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Homework
• WebAssign: Simple Electricity(1)
• Read Chapter 22 (Notes)
Electricity
A flow of charged particles in
a closed system. (Which can
be very large…)
Electron
Energy
Kinetic
Energy
V: Definition
Potential
Difference
Work
Current (I):
Definition
Resistance (R):
Definition
Positive and
Negative
Current
Ohms / Mhos
Franklin
SuperConductor
Schematic
V = IR
Parallel
Circuit
P = IV
RT = 1/R1+1/R2
+1/R3 + …
P = I 2R
Ohm’s
Law
Series
RT = R1+ R2 +
…
R = r(T)L/A
Material
Resistivity /
Reference
Tables
R=R(T)
Battery
kWH and Usage
Internal
Resistance
Graphs:
VI(Linear)
IV(Linear)
V=IR Light bulb
R(t)
Tools:
Voltmeter
Ammeter
VOM
DMM
Rheostat
Potentiometer
Battery
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
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
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
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?
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.
How do I visualize what is going on in an electric circuit?
How do I visualize what is going on in an electric circuit?
What is driving the flow?
What is driving the current?
Ancient Video
http://www.stmary.ws/highschool/physics/home/
notes/electricity/circuits/default.htm
http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/
OhmsLawEquation.htm
http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/
CurrentBasics.htm
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.
Linear equation with zero intercept:
y = mx
V = IR
Resistance
• Resistance
– 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!
Which is the dependent and which the independent
variable?
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.
Ohm’s Law Mnemonic
V = IR
V
----- = I
R
V
----- = R
I
Ohm’s Law
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
Example
Units:
Volt
Current
Resistance
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?
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
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 of its
resistance is independent of the Voltage
Find Resistance of a material
• Depends on four factors
– R ∞ Temp
– R ∞ RHO (ρ) resitivity
– R ∞ Length
– R ∞ A-1 (cross-sectional Area)
Low resistance
• Short
• Fat
• cold
High Resistance
• Long
• Thin
• Hot
Problem
• Find the resistance of a 20 meters length
of Aluminum with a diameter of 12 mm
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
Diagramming circuits
• Series Connection:
– Within the circuit,
– ammeters are always in the circuit, they measure flow of
charge
• Ammeters should always have low resistance
• 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
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
Kirchoff’s Junction Rule
• Conservation of
charge
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.
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
Series Diagram
VIRP Table
•
•
•
•
Vt =
It =
Rt =
Pt =
V1 =
I1 =
R 1=
P1 =
V2 =
I2 =
R2 =
P2 =
Practice Series Circuits
/www.stmary.ws/physics/home/animations3/
electricity/ElectricPowerChallenge.html.net
/ed1_files/circuits1.html
http://www.stmary.ws/highschool/physics/ho
me/notes/electricity/circuits/default.htm
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
Parallel Equations
Parallel Diagram