Download Unit 11: Electric Current

Unit 11: Electric Current
Many practical devices and applications
are based upon the principles of static
electricity.
 Electricity became an integral part of
our daily lives when scientists learn to
control the movement of electric
charge.

Unit 11: Electric Current

Electric Current is the rate at which
electric charges pass through a given
area.

Current is measured in amperes which
are equivalent to a Coulomb per second.
Unit 11: Electric Current

Conductors transmit charges easily and
are used for electric circuits.

Conductor (freely
flowing electrons)

Insulator (electrons
cannot easily flow)
Unit 11: Electric Current

What makes Copper a good conductor?
◦ Copper has two electrons in the innermost
shell, eight in the next shell, eighteen in the
third shell, and one in the fourth shell
 The first three shells each have as many electrons
as they can hold
 The fourth shell has one lonely electron
 This one lonely electron can easily separate from
the rest of the atom and go roaming around
Unit 11: Electric Current

Batteries [pictures on next slide] provide a
chemical reaction between two terminals
(cathode and anode) in some form of
medium (electrolyte)
◦ In an Alkaline Battery
 Cathode: Manganese Dioxide
 Anode: Zinc powder
 Electrolyte: Potassium hydroxide

The chemical reaction causes the build-up
of electrons on one terminal, and a
deficit of electrons on the other
Unit 11: Electric Current
Equivalent
pictures
Unit 11: Electric Current

EMF () – Electromotive Force
◦ The maximum potential difference (voltage)
a power source can have
 The potential difference between the terminals of
a battery when it’s not connected to anything
 Batteries maintain a constant voltage
◦ Not actually a force
◦ Measured in J/C (Volts)
Unit 11: Electric Current
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Each cell can produce a fixed amount of
EMF
Bigger cells can produce EMF for a longer
time than smaller cells
Batteries run out of EMF when all of the
free electrons have been moved from the
cathode to the anode
Rechargeable batteries can reverse
the chemical reaction to re-deposit
negative charges back on the cathode
Unit 11: Electric Current
Batteries convert chemical energy into
electrical energy
 The actual voltage of a battery is always
slightly less than the EMF

◦ Batteries have internal resistance
◦ Energy is lost as heat
Unit 11: Electric Current

A basic circuit has three main parts:
◦ A source of energy
 Sometimes called a “potential difference”.
◦ A closed path
◦ A device which uses the energy
Circuits are very similar to water
in a pipe

Pumps
◦ There is a pressure
difference where the
flow begins and ends
◦ A certain amount of
flow passes each
second

Circuits
◦ There is a potential
difference (voltage)
were the charge
begins and ends
◦ The amount of
charge that flows per
second is called
current
Unit 11: Electric Current

Resistance: The opposition presented
to electric current by a material/device.
◦ Units are ohm (Ω) = volt/ampere.

Ohm’s Law:
Unit 11: Electric Current

Resistance depends upon length, area,
material and temperature.
Unit 11: Circuits

Electric Circuit: A set of electrical
components such that they provide one
or more complete paths for the
movement of charges.
Unit 11: Circuits

Ohm’s Law:
Unit 11: Circuits

Series: describes two or more
components of a circuit that provide a
single path for current
Unit 11: Circuits
Equivalent resistance in a series circuit
is the sum of the resistances.
 In our picture below, R = R1 + R2 + R3

Unit 11: Circuits

Example: A 24.0 V battery is connected
in series with R1 = 2.0 Ω, R2 = 5.0 Ω and
R3 = 5.0 Ω. Find the equivalent
resistance and the current in the circuit.
Unit 11: Circuits

Parallel: describes two or more
components of a circuit that provide
separate conducting paths for current
because the components are connected
across common points or junctions.
Unit 11: Circuits
Equivalent resistance in a parallel
circuit is the sum of the reciprocals of
the resistances.
 In our example below,
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Unit 11: Kirchoff’s Rules
A junction or node is a point where three
or more wires are joined.
 A branch is a path connecting two
junctions.
 Below are 4 nodes and 4 branches.

Unit 11: Kirchoff’s Rules
Kirchoff’s 1st rule: ΣI at a junction = 0
 Kirchoff’s 2nd rule: ΣV in closed loop = 0

Unit 11: Electric Power

Batteries maintain a potential
difference across their terminals by
converting chemical energy to electrical
potential energy.
◦ Electrons are moving and thus, they have
kinetic energy.
◦ They collide with other charges,
transferring energy as heat
Unit 11: Electric Power
Sometimes batteries must be replaced
or recharged (when all electrons move
from the cathode to the anode).
 Generators do not run out of energy.

◦ They convert mechanical energy into
electrical energy
◦ Generators are the source of current to a
wall outlet in your home.
Unit 11: Electric Power
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https://www.youtube.com/watch?v=GI7A
hajfhWE
Unit 11: Electric Power
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Calculating electrical power:
Unit 11: Electric Power

Electric companies measure energy
consumed in kilowatt-hours.

Remember: Power = Energy/Time = J/s
Unit 11: Electric Power
Some current, like in a battery, is Direct
Current.
 Some current, like that which is
generated by the power company, is
Alternating Current.
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