electrostatics. section 1: intro to static electricity

Electrostatics

Section 1: Intro to Static Electricity

• Electrostatics- Physics that deals with the attractions and repulsions of electrical charges not dependent on their motion. (Electricity at rest)

• Electrical forces arise from charged particles in the atoms.

• What are the charged particles called?

- ____________

+ ____________

• Electrical forces arise from charged particles in the atoms.

• Proton (+)

• Electron (-)

• Charge- The fundamental electrical property to which mutual attractions or repulsions between electrons or protons is attributed.

• Neutral atoms contain equal numbers of positive protons and negative electrons. (net 0 charge)

• Only the electrons move to create unbalanced charges.

• When atoms lose electrons they become positively charged ions.

Becomes

Neutral Sodium (Na)

11 protons(+) and 11 electrons(-)

11p12 n

Positive Sodium Ion (Na+)

11 protons(+) and 10 electrons(-)

11p12 n

• When atoms gain electrons they become negatively charged ions.

Becomes

Neutral Chlorine (Cl)

17 protons(+) and 17 electrons(-)

Negative Chlorine Ion (Cl-)

17 protons(+) and 18 electrons(-)

17 p18 n

17 p18 n

• Electrical charges are conserved!– When one atom becomes a positive ion

another one/few must have accepted those electrons and become equally negative

• Ex sodium is +1 because it gave its extra electron to chlorine

17 p18 n

Becomes Negative Chlorine Ion (Cl-)

17 protons(+) and 18 electrons(-)

Becomes Positive Sodium Ion (Na+)

11 protons(+) and 10 electrons(-)

11p12 n

• Interaction between charges– Like charges repel and opposite charges

attract.

• Static Electricity - Electricity at rest

• Electric charges that can be confined to an object I hate static

electricity

Some materials have a greater affinity for electrons

• Greater affinity for e- : stick to electrons more and tend to gain electrons becoming negative

• Less affinity for e- : don’t hold electrons as tight and are more likely to loose electrons become positive

• Friction can cause charge separation

• Electrons are stripped from one material and added to the other when rubbed together

Charging by Friction

•A wool cloth does not have much affinity for electrons.

•Becomes Positive

•PVC becomes negative

+ + +

+ +

+ +

+

+ + +

+ +

+ +

+

+ + +

+ +

+ +

+

- - - - -

Activity 1

List some examples of charging by frictionWhen have you noticed static electricity or attraction of one object to another

•Static cling from dryer (cotton socks w/ nylon pants)

•Balloon rubbed in hair

•Walking on carpet

Attraction for electrons

(Hold electrons tightly)

Most likely to gain electrons and become negative

(Hold electrons loosely)

Most likely to loose electrons and become positive)

PVC

Rubber

Cotton

Paper

Silk

Fur

Wool

Nylon

Hair

Acetate

Glass

Question Set 11. A girl pulls a wool cap off her head. What charge

will be produced:a) on her hair?b) on her cap?

2. Which will produce the most static cling with a cotton t-shirt in a dryer. Wool socks or a nylon nightgown?

3. Can there be static cling if only cotton items are placed in a dryer?

4. Predict the charges on the underlined objects:a) A rubber rod rubbed with furb) A glass test tube rubbed with silkc) A PVC pipe rubbed with nylon

1. A girl pulls a wool cap off her head. What charge will be produced:a) on her hair? positiveb) on her cap? negative

2. Which will produce the most static cling with a cotton t-shirt in a dryer. Wool socks or a nylon nightgown? Nylon

3. Can there be static cling if only cotton items are placed in a dryer? No

4. Predict the charges on the underlined objects:a) A rubber rod rubbed with fur negativeb) A glass test tube rubbed with silk positivec) A PVC pipe rubbed with nylon negative

Question Set 1

Conductor:

• Material through which electrons move freely

• Examples (gold, silver, copper, and aluminum)

• The general rule is that good thermo conductors are good electric conductors

• Metals tend to share electrons in electron clouds

• electrons are free to move around making them better conductors.

Electrical Conductor

Electrical Insulator

Insulator:

• Material through which electrons do not freely move

• Examples: rubber, paper, plastic, air

Grounding• Removing a static charge by

producing a path to the ground

• Electrons move from a negatively charged objects to the ground until the object is neutral

• Electrons move from ground to neutralize positively charged objects

• The earth both accepts and gives electrons while remaining overall neutral

Grounding wand for Van De Graaff generator

It’s easy to ground conductors since electrons transfer readily

It’s hard to ground insulators since charges don’t move away easily

Grounding

Section 2: Charging Objects

Three Ways of putting a charge on an object

1.Friction

2.Induction

3.Conduction

1. Charging by Friction

•Charging by rubbing objects that have different affinities for electrons together

+ + +

+ +

+ +

+

+ + +

+ +

+ +

+

+ + +

+ +

+ +

+

- - - - -

Induction (charging without contact)

1. Bring a charged object (rod) close to a neutral one (ball) without contact

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

- - - - - -- - -

+ + + + + ++ + +

- - - - - -- - -

Induction (charging without contact)

1. Bring a charged object (rod) close to a neutral one (ball) without contact

2. The electrons in the ball will be repelled leaving a positive side

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

- - -- - -- - -

Induction (charging without contact)

1. Bring a charged object (rod) close to a neutral one (ball) without contact

2. The electrons in the ball will be repelled leaving a positive side

3. The now positive sided ball with be attracted to the negative rod

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

- - -- - -- - -

+ + + + + ++ + +

- - - - - -- - -

Induction (charging without contact)

• Induction is only a temporary change without contact therefore electrons are not transferred

• The charge induced is opposite • Take away the rod and a neutral charge will

return

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

- - -- - -- - -

Conduction (charging with contact)

• Conduction is a more permanent change with contact; electrons are transferred and then isolated.

