1. PHYSICS UNIT 7: ELECTRICITY

2. ELECTRIC CHARGE Static Electricity: electric charge at rest
due to electron transfer (usually by friction) + –
neutral: electrons equal protons (no net charge) + –
positive charge: deficiency (loss) of electrons + –
negative charge: excess (gain) of electrons

3. ELECTRIC CHARGE
law of conservation of charge: total charge stays constant (for every + charge produced, there is a – charge produced) + – + –

4. ELECTRIC CHARGE
law of conservation of charge: total charge stays constant (for every + charge produced, there is a – charge produced) + – – + –

5. ELECTRIC CHARGE
law of electrostatics: like charges repel, unlike charges attract

6. ELECTRIC CHARGE Charge transfer
conductor: readily transfers charge (free electrons)
insulator: doesn’t transfer charge (electrons in bonds)

7. ELECTRIC CHARGE Charging by Conduction direct contact same sign
permanent
charge divides evenly between objects

8. ELECTRIC CHARGE Charging by Induction no contact opposite sign
temporary unless grounded

9. 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.

10. ELECTRIC CHARGE
Charging by conduction & induction

11. 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

12. 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/r2
Fe: electric force q: charge r: distance between charges k: 8.99×109 Nm2/C2
+Fe: repulsion, –Fe: attraction

13. 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)

14. 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

15. 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.

16. 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.

17. 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

18. Electric Potential Q q
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

19. 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 = 5V
The Electric Potential Difference is called VOLTAGE! B A

20. 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

21. 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

22. 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

23. 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 C) (1V) = 1.6 X J
So X J is defined as an electron volt. This unit can be used as an energy unit for situations dealing with small charges.

24. PHYSICS
UNIT 7: ELECTRICITY

25. ELECTRIC CIRCUITS load: energy user (bulb, resistor, heater, motor)
Basic Circuit: conductor loop for transferring energy
load: energy user (bulb, resistor, heater, motor)
source: energy provider (battery, generator)

26. ELECTRIC CIRCUITS Current, I: rate of flow of electric charge
unit: ampere, A
I = Q/t A = 1 C/s
conventional current flow: positive to negative
(real current is electrons, flowing negative to positive)

27. ELECTRIC CIRCUITS
Potential Difference or Voltage Drop, V: work done per coulomb of charge between two points, unit: volt, V
V = W/q V = 1 J/C
12 V gives 12 J/C to the electrons

28. ELECTRIC CIRCUITS Sources of Potential Difference
capacitor: stores charge
anode
cathode
cell: stores chemicals; reactions produce V
battery: cells connected in series
for cells in series, battery voltage is the sum of cell voltages

29. ELECTRIC CIRCUITS
Resistance, R: opposition to charge flow, unit: ohm, W
resistance limits the flow of current
resistance turns electric energy into heat (& light)
resistor: fixed resistance, symbol:

30. ELECTRIC CIRCUITS

31. ELECTRIC CIRCUITS resistance of a length of wire, R = rL/A
r: resistivity (W·cm), L: length (cm), A: cross-section (cm2)
rsilver=1.59×10–10 rcopper=1.68×10–10
rcarbon=3.00×10–7 rsilicon=
for solids, as T increases, r increases and vice versa

32. 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, W
applies to circuit as a whole: VT = ITRT
applies to each part of a circuit: V1 = I1R V2 = I2R2

33. 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 R1 R2 R3

34. 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

35. 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

36. 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)

37. 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

38. ANALYZING CIRCUITS EXAMPLE CIRCUIT 1 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 W V1=____ I1=____ P1=____
R2= 8 W V2=____ I2=____ P2=____

39. ANALYZING CIRCUITS EXAMPLE CIRCUIT 2 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 W V1=____ I1=____ P1=____
R2= 16 W V2=____ I2=____ P2=____

40. ANALYZING CIRCUITS EXAMPLE CIRCUIT 3 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 W V1=____ I1=____ P1=____
R2= 8 W V2=____ I2=____ P2=____

41. ANALYZING CIRCUITS EXAMPLE CIRCUIT 4 - assume 4 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 8 W V1=____ I1=____ P1=____
R2= 16 W V2=____ I2=____ P2=____

42. ANALYZING CIRCUITS EXAMPLE CIRCUIT 5 - assume 5 V per cell
RT=____ VT=____ IT=____ PT=____
R1= 1 W V1=____ I1=____ P1=____
R2= 6 W V2=____ I2=____ P2=____
R3= 12 W V3=____ I3=____ P3=____

43. CIRCUIT BOARD INTRO

44. 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.

45. 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

46. 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

47. ELECTRIC ENERGY & POWER
Electric Hazards
body resistance 104 to 106 W dry, 103 W wet
short circuit: low resistance path
low resistance = large current  shock, fire
fuses & circuit breakers: disconnect circuit above a specific current level

48. 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 = rL/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