1. Charging by Conduction (in conductors)
Like charges repel each other
Electrons want to travel to the ground or to some place less crowded so they can spread out
Before
After
© RHJansen

2. Charging by Friction
Rubbing two substances together can transfer outer electrons from one material to the other. (Example balloon and wool)
Electrons are pulled from the wool to the balloon. – + –
The balloon has more electrons than usual.
The balloon: – charged,
The wool: + charged
wool

3. Remember when Charging by Friction …
Rubbing materials does NOT create electric charges. It just transfers electrons from one material to the other.

4. Polarization Separation of charges in a neutral object.
Net charge of object remains zero!

5. Equipotential Lines
Electric Fields &
(VOLTAGES)
Point Charge
Di-Pole

6. Electric Fields & Equipotential Lines
Two Neg Point Charges
Equipotentials
& E-field Arrows
Equipotential
Lines

7. Electric Fields & Equipotential Lines
Lines of Charge:
Charged Parallel
Conducting Plates

8. Capacitors
Charged Parallel
Conducting Plates

9. WHAt capacitors really look like
Some typical capacitors. Size and value of capacitance are not necessarily related. (credit: Windell Oskay)

10. Dielectric is Insulator
Capacitors
Dielectric is Insulator
Placed Between
Conducting Plates.
Charged Capacitor
POLARIZES
the Dielectric

11. Conductors, Charge, & Electric Fields
Charges are free to move on conductors
The E-Field from on charge pushes the other charges away
Charges on a Conductor are pushed to the outside of the conductor
On a charged conductor, the E-field lines are perpendicular to the conductor surface

12. Conductors in External Electric Fields
Electric fields polarize charges. The charges move to counteract the field that’s acting on them.
This cancels the effect of the E-field inside the neutral conductor

13. Conductors and Electric Fields
The electric field inside a conductor is always zero for electrostatic equilibrium.
The electric field outside a conductor is always perpendicular to the surface
It is stronger for sharper points, and weaker for flatter surfaces.

14. Current, Resistance, & Voltage Video

15. Current (Ch. 21.1) Charges Moving (Dynamic NOT Static)
Current = Rate at which Charges Flow
Symbol: I Units: Amp (A) –short for Ampere

16. How Do We Get Charges to Move?
Connect a Charged Capacitor
with a conducting wire
This will create a conducting path form high to low electrical potential.
A current (moving electric charges) will flow through the wire.

17. How Do We Get Charges to Move?
The Electric Field in the Wire
Applies a Force that Pushes Charges in the Wire from the + side to the – side.

18. Current Direction:
Current Flows in SAME direction as Electric Field in the Conductor: + to 
electrons flow from  terminal to + terminal

19. Practice Problem 1: (Ex. 20.2)
If a mA current through a calculator is carried by electrons, how many electrons per second pass through the calculator?
Givens:
I = 0.3x10-3 A
Equation:
Conversions:
1e- = 1.6x10-19 C
Use Logic to Put it Together:
=1.881015 e-/s

20. Current Flow Exercise: Everyone, please stand and form a loop
Cotton Balls are Electrons
Due to Coulomb Repulsion, CAN NOT HAVE TWO COTTON BALLS IN HAND AT SAME TIME!!!
Only pick up neighboring ball IF your other hand is Empty.

21. Current Flow Exercise:
Drift Velocity:
Current Flow Exercise:
Current Moves almost Instantaneously when Voltage (Electric Potential) is applied to a conductor
Individual electrons move MUCH SLOWER in the conductor
How fast did the cotton balls move?
How long did it take to STOP the cotton balls moving?

+++

22. How can we detect current?
connecting wire gets warm
A lightbulb glows. The bulb filament is part of connecting wire.
A compass needle gets deflected (electromagnetism)

23. Ohm’s Law: Resistance & Simple Circuits
Resistance Symbol: R
Unit = Ohms,  - the abbreviation symbol for Ohms
Definition Resistance: determines how easily current can flow through a material

24. On a CONDUCTOR, electrons are free to move
Electrons on an INSULATOR do NOT move easily

25. Resistance in a circuit is a natural thing.
ALL conductors have some resistance.
Good conductors, like copper, have a small resistance values. Insulators have VERY high resistance values.
Electrical Resistance is like rocks in the stream, the slow down the current flow.

26. Resistance
Resistance in a Wire is like trying to walk down a crowded hallway
How do you increase current?
More people move in your direction (increase Voltage)
Reduce # of people in hallway (better conductor)
Increase the width of the hallway ( diameter wire)
Shorten hallway – get to other side faster (shorter wire)
Reduce temperature (harder to move past people or molecules with a lot of internal kinetic energy)

27. Resistivity,  (units: /m)
(Ch. 21.3)
Resistance in a wire depends on:
length of wire
cross-sectional area of wire
resistivity of material (Table 20.1)

28. In electric circuits it is important to control the flow of current
To do this we use little resistors that can effect the flow of electrons.
Picture of Resistors
Symbols for Resistors

29. Circuit Drawing with Resistors

30. OHM’S LAW
V = IR V I R

31. OHM’S LAW FORMULAS V = I R
Find Current
Find Voltage
Find Resistance
I = V R
R = V I
V = I R
Current equals
voltage divided
by resistance
Voltage equals
current multiplied
by resistance
Resistance equals
voltage divided
by current

