400,74

)5.1(

)102.6()103()109(

2

332

29

221

Nm

CxCxC

mNx

r

qqkFE 9.0

)1(

)1010()1010()109(

2

662

29

221

9. A -6X10-6C charge exerts and attractive force of 65N of a 2nd

charge .05m away. Find 2nd charge.

10) A +1C charge & +2C charge at 1.0m,2.0m,3.0m,4.0m,5.0m.

Find force.

9. A -6X10-6C charge exerts and attractive force of 65N of a 2nd

charge .05m away. Find 2nd charge.

10) A +1C charge & +2C charge at 1.0m,2.0m,3.0m,4.0m,5.0m.

Find force.

Cxqm

qxxN 6

222

69

103 )05.0(

)()106()109(65

Ch21.1 – Electric Fields

E-fields - test with a small (+) test charge (+qo)1. 2. 3.

4. 5.

6.

– – – – – – – – – – – – – – – –

+ +

+ -+

– –

+ + + + + + + + + + + + + + +

Units:

Ex1) A +1μC test charge experiences a force of .01N to the right,

as show. What is the magnitude of the E-field?

Label the charge as (+) or (-) FE

Ex2) A +2nC test charge experiences a force of 2.5mN to the left.

What is the mag of the E-field. Label the E-field creater as (+) or (-).

FE

oq

FE

2r

qQkF oE

2r

QkE

+

+

Ex3) What is the strength of an E-field created by a +3.0 mC charge, at

a distance of 85cm?

Ch21 HW#1 1 – 7

Ch21 HW#1 1 – 7

1. A +2.0x10-8C test charge experiences a force to the right of 0.060N by an E-field. Find it, label charge.

FE

2. A +4μC test charge experiences a force of 0.10 N to the left in an E-field. Find it and label it.

FE

3. A 5.5μC test charge experiences a force of 2mN to the left in an E-field. Find it and label it.

+

+

Ch21 HW#1 1 – 7

1. A +2.0x10-8C test charge experiences a force to the right of 0.060N by an E-field. Find it, label charge.

FE

2. A +4μC test charge experiences a force of 0.10 N to the left in an E-field. Find it and label it.

FE

3. A 5.5μC test charge experiences a force of 2mN to the left in an E-field. Find it and label it.

FE

oq

FE

+

+

–

+

+

– oq

FE

oq

FE

4. A positive test charge of 5x10-4C is in an E-field that exerts a force of

2.5x10-4N on it. Find the E-field at that location.

5. You probe the field of an unknown charge. 1st with a 1μC test charge then with a 2μC.

a.)Do they experience the same force?

b.)Same field?

c.) If E-field is 1x10-5N/C, what is the force on each?

4. A positive test charge of 5x10-4C is in an E-field that exerts a force of

2.5x10-4N on it. Find the E-field at that location.

5. You probe the field of an unknown charge. 1st with a 1μC test charge then with a 2μC.

a.)Do they experience the same force?

b.)Same field?

c.) If E-field is 1x10-5N/C, what is the force on each?

a. b.

c. 2

22

21

1

r

qQkF

r

qQkF

E

E

2r

QkE

NCxEqF

NCxEqF

CN

CN

2.0)2)(101(

1.0)1)(101(5

2

51

oq

FE

6. What is E-field at a distance of .25m from +1μC charge?

7. An E-field of 6X10-7 N/C at a distance of 2.5m.

What charge creates it?




CN

m

CxC

mNx

r

QkE 000,144

)25.0(

)101()109(

2

62

29

2




CN

m

CxC

mNx

r

QkE 000,144

)25.0(

)101()109(

2

62

29

2

CxQm

QC

mNx

x CN 16

2

2

29

7 105.1 )5.1(

)109(106

Ch21.2 – Electric Potential

Where is PEG greatest?

Greatest PEE

Earth

+

+

+

+

+

Ch21.2 – Electric Potential

Where is PEG greatest? High PEE

Low PEE

Greatest PEE

Low PEE High PEE

High PEE Low PEE

Potential Difference:

Units:

Earth

+

+

+

+

+

oq

WV

Equipotentials - areas where the potential doesn’t change.

ΔV = E.dEx1) 2 parallel plates are charged up. A voltmeter measures the potential

diff to be 60.0V. The plates are 3.0cm apart. What is the magnitude of the E-field between them?

