Transcript Current Electricity

Current Electricity
Electricity
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Electricity is the flow of electrons through a
conducting material.
The flow of electricity is called current.
Current is measured by counting the number
of electrons that flow past a point in a given
time.
Current
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An electric current is moving charges (electrons)
The brightness of a bulb depends on the
current through the bulb (qualitative measure)
As the electrons move through the conductor,
they collide with the fixed particles of the
conductor and the kinetic energy is transferred
into heat and light in the resistance
Current
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More current means more electrons collide and
therefore, more energy is dissipated in the
resistance
Resistance should be considered an obstacle to
flow. Less resistance implies more flow and vice
versa
Current is conserved. What goes in one end
must come out the other end
Electrical Charge and Current
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Electrical charge is measured in Coulombs (C)
1 coulomb is equal to the amount of charge
in 6.25 x 1018 electrons
1 coulomb of charge moving past a point in
1 second is equal to 1 Ampere of current
1 Ampere = 1 coulomb/1 second
Calculating Current
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To calculate current we use the formula;
I = Q or Q = I x t
t
I = current and is measured in Amperes (A)
Q = charge and is measured in coulombs (C)
t = time and is measured in seconds (s)
Examples
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If a bulb has 0.25A, how many electrons pass
through the bulb in one second?
I = Q/t
and
Q = It
therefore
Q = (0.25A)(1s) or 0.25C, but 1C = 6.25 x 10^18
So… (0.25A)(6.25 x 10^18) = 1.56 x 10^18 e
That is a lot of electrons!
Voltage
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Voltage (electric potential difference) is
measured in Volts (V).
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Voltage is the energy per unit charge between two
points along a conductor
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V = E/Q
Five Sources of Electricity
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1. Chemical – chemical sources accumulate
charge through chemical processes.
Ex. batteries
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2. Photoelectric (light) – photoelectric materials
emit electrons when they are struck by light.
Ex. Solar powered calculators
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3. Electromagnetic – a wire moving in a
magnetic field generates a current.
Ex. generator
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4. Thermoelectric (heat) – the temperature
difference between different materials can create
a current.
Ex. A thermocouple in a furnace
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5. Piezoelectric (crystals) – some crystals will
create a current when put under mechanical
stress (force).
Ex crystals found in quartz watches.
The Electric Cell
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The electric cell is a device that has the ability to
produce an electric charge for a longer period of
time than an electrostatically charged object.
The History
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1794 – Luigi Galvani
researched the link
between muscle
contractions and electric
“sparks”
1800 – Alessandro Volta
Discovered that if two
different metals are
placed in a salt or acid
solution a charge was
created.
The charge produced
seemed like it was
continuous. This type of
cell is called a Volta Cell
or Wet Cell
The Wet Cell
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Two different metals called electrodes are placed
in a salt or acid solution called an electrolyte.
Able to give off sparks without being recharged
Produces an electric current – a continuous
supply or “flow” of electrons.
Example
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A zinc electrode and a
copper electrode are
placed in an electrolyte
solution of NaCl. This
solution contains (Na+)
sodium ions and (Cl-)
chloride ions.
How this works
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The copper electrode gives electrons to the
positive sodium ions – because it has given away
electrons it becomes positively charges.
The zinc electrode attracts the negatively
charged chloride ions and becomes negatively
charged.
The conducting loop (usually a metal) provides a
pathway for the negative charges (electrons) to
flow from the negative Zn electrode towards the
positive Cu electrode.
Example
The Dry Cell
Works the same way as the wet cell but the
electrolyte solution used is in a paste not a liquid
form.
Current Flow in Metal Conductors
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Electric current is the movement of electrons
through a conductor at a certain rate when
something is “pushing” them.
When an electric cell is attached to a conductor
it moves electrons into the conductor
The electric cell pushes electrons from the –ve
elecrode (by repulsion) into the conductor which
pushes the rest of the electrons towards the +ve
electrode (by attraction)
Circuits
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The conducting loop in a cell is often referred to
as a circuit.