32. Practice Problem (example 20
Practice Problem (example 20.4) What is the resistance in an automobile headlight through which 2.50 Amps flows when 12 Volts is applied?
Givens:
I = 2.5 A
V = 12V
R = ?
Equation:
Conversions:
Use Logic to Put it Together:
=4.80 

33. White Boards: Solve using Ohm’s Law: V=IR
ANS: I = 0.36 A
ANS: V = 120 V
ANS: R = 1.5x104 

34. Whiteboards Solve using Ohm’s Law: V=IR1. 2. 3.
V = IR so I = V/R
I = (12 V)/(33) = A
I = 0.36 A
R=33
V = 12 V
V = IR
V = (3.8 A)(32) = V
V = 120 V
R=32
I = 3.8 A
V = IR so R = V/I
R = (3.0 V)/(2.0x10-4 A) = 15,000 
R=1.5 x104  or
R = 15 k
I = 2.0x10-4 A
V = 3.0 V

35. Ohm’s Law Does Not Always Apply
Δ𝑉=𝐼𝑅
Ohmic devices
Devices that obey Ohm’s law
Non-ohmic devices
Capacitors
Diodes
Batteries

36. Quick QUIZ: In which circuits will the bulbs light up
Quick QUIZ: In which circuits will the bulbs light up? There must be a complete unbroken path so electrons can flow for bulbs to light! A B C

37. Quick QUIZ: In which circuits will the bulbs light up
Quick QUIZ: In which circuits will the bulbs light up? There must be a complete unbroken path so electrons can flow for bulbs to light! A B C

38. Series Resistors What are resistors in series?
Connected one after the other
There is only one path
One path means there can be only ONE CURRENT
Each resistors has same current (I)
If one breaks, all stop working

39. Current for SERIES Resistors is the SAME!
Series Resistors – the current through each resistor is the same.
The voltage across each resistor can be different! I I I I

40. A new idea: Equivalent Resistance (Req)
Equivalent Resistance – you can replace any number of resistors with one equivalent resistor called Req

41. Equivalent Resistance (Req) for Series Resistors
Just add them up!

42. Practice Problem: Series Resistors
25 Ω
150 Ω
125 Ω
30 V
What is the equivalent resistance for the circuit?
How much current goes through the 25 Ω resistor?
How much current goes through the 150 Ω resistor?

43. Practice Problem: Series Resistors
25 Ω
150 Ω
125 Ω
30 V
What is the equivalent resistance for the circuit?
How much current goes through the 25 Ω resistor?
How much current goes through the 150 Ω resistor?
Req = 25+150+125 = 300 
I =V/Req = 30V/300  = 0.1 A
ONLY 1 CURRENT so I = 0.1 A

44. Voltage Drops in a Series Circuit
Vbatt
Vbatt V1 R1
Vbatt- V1
Vbatt R2 V2
Vbatt- V1-V2 = 0 V
0 V
The electric potential drops as energy is used/dissipated by each resistor

45. Voltage Drop across Series Resistors
25 Ω
150 Ω
125 Ω
30 V
Use Ohm’s Law to find V across each resistor
Vn = IRn
where n is the number identifying each resistor

46. Practice Problems:
25 Ω
150 Ω
125 Ω
30 V
0.1 A current flows through the circuit shown. Find the voltage drop across each resistor.
What is the total voltage drop for this circuit?
V1 = IR1=(0.1A)(25) = 2.5V
V2 = IR2=(0.1A)(150) = 15V
V3 =(0.1A)(125) = 12.5V
Vtot = V1 + V2 + V3
=Vbatt
= 2.5V + 15V V = 30V

47. Parallel Resistors What are resistors in parallel?
Each Resistor is Connected to the Battery
There is more than one path
Each Path has DIFFERENT CURRENT (I)
Each resistors feels the same voltage (V)
If one breaks, the rest keep working!

48. The total current is equal to the sum of the currents
Parallel Resistors – the voltage difference across each resistor is the same Vtotal = V1 = V2 = V3
The total current is equal to the sum of the currents
Itotal = I1+I2+I3
Itotal I1 I2 I3 V R1 R2 R3

49. Work in your notes:
Example problem:
Find Itot and Req 2) Find P across each Resistor
Step 1: find I for each resistor
I1 = V/R1 = (10V)/(2) = 5 A
I2 = V/R2 = (10V)/(5) = 2 A
I3 = V/R3 = (10V)/(10) = 1 A
Step 2: Add the currents to get the Itot in the circuit
Itot = I1 + I2 + I3 = 5A + 2A + 1A = 8 A

50. Example problem:
Find Itot and Req 2) Find P across each Resistor
Step 3: Find Req
Req = Vbatt/Itot = (10V)/(8A) = 1.25 
Step 4: Use P=V2/R to find Power used by each resistor
P1 =(10V)2/(2) = 50 Watt
P2 =(10V)2/(5) = 20 Watt
P3 =(10V)2/(10) = 10 Watt
Step 5: Ptot = P1 + P2 + P3 = 80 W
or Ptot = (Itot)(Vbatt) = (8A)(10V) = 80 W

51. Parallel Circuits
In a parallel circuit, there is more than one current path.
To find the equivalent resistance: