Electrostatics

March 17 and 18, 2014

Objectives

• 5A Research and describe the concepts of electromagnetic forces.

• 5E Characterize materials as conductors or insulators based on their electrical properties.

• 5C describe and calculate how the magnitude of the electrical force between two objects depends of their charges and the distance between them.

Electric Charge

Particle Charge (C) Mass (kg)electron -1.60 x 10-19

protonneutron

Electric Charge

Particle Charge (C) Mass (kg)electron -1.60 x 10-19

proton +1.60 x 10-19

neutron

Electric Charge

Particle Charge (C) Mass (kg)electron -1.60 x 10-19

proton +1.60 x 10-19

neutron 0

Coulomb or C

• C: Coulomb Unit of Charge• μC: micro Coulomb 10-6 C• nC: nano Coulomb 10-9 C• pC: pico Coulomb 10-12 C

Let’s Practice.

• Convert 6.0 nC to C.• 1nC= 10-9 C• 6.0 nC= 6.0 x 10-9 C

Let’s Practice.

• Convert 12.5 pC to C.• 1pC= 10-12 C• 12.5 pC= 12.5 x 10-12 C = 1.25 x 10-11 C

Let’s Practice.

• How many electrons does a 6.0 nC charge represent? (note that an electron has the charge of 1.6 x 10-19 C )

• Total charge = charge of an electron x number of electrons

• 6.0 x 10-9 C = 1.6 x 10-19 C x number of electrons• 6.0 x 10-9 C / 1.6 x 10-19 C = number of electrons• 3.8 x 1010 = number of electrons

Charging by contact

• The net charge of a balloon after being rubbed against a wool sweater is ( ).

• The net charge of a sweater after being rubbed by a balloon is (positive).

negative

Balloons and Static Electricity

• The sweater ( ) electrons, and the balloon ( ) electrons.gained

lost

Affinity for Electrons

• Materials have different affinity for electrons.• Fur and a rubber rod rubbed together Fur

lost electrons• A glass rod and silk rubbed together Glass

lost electrons

John Travoltage

• When John moved his foot on the carpet, what happened?

• Electrons moved from the carpet to John and gathered in his body.

• When John turn his finger on the door, what happened to the built-up electrons in his body?

• Electrons moved from his body to the door handle.

Grounding

• The process of touching the door knob to drain the charge is called ( ).

• Describe two situations where you collected and then discharged static electricity through grounding.

• When you touch other people• When you touch a car

grounding

Charging by induction

• The process of charging a conducting object without contact

Induction

Coulomb’s Law

q1 q2d

q1: +, q2: + force of repulsion

q1: -, q2: + force of attraction

Coulomb’s Law

Coulomb constant k = 9.0 x 109 N•m2/C2

Gravitational force vs. electrical force

• Compare two formulas.• electrical force

Let’s practice.

• Two identical charges of 5.2 pC are separated by a distance of 1m. What is the force between them?

• F= 9.0 x 109 x 5.2 x 10-12 x 5.2 x 10-12 / (1 x 1)• F=2.4 x 10-13 N

Force and charge

• Electric force varies as the product of charges.• If one charge doubles, then the force ( ).• If both charge doubles, then the force ( ).• If both charge are halved, then the force

decreases to ( ).• If one charge is halved, then the force

decreases to ( ).

Force and distance

• Electric force inversely varies as the square of distance.

• If the distance doubles, then the force decreases to ( ).

• If the distance triples, then the force decreases to ( ) as great.

• If the distance is halved, then the force increases to ( ) as great.

Force, distance, and charge

• If the charge on each particle and the distance between the two particles doubles, then the force ( ).

• If the charge on each particle triples and the distance between the two particles doubles, then the force ( ).