Electricity: The Mouse and Cheese
Analogy
• All matter is made up of positive
charges and negative charges.
– The positives have mass and are not usually
free to move.
– The negatives have no mass and are free to
move through some materials (conductors).
Mouse and Cheese Analogy
• Negative charges are attracted to
positive charges the same way mice are
attracted to cheese.
– Any time there is a natural attraction between two
things we can use it to make the objects do work.
– The negative charges (mice) will gladly do work in
order to get to the positive charges (cheese).
Electric Current
• Continuous flow of electric charge.
– SI unit of electric current is the AMPERE (A) or amp.
– 1 amp = 1 coulomb per second.
• Two types of electric current:
– Direct Current (DC)
– Alternating Current (AC)
Mouse and Cheese Analogy
• Current:
– The number of charges (mice) passing a
point per second. The rate of flow of
charges.
Direct Current
• DC
• Charges flow in
one direction.
– Most battery
operated items use
DC.
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Alternating Current
• Flow of electric charge that regularly
reverses its direction.
Most electric current at
home and school.
Dotted lines are from AC
Conductor and Insulator
• Electrical Conductor
– Material through which charge
can flow easily.
• Copper and Silver
• Electrical Insulator
– Coating around a wire that
controls the current and keeps it
where its needed.
• Wood, plastic, air
Rubber
Insulating Material
(plastic)
Conducting Wire
(copper)
Superconductor
• Material that
has almost
zero resistance
when cooled to
low
temperatures.
– Best superconductors
must be cooled to
138K.
Resistance
• Opposition to the flow of charges in
a material.
– SI unit of resistance is ohm (Ω).
• Four factors affect the resistance in
a wire:
– Material type
– Thickness
– Length
– Temperature
Mouse and Cheese Analogy
• Resistance:
• The opposition to the flow of charge.
• Any appliance that asks the charge (mouse) to
do work will slow it down.
Voltage
• The difference of electrical
potential between two points of an
electrical circuit.
• Also called the potential difference.
– SI unit for voltage is a VOLT
– One volt = one joule per coulomb
Mouse and Cheese Analogy
• Voltage:
–The amount of work that each charge
(mouse) will do as it goes through
the circuit.
–Can also be thought of as
the amount of push on the
charges or how hungry the
mice are.
Voltage Sources
• Three common voltage sources:
–Battery
–Solar Cell
–Generator
Battery
• Device that converts chemical
energy to electrical energy
Ohm’s Law
• Georg Ohm (1789-1854)
– Found a relationship between voltage, current,
and resistance.
• Ohm’s Law:
– The voltage (V) in a circuit equals the product
of the current (I) and the resistance (R).
– V=IxR
I=V
R=V
R
I
Chapter 20.3
ELECTRICAL CIRCUITS
Circuit Diagrams
• Use symbols to represent parts of
a circuit.
• Include:
– Sources of electrical
energy
– Devices that are run by
energy
electrical
TYPES OF
CIRCUITS
Series Circuits
• Charge has only ONE path through
which it can flow.
• If one element stops functioning in a
series circuit, NONE of the elements
can operate.
–The bulbs in a circuit are a source of
resistance. The more bulbs, the more
resistance.
Parallel Circuit
• Charge has TWO OR MORE
paths through which the
charge can flow.
• If one element stops
functioning, the rest
of the elements can still
operate.