+ + + + + + + + + + + + + + + +

– – – – – – – – – – – – – – – –

+ + + + + + + + + + + + + + +

Ex2) 2 large, charged parallel plates are 4.0 cm apart. If the E-field

measures between the is 625N/C:

a) What is the electric potential?

b) How much work is done moving a proton (+1.6X10-19C)

from the (-) plate to the (+) plate?

Ch21 HW#2 6 – 9

Ch21 HW#2 6 – 9

6. What is the change in potential when an external force does 10J

of work to move a +1μC charge?

7. How much work is done by the E-field to move a +10mC charge

to a location where the potential is 10V lower?

8. A charged PVC pipe creates an E-field of roughly 1x103N/C at

a distance of 0.05m from it. What is the potential?

Ch21 HW#2 6 – 9

6. What is the change in potential when an external force does 10J

of work to move a +1μC charge?

7. How much work is done by the E-field to move a +10mC charge

to a location where the potential is 10V lower?



Cx

J

q

WV

o6101

10

)1010)(10( 7CxqVW CJ



9. 2 charged parallel plates have a potential diff of 12.0V when separated by 1.0cm. E-field?

)05.0)(101( 3 mxdEV CN



9. 2 charged parallel plates have a potential diff of 12.0V when separated by 1.0cm. E-field?

)05.0)(101( 3 mxdEV CN

m

V

d

VE

01.0

0.12

Ch21.3 – Capacitance

- the ability to store charge

All objects can store some amount of charge.

Q = V . CUnits:

charge potential capacitance

stored diff

Most capacitors are made of 2 metal sheets, separated by an insulator,

and rolled up into a cylinder.

Ex1) In your next lab, you will use a 22,000μF cap that is connected

to a 6V battery. How much charge should it store?

Ex2) A sphere has an electric potential difference between it and the earth of 60V, when it is charged to 3μC. What is its capacitance?

Circuit symbols: Battery Capacitor Resistor Switch Light Emitting Diode (LED)

or

Make these circuits: R R R

6V 6V 40,000μF 40,000μF

Ch21 HW#3 10 – 14

Lab21.1 – Capacitance

- due tomorrow

- Ch21 HW#3 due at beginning of period

Ch21 HW#3 10 – 14 10. A 27μF capacitor has an electric potential difference of 25V across it.What is its charge?

11. A 6.8μF capacitor is connected to a 15V battery. What is its charge?

12. A 3.3μF cap is charged to 2.5x10-7C. What is its potential?

Ch21 HW#3 10 – 14 10. A 27μF capacitor has an electric potential difference of 25V across it.What is its charge?

Q = V.C = (25J/C)(27x10-6C) =

11. A 6.8μF capacitor is connected to a 15V battery. What is its charge?

Q = V.C = (15V)(6.8x10-6C) =

12. A 3.3μF cap is charged to 2.5x10-7C. What is its potential?

Fx

Cx

C

QV

6

7

103.3

105.2

13. A 1.5C test charge is moved by an external force that does 25J of work to move the test charge to a higher potential. What is the change in potential?

14. A +2.5C test charge is moved by an E-field that does 10J of work to move the test charge to a lower potential. What is the change in potential?

13. A 1.5C test charge is moved by an external force that does 25J of work to move the test charge to a higher potential. What is the change in potential?

14. A +2.5C test charge is moved by an E-field that does 10J of work to move the test charge to a lower potential. What is the change in potential?

C

J

q

WV

5.1

25

C

J

q

WV

5.2

10

Ch20,21 Review1. 2 charges qA and qB are separated by a distance d and exert a force F

on each other. How does the force change if:a. qA and qB are doubled?

b. d is cut in half?

c. qA is doubled and d is doubled?

2. Find net force:

2. Find net force:

A = 2C B = 4C----- 3m -------

| 2m |

C = 3C

+ +

+

3. A positive test charge of 1.0x10-5C experiences a force of 0.20Nwhen located at a certain point. Find the E-field.

4. Sketch the fields:

a) b)

c)

+1μC -2μC

+ –

5. If 120J of work are performed to move a +5.5C chrage from a (-) plate to a (+) plate. Find the ΔV. + or – work?

6. A voltmeter indicates that the electric potential difference between 2 plates is 50.0V. Plates are 0.020m apart. Find E-field.

7. What is the charge stored in a 15.0pF capacitor when it is connected to a 75V source?

– – – – – – – – – – – – – – – –

+ + + + + + + + + + + + + + +

– – – – – – – – – – – – – – – –

+ + + + + + + + + + + + + + +

8. 2 pith balls with masses of 1.0g and charges of 2.4x10-8C, as shown.Find force.

----3cm--- + +