# Electric Circuits - Foothill High School

```Electric Circuits
Electric Circuit
Diagram of Electric Circuit
Electric Circuits
• Electric Current – Flow of charged
particles in a conductor (electrons)
• Conductor – Allows the flow of current
i.e. metal, water with ions
• Insulator – Prohibits the flow of
current i.e. wood, plastic
While the switch is open:
• Free electrons (conducting electrons) are
always moving in random motion.
• There is no net movement of charge across a
cross section of a wire.
What occurs in a wire when the
circuit switch is closed?
http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/imgele/micohm.gif
What occurs in a wire when the
circuit switch is closed?
• An electric field is established
instantaneously
• Free electrons, while still randomly moving,
immediately begin drifting due to the
electric field, resulting in a net flow of
charge.
Electric Potential EnergyTwo Unlike Charges
Higher Potential
Energy
+
Lower Potential
Energy
-
•To cause movement of a charge, there
must be a potential difference.
Battery (Chemical Cell)
• A device that converts chemical energy to
electricity.
A battery provides the potential
energy difference (voltage source).
• A simple circuit
• Current flow is created
by a battery, solar panel,
etc
Through a
resistance
• A water based
analogy
• So which way do the electrons “flow” ?
Conventional Current
direction in
which
“positive
charges”
would flow.
• Direction is
opposite of
electron flow
- to +.
• OOPS!!!!!!!!
Closing the switch establishes a potential difference
(voltage) and causes current to flow
• In reality
electrons
flow in a
net
direction
away
from the
(-)
terminal.
Low
Potential
High
Potential
Definitions
• Resistance:R - Depends on type of
material, size and shape,
temperature.
• Coulomb:C - a measure of charge
• Electron has 1.6 X 1019 coulombs of
charge
• Current : I = Coulombs/sec = 1 amp
Definitions
• Voltage: V- potential difference
(J/C) = volts
• Power = I • V Units are watts; 1
watt = 1J/1sec
• Energy = P • t Units are joules
Question:
What is required in order to have an
electric current flow in a circuit?
1. A voltage source.
2. The circuit must be closed.
Voltaic Cell
• Alessandro Volta (1800’s)
• Battery
Cu and Zinc Electrodes. Why?
Resistor
• An object that
has a given
resistance.
A Battery Provides Energy
• The battery
“pumps” positive
charges from
low (-) to
high (+) potential.
Electric Circuit
Resistors use up Energy
• A resistor uses up
energy.
• When the current
goes through the
resistor it goes to a
lower potential.
Electric Circuit
Question:
• Which point has a
lower potential, A
or B?
Electric Circuit
Resistance
• Depends on type of material, size and
shape, temperature.
R=ρ L
A
L: length of the wire
A: cross-sectional area
ρ: resistivity (inherent to material)
Example:
• What happens to the resistance when the
length is doubled and the area is
• Answer: It changes by 1/2
Temperature Dependence of
Resistance
• For metals: as temperature increases
the resistance increases. At very low
temperatures resistance can become
zero: superconductivity.
• For semiconductors: the opposite
occurs.
Potentiometer
• A variable resistance.
• Used for dimmers, fan
speed controls, etc.
Potentiometer Symbol
Voltmeter
• Measures the voltage between
two points in an electric circuit.
• Must be connected in parallel.
A voltmeter is connected in
parallel.
Ammeter
• Measures electric current.
• Must be placed in series.
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