physics unit 7: electricity. electric charge static electricity: electric charge at rest due to...
TRANSCRIPT
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
2
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
Q q
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
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
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
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
anode
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 & 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
ANALYZING CIRCUITS
EXAMPLE CIRCUIT 1 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 V1=____ I1=____ P1=____
R2= 8 V2=____ I2=____ P2=____
ANALYZING CIRCUITS
EXAMPLE CIRCUIT 2 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 V1=____ I1=____ P1=____
R2= 16 V2=____ I2=____ P2=____
ANALYZING CIRCUITS
EXAMPLE CIRCUIT 3 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 V1=____ I1=____ P1=____
R2= 8 V2=____ I2=____ P2=____
ANALYZING CIRCUITS
EXAMPLE CIRCUIT 4 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 V1=____ I1=____ P1=____
R2= 16 V2=____ I2=____ P2=____
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=____
CIRCUIT BOARD INTRO
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.
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
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
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
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