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Chapter 34
Electric Current
The Big Idea
Electric current is related to the voltage
that produces it and the resistance that
opposes it.
Ohm’s Law
V  IR
Voltage equals the current times the resistance
Basic Facts
• We use the letter I to represent current.
• Current is measured in Amperes.
• Ampere or Amps or the Letter A.
• We use the letter R to represent resistance.
• Resistance is measured in Ohms.
• Ohms or the Greek letter omega .
Some Examples
with
V  IR
1. A Battery with 10Volts is hooked up in a circuit with
a resistor at 100. Calculate the current that
flows through the circuit.
2. You want 1 A to flow through a resistor of 1,000.
What Voltage battery will you need?
3. You have a battery with 20V. You have 3 different
resistors, 10 , 100 , 1000 . Which resistor will
give the largest current? Which will give the
smallest current?

Power!
• To calculate the power used in a circuit is
very similar to Ohm’s Law.
V  IR
P  IV or P  I R
2
• Units for power are Watts (W).
Circuit
• A Complete Path
We hook the battery up and
make a circuit.
+
_
How to Draw a Battery
Resistors
• Resists the flow of electrons.
• They get warm when they do this.
• Ex. A Light bulb.
Series and Parallel
• Series
– More then one component in a row
• Parallel
– One component, multiple paths.
Equivalent Resistance for
Series & Parallel
Series
Parallel

Rtotal  R1  R2
1
Rtotal
1
1
 
R1 R2
Remember Charge.
• Battery has a positive (+) and a negative (-)
side.
• The plus (+) side does not have enough
electrons.
• The minus (-) side has too many electrons.
• This difference is called a difference in
electric potential or more commonly a
Voltage.
Why Electrons Move
Electric current is the flow of electrons.
Remember: Opposites attract.
1. Negative Charges want to move towards
positive charges.
2. Electrons move freely through conductors.
3. If there are too many electrons in a place
and they have a path to where there aren’t as
many electrons, they will move towards that
place.
AC/DC
• Direct Current (DC) Vs. Alternating
Current (AC)
Direct current stays the same while alternating
current alternates with a given frequency.
Capacitors
• Another component of electric circuit is
a capacitor.
• Commonly called a cap.
• They store charge and can be deadly!
• The units for caps are called Farad (F).
A capacitor is just two conductors
separated by an insulator.
Modern capacitors
Old Style Cap.
Transistors
• Transistors are used in circuits in two ways.
• 1. As an amplifier.
– Transistors can amplify signals.
• 2. As a switch. Transistors can act just like
your light switch.
– On, current flows,
– Off, current does not flow.