Electrostatics(electricity + at rest)Study of charges at rest

•Electrostatic Force• Opposites attract• Like repel•All materials are neutral (p+=e-)•Charging- process of removing or adding electrons• Conduction- direct contact• Induction- indirect contact

•Grounding- Connection with free flow of electrons to or from the Earth

Electroscope

• Instrument used to detect electrical charge.

• Operates through charging by induction.

• If the foil leaves move apart, a charge is present (positive or negative).

Charging by…•charging materials through direct contact- conduction

•charging materials through indirect contact- induction

Charging with two spheres

Question # 1

Which method of charging is caused by friction?

A. Convection C. RadiationB. Conduction D. Induction

Johntravoltage

Question # 2

A neutral metal sphere is touched by a negatively charged metal rod. As a result, the sphere will be ____ and the metal rod will be neutral.

a. positively chargedb. negatively chargedc. neutrald. much more massivee. ... not enough information to tell

Question# 3

A positively charged balloon is brought near a neutral conducting sphere. While the balloon is near, the sphere is touched (grounded). At this point, there is a movement of electrons. Electrons move ____ .a. into the sphere from the ground (hand)b. out of the sphere into the ground (hand)c. into the sphere from the balloond. out of the sphere into the balloon

Question # 4Which of these choices MOST ACCURATELY describes the process of

charging by induction?

A. A charged rod is brought close to, but does not touch, an uncharged sphere and then is removed. The sphere is now charged.

B. A charged rod is brought close to, but does not touch, an uncharged sphere, while a third object touches and is then removed from the sphere, leaving behind an unbalanced charge.

C. A charged rod is brought into contact with an uncharged sphere and then removed. Both objects share the original charge between them.

D. A charged rod is brought into contact with an uncharged sphere and then removed. The rod is now neutral, and the sphere now carries the charge of the original rod.

Insulators• Insulators – Materials do

not distribute charge across surface.

• Don’t allow electrons to move from molecule to molecule.

Conductors• Conductors – materials that

allow electrons to flow freely from atom to atom and molecule to molecule

• Charge is distributed across entire surface of object.

Question # 51. One of these isolated charged spheres is copper and the other is rubber. The diagram below depicts the distribution of excess negative charge over the surface of two spheres. Which sphere is copper?

RUBBER COPPER

Question # 6A Van de Graff generator becomes positively charged.

Which best describes the charge distribution on the sphere?

A. charge is evenly distributed on the surface of the sphere

B. charge is evenly distributed throughout the sphereC. charge is concentrated at the highest point of the

sphereD. charge is concentrated close to the belt where

charge is generated

Electrical Fields• Electrical fields around charges• Opposites attract/ like repels= creates a force

• Test charge (q+)- – used to determine an electric field – small and positive

# 7 Electric Field LinesElectric field lines are lines of force.

Q+

q+

q+q+

q+

test charge (always positive w/ small magnitude)

Radial field- field lines are not uniform (not pointing in same direction)

Field lines point away from positive charges!

# 8 Electric Field Lines

Q-

q+

q+q+

q+

test charge (always positive w/ small magnitude)

Radial field- field lines are not uniform (not pointing in same direction)

Field lines point toward negative charges!

• What charges do A and B have?

A B

# 9 Electric Field Equation

Radial field- field lines are not uniform (not pointing in same direction)

Uniform field- field lines are orderly (pointing in same direction)

E= Magnitude of the electrical field (N/C)F= force (N)q= test charge (C)

#10 Identify the charge

A. B. C.

D. E.

Radial or Uniform Field?

Uniform

Radial

Capacitors – device that stores charges (uniform field)

Positive or Negative Charge?

Question # 11

Which test charge experiences the greatest force within the electric field? The least?

A

B

C

D

THE CLOSER THE LINES, THE GREATER THE FORCE!

Question # 12

A glass sphere, a copper sphere, a wood sphere and a rubber sphere are placed on individual insulating stands. Which sphere will distribute electrical charge over its surface most quickly?

A glass B copper C wood D rubber

Question # 13

2. A conductor differs from an insulator in that a conductor ________.a. has an excess of protonsb. has an excess of electronsc. distributes charges evenly throughout the surfaced. has faster moving moleculese. does not have any neutrons to get in the way of electron flowf. none of these

F= electric forceq= chargek= Coulomb constant

•F is directly proportional to charge (F α q)•F is inversely proportional to r2 (F α 1/r2)

Coulomb’s LawThe electric force between 2 objects is symbolic of the

gravitational force between 2 objects.

Law of Universal Gravitation

•F is directly proportional to mass (F α m)•F is inversely proportional to r2 (F α 1/r2)

RECALL:

Coulomb’s Law

2

2910998 constant CoulombC

Nmx.k

Coulomb’s Law

Situation q1= q2= r= Fe= (keep kin the equation)

1. Control 1 1 1

2. Double the charge 2 1 1

3. Half the charge 1 1/2 1

4. Double the distance 1 1 2

5. Half the distance 1 1 1/2

What happens to the Force when-A. Both charges are doubled C. distance is tripledB. One charge is 1/3rd as big D. distance is decreased by

1/3rd

Question # 1Which has a greater charge?

____ 2 m ____

OR

__1m_

+ ++ +

+ ++ +

+ ++ +

+ ++ +

Question # 2

Which has a greater charge?

- - ___2 m___ - -

OR

- ___2 m___ -

Coulomb’s Law

A sock with a charge (q1) of 7.2x10-7C is held a distance of 0.02m from a second sock having a charge (q2) of 4.8x10-7C. Use the following equation to calculate the magnitude of the repulsive force. (K= 9.0 x 109 N•m2/C2)

G U E SS

Coulomb’s Law Calculations1. Suppose that two point charges, each with a charge of +1.00 Coulomb are separated

by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them.

2. Two balloons are charged with an identical quantity and type of charge: -6.25 nC (-6.25 x-9C). They are held apart at a separation distance of 0.617 m. Determine the magnitude of the electrical force of repulsion between them.

3. Determine the electrical force of attraction between two balloons with separate charges of +3.5 x 10-8 C and -2.9 x 10-8 C when separated a distance of 0.65 m.

4. Determine the electrical force of attraction between two balloons that are charged with the opposite type of charge but the same quantity of charge. The charge on the balloons is 6.0 x 10-7 C and they are separated by a distance of 0.50 m.

5. Joann has rubbed a balloon with wool to give it a charge of -1.0 x 10-6 C. She then acquires a plastic golf tube with a charge of +4.0 x 10-6 C localized at a given position. She holds the location of charge on the plastic golf tube a distance of 0.50 m above the balloon. Determine the electrical force of attraction between the golf tube and the balloon.

a. the leaves of the electroscope are neutral

b. the leaves of the electroscope have received a charge

c. no charge is moving through the electroscope

d. there is static electricity in the electroscope

Question #3

If the leaves of an electroscope spread apart, it indicates that ____.

Charging Weather

• Charging occurs best in dry conditions.

• Why? Air is more easily ionized without moisture.

• When you hear the crackling sound and a flash of light, that is charge moving through the air.

Question # 45. During a physics lab, a plastic strip was rubbed with cotton and became positively charged. The correct explanation for why the plastic strip becomes positively charged is that ...a. the plastic strip acquired extra protons from the cotton.b. the plastic strip acquired extra protons during the charging process.c. protons were created as the result of the charging process.d. the plastic strip lost electrons to the cotton during the charging process.

Question #5

6. Saran Wrap has a larger electron affinity than Nylon. If Nylon is rubbed against Saran Wrap, which would end up with the excess negative charge?

A. Saran WrapB. Nylon