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PHYSICS UNIT 7: ELECTRICITY

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Page 1: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

PHYSICS UNIT 7: ELECTRICITY

Page 2: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction)

+

–+–+

+ +–+

+

–+–+

–– negative charge: excess (gain) of electrons

positive charge: deficiency (loss) of electrons

neutral: electrons equal protons (no net charge)

Page 3: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE law of conservation of charge:

total charge stays constant (for every + charge produced, there is a – charge produced)

+

+

+

– –

+

+

Page 4: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE law of conservation of charge:

total charge stays constant (for every + charge produced, there is a – charge produced)

+

+

+

– –

+

+

Page 5: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE law of

electrostatics: like charges repel, unlike charges attract

Page 6: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE Charge transfer

conductor: readily transfers charge (free electrons)

insulator: doesn’t transfer charge (electrons in bonds)

Page 7: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE Charging by

Conduction direct

contact same sign permanent charge

divides evenly between objects

Page 8: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE Charging

by Induction no

contact opposite

sign temporary

unless grounded

Page 9: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE

Conductor that has induced charge by neighboring positive wall. Free electrons move towards the wall.

Insulator that has induced charge by neighboring positive wall. Molecules are polarized.

Page 10: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CHARGE Charging

by conduction & induction

Page 11: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC FORCE electric force is a fundamental force

of nature: holds atoms together, holds molecules together, causes friction & most forces (except gravity)

Amount of charge, q or Q: measured in coulombs, C 1.00 C = 6.25×1018 electrons charge on one proton or electron, e = ±1.60×10–19 C

Page 12: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC FORCE Coulomb’s Law: force between

charges depends on amounts of charge and distance between them inverse square law like the force of gravity Fe = kq1q2/r

2

Fe: electric force q: charger: distance between charges k: 8.99×109 Nm2/C2

+Fe: repulsion, –Fe: attraction

Page 13: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC FORCE electric fields exert force on

charged objects electric field strength, E: force

exerted on a charge by an electric field

E = F/q unit: N/C (Newtons/Coulomb), or V/m

(Volts/meter)

Page 14: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC FORCE Electric field: region around a

charge where it exerts electric force on other charges

field lines: show direction & amount of force (by how close the lines are) on a + test charge

Page 15: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Electric Field Lines E field lines are

constructed by determining what a positive charge would do if placed in the field

The denser the lines the stronger the field.

Lines always emanate from positive charge and end at negative charges.

Page 16: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Lines of Equipotential The grey dotted

lines represent places where the Net E-field magnitude is equal.

Note how on the parrallel plate scenario The E-field is equal for any point within the plates.

Page 17: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC FORCE constant electric fields are used to

accelerate charged particles field is constant between parallel plates

force F = qE change in Kinetic Energy = Work Kf-K0 = Fd (Work done by the field)

d: distance traveled in electric field K = ½mv2

Page 18: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Electric Potential Imagine that a positive

charge q is released from contact with Q and is allowed to accelerate to an infinite distance away picking up KE as it goes. The Change in KE is the Work required to bring the test charge from an infinite distance back to Q.

Electric Potential, V, is work per unit charge that is needed to bring q toward Q

V = W/q Units are

Joules/Coulomb

Q q

Page 19: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Examples

What is the electric potential at point A of a 2 Coulomb charge that requires 10 J of work to move from B to A?

V= W/q = 10/2 = 5 J/C = 5VThe Electric Potential Difference is

called VOLTAGE!

BA

Page 20: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Potential Energy Is PE increasing or decreasing as q, 5

C, moves towards Q?

IF Vb=0 and Va=10, Then Voltage is 10 volts at A. Potential Energy is equal to work needed to move q to A. So…

W = qV = U = 50 Joules!

A B

Page 21: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Negative Charges? What happens if negative q, -5C, is

moved from A to B? Assume Vb=0 and Va=10

Then as q moves to A PE decreases. U=qVa=(-5)(10)= -50 J

A B

Page 22: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

Conclusion F = qE E = kQ/r2

W = q V U = q V Positive charges move naturally from high

electric potential to low electric potential Negative charges move naturally from low

electric potential to high electric potential All charges move from high PE to low PE

Page 23: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

The Electron Volt The Joule is a huge unit of energy when

dealing with electrons moving across electric potentials.

How much energy would an electron gain if it moved across a potential difference of 1 V?

U = qV = (1.6 X 10-19 C) (1V) = 1.6 X 10-19 J So.. 1.6 X 10-19 J is defined as an electron

volt. This unit can be used as an energy unit for situations dealing with small charges.