• Charge conducted is the same

• After conduction the ball and rod will repel each other

- - - - -

+ + + + + ++ + +

- - - - - -- - -

- - -- - -- - -

Make this table in your notes

Conduction Induction

Contact? Contact No Contact

Permanent? Permanent Temporary

Charge vs. Charging Device

Same Opposite

Section 3: Coulomb’s Law

Electric Charge

• Symbol is Q or q• The MKS unit is the coulomb (C)• 1 C = the charge on 6.25 x 1018 electrons

Extra info to help you with problems• 1 electron = 1.60 x 10-19 C• A coulomb is a huge charge. Static charge is

usually stated in µC which is 1x10-6 C.

Magnitude of force

3 factors affecting the magnitude of the force between two charged objects:

1. Charge on the objects

2. Distance between objects

3. Material separating objects

Coulomb’s Law

• F: electrical force• Q1: charge 1• Q2: charge 2• d: distance between charges• k: constant depending on materials separating objects• For air, k = 8.99 x 109 N·m2/C2

When using this equation:

•A positive force (F) signifies repulsion– Both charges (Qs) must be positive or both

negative

•A negative force (F) signifies attraction– One charge (Q1 or Q2) must be positive and

the other negative

Example 1

a. What is the electrostatic force between two objects, +13 μC and -22 μC which are 0.055m apart (μC = x 10-6 C)

b. Is it an attraction or a repulsion?

Example 1

a. What is the electrostatic force between two objects, +13 μC and -22 μC which are 0.055m apart (μC = x 10-6 C)

b. Is it an attraction or a repulsion?

Attraction (Q1 and Q2 are opposite signs)

• Coulomb’s Law is similar to Newton’s Law of Gravity

Similarities:

– They both are used to calculate a field force

– Both forces have an inverse square relationship to distance

– They are both related by a constant

Differences:

– Force of gravity is always attractive

– Electrostatic force can be either attractive or repulsive

– Gravities constant is very small since gravity is a very weak force

– Fg relates force created by a masses, Fel relates force created by charges

el

• Both electric and gravitational forces are field forces because objects do not have to touch to be subjected to the force.

el

• Electrical Field (E): an area of electrical influence around a charged object.

• Variable E

• Unit: newton per coulomb (N/C)

Section 4: Electrical Fields

Drawing electrical fields-• Arrows point away from the positive and

toward the negative

• In the direction a positive charge would travel in the field

• Spacing of lines show field strength

Drawing electrical fields-• Arrows point away from the positive and

toward the negative

• In the direction a positive charge would travel in the field

• Spacing of lines show field strength

This is what it would be seen if you used iron filings to see the field

Common point charge examples

Electric field between two parallel plates

• All charge lies on the surface of a conductor

• Electrical field inside a conductor is zero

E = 0 inside conductor

True or false: A cars tires protect you from being struck by lightning

False: Electrical shielding does

• The metal outside the car gives the car a path to the ground

Shielding

• Here is more proof of shielding

Section 5: Voltage

I.

• Work must be done on a positive charge to move it away from a negative sphere.

• The electric PE of the charge will increase

• When the charge is released, it will move closer to the negative sphere. Its electric PE will decrease and work can be done by the charge.

II.

• Work is required to push the small + charge against the electric field around the + sphere.

• Since work is done on the small charge, its PE increases.

• The closer it gets, the more it is repelled by the field and the more work is required.

III.

• Once the little charge is placed on the sphere, the charge on the sphere increases, and the field around it becomes stronger.

• Moving the next + charge toward the sphere will take even more work and give the small charge more PE.

Potential Difference (Electric Potential)

• Potential difference (or electrical potential) is work done as a single charge is moved in an electric field.

• Unit is the volt1V = 1J/1C

• Potential difference is measured in volts and commonly called voltage (V).

• Which positive charge has more potential energy?

• Which positive charge has more potential energy?

Its closer and therefore has a greater repulsion

• Which rock has more potential energy?

• Which rock has a greater potential difference?

A

B

Activity 2

• Both rocks have the same potential difference (potential energy per charge)

but

• Rock B has more charges and therefore more potential energy (PE)

A

B

• Electric potential is not the same as electrical potential energy. Electric potential is electric potential energy per charge.

Electric Potential (PE of one charge)

Potential Energy (PE Total)

Section 6: Electric Storage and Discharge

Electrical Energy Storage

• Capacitor- simple device used to store electrical energy. The simplest form is a pair of conducting plates separated by a small distance.

• The plates hole equal and opposite charges• The electrical energy in a capacitor comes from

the work done to charge it.

These capacitors consist of thin metal foils rolled up into a cylinder

Electric Discharge

• Discharge occurs when the electric field around a conductor becomes so strong. The air is ionized helping the charge make a break for the ground.

Arc Discharge

• Arc- a rapid discharge producing heat, light, and sound.

Lightning

Storm clouds cause a separation of charges trough updrafts and downdrafts. The bottoms of clouds become negative.

The negative bottom of the clouds induces the top of the ground to become positive

Moist air is ionized by the strong electrical field creating a conducting path. Lightning is the arc formed.

1 2

3

Facts about lightning

• Charges are separated in storm clouds; + top and – bottom

• Ground under the cloud becomes positively charged by induction

• V = millions of volts; causes arc discharge with tremendous energy.

Corona Discharge

• Corona- a slow discharge of static electricity from a pointed conductor

Lightning Rods-

• Prevent strike by allowing induced charge to leak off building in a corona discharge

• Provide a path to the ground in case of a strike