Electric Current and
Ohm’s Law
Chapter 20.2
• Key Concepts:
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What are two types of current?
What are some examples of conductors and insulators?
What factors affect electrical resistance?
What causes an electric current?
How are voltage, current, and resistance related?
• Vocabulary:
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Electric Current
Resistance
Direct Current
Superconductor
Alternating Current
Potential Difference
Electrical Conductor
Voltage
Battery
Ohm’s Law
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).
Current (I)
• 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.
http://www.geocities.com/capecanaveral/hall/6645/electmag/electricmotor.htm
Alternating Current
• Flow of electric charge that regularly
reverses its direction.
Most electric current at
home and school.
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 an 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.
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
• SI unit for voltage is a VOLT (V)
• One volt = one joule per coulomb
• V = J/C
Voltage and Current Video
• Link:
– Click on the batter below to view video.
– http://www.youtube.com/watch?v=1xPjESsHwg&feature=related.
Voltage Sources
• Three common voltage sources:
–Battery
–Solar Cell
–Generator
Battery
• Device that converts chemical
energy to electrical energy
Questions
• Page 607
• Questions 1 - 4
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
Ohm’s Law
• Calculate the potential difference
(voltage) when the current is 4.0 amps
and the resistance is 3.0 ohms.
Ohm’s Law
• Calculate the current when the
resistance is 6.0 Ω and the potential
difference is 12 V.
Ohm’s Law
• What is the resistance of a wire if the
potential difference is 24 V and the
current is 4 A?
• Key Concepts:
–
–
–
–
–
What are two types of current?
What are some examples of conductors and insulators?
What factors affect electrical resistance?
What causes an electric current?
How are voltage, current, and resistance related?
• Vocabulary:
–
–
–
–
–
–
–
–
–
–
Electric Current
Resistance
Direct Current
Superconductor
Alternating Current
Potential Difference
Electrical Conductor
Voltage
Battery
Ohm’s Law