Page 24: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

PHYSICS

UNIT 7: ELECTRICITY

Page 25: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS Basic Circuit: conductor loop for transferring

energy load: energy user (bulb, resistor, heater,

motor)

source: energy provider (battery, generator)

Page 26: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS Current, I: rate of flow of electric

charge unit: ampere, A I = Q/t 1 A = 1 C/s conventional current flow: positive to

negative (real current is electrons, flowing

negative to positive)

Page 27: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS Potential

Difference or Voltage Drop, V: work done per coulomb of charge between two points, unit: volt, V V = W/q 1 V

= 1 J/C 12 V gives 12 J/C

to the electrons

Page 28: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS Sources of Potential

Difference capacitor: stores charge

battery: cells connected in series

cell: stores chemicals; reactions produce V

for cells in series, battery voltage is the sum of cell voltages

anode

cathode

Page 29: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS Resistance, R: opposition to charge flow, unit: ohm,

resistance limits the flow of current resistance turns electric energy into heat (& light) resistor: fixed resistance, symbol:

Page 30: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS

Page 31: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC CIRCUITS resistance of a length of wire, R =

L/A : resistivity (·cm), L: length (cm),

A: cross-section (cm2) silver=1.59×10–10 copper=1.68×10–10

carbon=3.00×10–7 silicon=0.00100 for solids, as T increases, increases

and vice versa

Page 32: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS Ohm’s law: current is proportional

to voltage and inversely proportional to resistance: V = IR V: voltage, V I: current, A R:

resistance, applies to circuit as a whole: VT = ITRT

applies to each part of a circuit: V1 = I1R1 V2 = I2R2

Page 33: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS Resistances in Series:

IT = I 1 = I2 = I3

VT = V1+V2+V3

RT = R1+R2+R3

adding resistors in series increases RT, decreases IT

removing one resistor stops current in the whole circuit

R1R2 R3

Page 34: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS Resistances in Parallel:

IT = I1=I2+I3 VT = V1 = V2 = V3

1/RT = 1/R1+1/R2+1/R3

adding resistors in parallel decreases RT, increases I

removing one resistor stops current only in that branch

R1 R2 R3

Page 35: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS Kirchoff’s 1st Rule:

total current entering a junction equals total current leaving a junction (conservation of charge)

Kirchoff’s 2nd Rule: total voltage change around any closed loop of a circuit is zero (conservation of energy)

I1 = I2 + I3

Page 36: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC ENERGY & POWER

Electric Power: rate of electric energy supply or use, in Watts, W power supplied or used, P = VI, 1 W

=1 J/s power used, P = I2R (appliance and

light bulb ratings)

Page 37: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC ENERGY & POWER

Electric Energy: work done (energy transferred) by electric current, in Joules, J (electric companies bill for energy, not power) energy, E = Pt electric bill in kilowatt-hours, 1.00

kWh = 3.60×106 J

Page 38: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 1 - assume 4 V per cell

RT=____ VT=____ IT=____ PT=____

R1= 8 V1=____ I1=____ P1=____

R2= 8 V2=____ I2=____ P2=____

Page 39: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 2 - assume 4 V per cell

RT=____ VT=____ IT=____ PT=____

R1= 8 V1=____ I1=____ P1=____

R2= 16 V2=____ I2=____ P2=____

Page 40: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 3 - assume 4 V per cell

RT=____ VT=____ IT=____ PT=____

R1= 8 V1=____ I1=____ P1=____

R2= 8 V2=____ I2=____ P2=____

Page 41: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 4 - assume 4 V per cell

RT=____ VT=____ IT=____ PT=____

R1= 8 V1=____ I1=____ P1=____

R2= 16 V2=____ I2=____ P2=____

Page 42: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 5 - assume 5 V per cell

RT=____ VT=____ IT=____ PT=____

R1= 1 V1=____ I1=____ P1=____

R2= 6 V2=____ I2=____ P2=____

R3= 12 V3=____ I3=____ P3=____

Page 43: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

CIRCUIT BOARD INTRO

Page 44: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

CIRCUIT BOARD INTRO Springs are

connectors for wires and components. Some springs are connected to devices on the board (like the D-cells). If a spring is too loose, squeeze the coils.

Page 45: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

CIRCUIT BOARD INTRO When you connect a circuit to a D-cell

note the polarity (+ or –). Only connect things long enough to

make your observations & measurements, then disconnect one wire so the D-cells don’t run down and resistors don’t overheat

Page 46: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC ENERGY & POWER

Electric Hazards effect of shock depends on location

skin: burns, muscles: spasms, nerves: pain, heart: disruption

effect of shock depends on current <10 mA: pain, no damage >10 mA: severe muscle contraction, paralysis 70 mA chest: heart fibrillation 1 A chest: heart stops completely, but may

restart

Page 47: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

ELECTRIC ENERGY & POWER

Electric Hazards body resistance 104 to

106 dry, 103 wet short circuit: low

resistance path low resistance =

large current shock, fire

fuses & circuit breakers: disconnect circuit above a specific current level

Page 48: PHYSICS UNIT 7: ELECTRICITY. ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction) + – + – + – +

UNIT 7 FORMULAS Fe = kq1q2/r2

k = 8.99×109 Nm2/C2

e = ± 1.60×10–19 C F = qE K-K0 = Fd I = Q/t V = W/Q

R = L/A V = IR P = VI = I2R E = Pt RT = R1+R2+R3

1/RT = 1/R1+1/R2+1/R3

1.00 kWh = 3.60×106 J