Unit 3: Characteristics of Electricity
Chapter 10: Electricity on the Move
p 322-351
Terms
positive terminal
negative terminal
circuit
switch
circuit diagram
battery
resistor
load
current
coulomb
ampere
ammeter
electric potential energy
potential difference
volt
voltmeter
resistance
ohm
Ohm's law
power watt
10.1 Pushing Charges Around
324
Circuits and Their Parts
Circuit
a closed path of electric current through conductors.
Electrochemical Cell a single set of chemicals that generate electrical energy using chemical reactions
Battery
a combination of electrochemical cells that generate electric energy.
Each battery has one positive and one negative terminal.
Terminals are the attachment points for the circuit.
Load
a device that uses electrical energy to perform a useful task by converting electrical
energy into another useful form of energy (motor, toaster, computer, lamp).
Resistor
any device that has a set resistance and uses up electric energy, often releasing heat.
Switch
a moving conductor that can open and close to control the circuit
Circuit Diagram
a representation of the parts of a circuit using symbols.
Copy Figure 10.1 (p324) into your notes
Electric Current
Variable
Symbol Units
Description
Electric Charge
Q
coulomb, C

the amount of electric charge (static or current)
Electric Current
I
ampere, A

the rate of flow of charge through a conductor

the amount of charge flowing past a point per unit time
Time Interval
Current

t
current 
second, s
amount of charge moving past a po int
time
I

Q
t
1A = 1C / 1 s
Ammeter
device used to measure the current in amperes
10.2 Energized and De-energized Charges
Energy
330
the ability of an object to exert a force on another object to change its motion.
Variable
Symbol Units
Energy
E
Description
joule, J

the ability to do work (exert a force to change motion)
Energy can be classified as being either kinetic or potential.
Kinetic Energy




the energy an object has based on its motion.
Machines, wind, waves, tides, and sound (moving objects or particles)
Electric currents and lightning (moving charges)
Light and other radiation (moving photons)
Heat (thermal energy)
Potential Energy




the “stored” energy based on its position.
Gravitational  hydroelectric power, roller coaster, wrecking ball
Elastic  springs, guitar strings, hockey sticks, soccer balls
Chemical (bonds between atoms or molecules) burning coal, gasoline
Electric Potential (build up of excess charge)  electrochemical cells, static electricity
Work, W
the change in energy
Electric Potential Energy the energy generated by a battery as it continuously separates charges when
connected to a circuit.
Potential Difference
Variable
Potential Difference
or
Voltmeter
the difference in potential energy per coulomb of charge between two points
Symbol
Units
Description
V
volt, V

the difference in potential energy per coulomb
Potential Difference = energy change
charge
V
Energy = Charge x Potential Difference

E = QV
E
Q
units: V = J / C
units: J = C V
device used to measure the potential difference between two points of a circuit in volts
10.3 Resisting the Movement of Charge
337
Resistance the property of a substance that opposes the motion of charge carriers in it
Resistance results in electrical energy being converted to other forms of energy, including heat.
Variable
Symbol
Resistance
R
Units
Description
ohm, 

the ratio of the potential difference to current across a load
Resistance, R, is the ratio of the potential difference, V, across a load to the current, I, passing through the load.
R
or

Ohm's law
V
I
V=IR
unit: ohm,  
V
A
volt = ampere x ohm

for a load to have a constant resistance, its potential difference to current ratio must stay constant
Factors Affecting the Resistance of a Conducting Wire
1. Temperature, T
2. Length of wire, L
3. Cross-sectional Area



As wires heat up, their resistance also increases.

Supercooled conductors can be “super conductors”

Superconductors have no resistance!
Longer wires have more resistance.

A lot of energy is lost over long power lines.

Electrical power plants should be located near cities
Larger diameter wires allow charge to pass through easier

Narrow wires are less expensive have more resistance

4. Material

Thicker wires are more expensive because they conduct
better (less resistance and less energy lost)
Some metals are better conductors than others

Gold, silver, and copper are very good conductors (low R)

Iron and carbon (graphite) are poor conductors (high R)
T  R
Direct relation
L  R
Direct relation
Area R
Diameter  R
Inverse relation
10.4 Powerful Charges
343
Power, P
Power is the rate at which energy is used. Power is the change in energy per unit time.
Variable
Symbol
Power
P
Power 
Units
Description
watt, W

Change in Energy
time
P
the change in energy per unit time.
E
t
Electrical Energy and Power
Electrical Energy = Charge x Potential

Difference
Substituting QV for E gives:
Efficiency

Units: watt
1W 
1J
1s
E = QV
QV
Q
P
but current, I  , so P = IV
t
t

Efficiency is a measure of the amount of useful energy compared to the amount of energy used.
Efficiency 
UsefulEnergy Out
Total Energy In
You never get more energy out than you put in.
Efficiency is usually expressed as a percentage.
