1. Homework
WebAssign: Simple Electricity(1)
Read Chapter 22 (Notes)

2. Electricity
A flow of charged particles in a closed system. (Which can be very large…)

3. Current (I): Definition Resistance (R): Definition
V: Definition
Electron Energy
Kinetic Energy
Current (I): Definition
Resistance (R): Definition
Potential Difference
Positive and Negative Current
Ohms / Mhos
Work
Franklin
SuperConductor
Schematic
V = IR
kWH and Usage
Parallel
Circuit
P = IV
RT = 1/R1+1/R2 +1/R3 + …
P = I2R
Ohm’s Law
Series
Material Resistivity / Reference Tables
Graphs:
VI(Linear)
IV(Linear)
V=IR Light bulb
R(t)
Tools:
Voltmeter
Ammeter
VOM
DMM
Rheostat
Potentiometer
Battery
RT = R1+ R2 + …
R = r(T)L/A
R=R(T)
Battery
Internal Resistance

4. Voltage, Current & Circuits
Voltage is the potential difference that drives the movement of charged particles. (Named after Volta, who rediscovered the battery)
Current is the flow of charged particles.
An electric circuit is composed of three elements:
1) conductor
2) potential difference (V), supplied by
battery, solar cell, thunderstorm, …
3) closed loop

5. Voltage, Current & Circuits
Voltage is the potential difference that drives the movement of charged particles.
Voltage is analogous to height in a gravitational field: gh
Multiply by m  mgh – and you now have gravitational potential energy (Joules).
Electrical potential energy is W = qV = Joules. The unit of V is Joules / coulomb

6. Current Electric current is the rate of flow of charge I = Δq/ Δt
Ampere = Coulomb per second (passing by a point in a circuit)
Ampere made discoveries regarding the interrelationships between electricity and magnetism in the early 1800’s

7. Current – which way does it go?
Two types of current:
Conventional current: Current flows from the positive terminal to the negative terminal
Electron flow: Follow the electrons! Which are going the wrong way (by definition)
Why are these not the same?
What famous American guessed wrong?
When was the electron discovered?

8. Current – which way does it go?
Two types of current:
Conventional current: Current flows from the positive terminal to the negative terminal
Electron flow: Follow the electrons! Which are going the wrong way (by definition)
Why are these not the same?
What famous American guessed wrong?
When was the electron discovered?
1898 (JJ Thompson) with his cathode ray tube. Long after current was defined.

9. How do I visualize what is going on in an electric circuit?

10. How do I visualize what is going on in an electric circuit?

11. What is driving the flow?
What is driving the current?

12. Ancient Video

13. Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.

14. Linear equation with zero intercept: y = mx V = IR
The slope is the resistance, R

15. Resistance Resistance Units: Ohms
the ratio of V/I
Units: Ohms
Its inverse is conductance, unit is mhos
Georg Ohm promulgated his famous law in 1827
He was a high school Physics teacher!

16. Which is the dependent and which the independent variable?
The slope is the inverse of the resistance, 1/R

17. Ohm’s Law V = IR Why do I care? Fix wiring problems in my home*
Fix wiring problems in my car
Fix my kid’s toys
Figure out why my circuit breaker / fuse blows
Do my job at work (the one I have now, not the one I had for the last 27 years)
*Disclaimer: Consult your parents or a qualified electrician before trying this yourself.

18. Ohm’s Law Mnemonic

19. V = IR

20. V
= I R

21. V
= R I

22. Ohm’s Law

23. Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.
When does it not apply?
Circuit elements that change temperature
Examples?
Circuit elements with large capacitance or strong magnetic fields (that are changing)
Semiconductors: materials that are natural insulators that are made to be somewhat conductive
Diode – like a one-way path

24. Example
Units:
Volt
Current
Resistance

25. Example
A 10V battery is placed across a 5 Ohm resistor. What is the current?
A 10A current flows through a 10 Ohm resistor. What is the voltage?
A 10V battery drives a 1A current through a resistor. What is the resistance?

26. Power Power is the rate of doing work (Watts) Power = Work / time
Power = Volt * q / t but q/t = I
Electrical Power = Voltage * Current = VI
Electrical Energy = Power * Time = VIt

27. Resistance and Ohm’s Law
Resistance is the impedance to the flow of charge
Resistance is defined as the ratio of potential difference to current
R = V/I Unit is the Ohm, Ω
A device is said to obey Ohm’s Law if its resistance is independent of the Voltage

28. Find Resistance of a material
Depends on four factors
R ∞ Temp
R ∞ RHO (ρ) resitivity
R ∞ Length
R ∞ A-1 (cross-sectional Area)

29. Units of ρ? (rho)
R = ρ L/A  ρ = RA/L 
ρ = (W)m^2/m  ρ = Wm

30. Low resistance
Short
Fat
cold

31. High Resistance
Long
Thin
Hot

32. Problem
Find the resistance of a 20 meters length of Aluminum with a diameter of 12 mm

33. Givens Length 20 m RHO =2.82 * 10 -8 Ω * m Area = π r 2
D= 12 mm = .012 m r = .006 m
A = π (.006m)2 = m2

34. Diagramming circuits

35. Ammeters should always have low resistance
Series Connection:
Within the circuit,
ammeters are always in the circuit, they measure flow of charge
Ammeters should always have low resistance

36. Parallel Connection Provides multiple paths for flow
Voltmeter is always placed parallel to the device you wish to measure the potential difference and have very high resistance

37. More Electric Energy Equations
P = W/t Watt
P = VI = V2/R =I2R
Electric Energy (W)= Pt = VIt = V2/Rt =I2Rt
Joules for all types of Energy

38. Kirchoff’s Junction Rule
Conservation of charge

39. Kirchoff’s Loop Rule Conservation of Energy
The sum of the changes in potential around any closed path (loop) of a circuit must be zero.

40. Series Circuit Current : One path for the flow of charge
It=I1=I2=I3
Resistance must increase because length increases
Rt-=R1+R2+R3
Voltage must add to Zero
Vt-V1-V2-V3=0 Vt=V1+V2+V3

41. Series Diagram

42. VIRP Table
Vt = V1 = V2 =
It = I1 = I2 =
Rt = R1= R2 =
Pt = P1 = P2 =

43. Practice Series Circuits/

44. Parallel Circuit More than one path for the flow of charge
More room for the flow of charge so Resistance goes down
Voltage must stay the same since all charges have the same drop

45. Parallel Equations

46. Parallel Diagram