physics unit 7: electricity. electric charge static electricity: electric charge at rest due to...

PHYSICS UNIT 7: ELECTRICITY

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)

ELECTRIC CHARGE law of conservation of charge:

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

– –

ELECTRIC CHARGE law of conservation of charge:

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

– –

ELECTRIC CHARGE law of

electrostatics: like charges repel, unlike charges attract

ELECTRIC CHARGE Charge transfer

conductor: readily transfers charge (free electrons)

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

ELECTRIC CHARGE Charging by

Conduction direct

contact same sign permanent charge

divides evenly between objects

ELECTRIC CHARGE Charging

by Induction no

contact opposite

sign temporary

unless grounded

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.

ELECTRIC CHARGE Charging

by conduction & induction

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

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

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

+Fe: repulsion, –Fe: attraction

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)

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

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.

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.

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

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

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!

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!

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

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

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.

PHYSICS

UNIT 7: ELECTRICITY

ELECTRIC CIRCUITS Basic Circuit: conductor loop for transferring

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

motor)

source: energy provider (battery, generator)

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)

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

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

cathode

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:

ELECTRIC CIRCUITS

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

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

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

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

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

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)

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

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 1 - assume 4 V per cell