The circuit (conducting loop) can be opened or
closed using a switch.
If the switch is OPEN the conducting loop is
“broken” so electrons cannot flow
If the switch is CLOSED the conducting loop is
complete so electrons can flow
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In an electric circuit, electrons are present in
ALL PARTS of the circuit all the time
In order to move electrons in a wire, we need a
potential difference between the ends of the
wire, which is created through an accumulation
of charge at the ends of the wire
As one electron is pushed into one end of the
wire, ALL electrons in the wire move
simultaneously and one electron moves off the
other end
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A circuit can be defined as a pathway used to
direct an electric current in order to perform a
“task”.
Every circuit must have 3 components
A Source of energy – electric cell or battery
 A Conducting loop – usually a metal wire
 A Load – something that uses potential energy to do
work such as a light bulb, resistor or motor.
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Measuring Current and Volts
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In order to measure current, an ammeter must
be placed in series such that the current flows
through the meter.
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If placed incorrectly (placed in parallel means it
behaves as a low resistance wire – shorts the circuit)
Voltmeters are placed in a circuit in parallel and
have a very high resistance
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If placed in series, the large resistance decreases the
current (effectively to zero) and the circuit will not
work
A
V
Resistance
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Resistance is the opposition to the flow of
electrons when a current passes through the
circuit.
Electron movement is slowed down
 Resistance converts electrical energy into other
forms of energy such as light or heat
 Devices called resistors are designed specifically to
provide resistance in a circuit.
 The unit used to measure resistance is the Ohm (Ω)
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Battery
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A combination of two or more dry cells that act
as an energy source for a circuit
2 types of batteries may be used in a circuit
1.
2.
Cells in series
Cells in parallel
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Cells in Series
Two or more dry cells connected end to end
 Connecting dry cells in series INCREASES the
voltage but DOES NOT CHANGE the current
produced by the battery
 Ex. If two 1.5V cells are connected in series we
would have a 3.0V battery
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Cells in Parallel (last longer)
2 or more dry cells connected side by side so that
their + electrodes are joined and their – electrodes
are joined
 Connecting dry cells in parallel DOES NOT
CHANGE the voltage produced by the battery but it
INCREASES the current produced by the battery
 Each cell only has to contribute a portion of the
current available to the circuit. This “shared” load
allows the current to be delivered for a longer period
of time.
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Series Circuits
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Circuits constructed in a way that all of the
current must pass through each of the loads in
the circuit
Resistors in Series
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Two resistors placed in series have a greater
resistance than a single resistor
The total resistance of resistors in series is the
sum of the resistors:
Rtotal = R1 + R2 + R3 …
Parallel Circuits
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Circuits constructed in a way in which the loads
are parallel to each other creating BRANCHES
in the circuit
Current has a choice of branches to travel through.
 The branch with the lower resistance will have the
higher current passing through it
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The current before the junction and after the
junction have the same value
The sum of the current running through each
branch is equal to the total current running
through the entire circuit
Resistors in Parallel
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When resistors are connected in parallel the total
resistance is less than that of a single resistor.
The electrons have an extra path to follow (like
opening another door)
The equivalent resistance of resistors in parallel
is given by the formula:
1/Req = 1/R1 + 1/R2 + 1/R3 …
Power, Energy and Time
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In physics the word Power is defined as the
energy per unit time
Electrical power describes the amount of
electrical energy that is converted into light,
heat, sound or motion every second.
The symbol for power is P. The equation that
defines power in mathematical form is
 P =E/t
Units of Power
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Since the unit for energy is joule ( J ) and the
unit for time is the second ( s ), power can be
expressed in joules/second
Joules/second has been named Watt ( W ) in
honour of James Watt.
When one joule of electrical energy is converted
into heat and light by a light bulb every second,
the power of the bulb is said to be one watt.
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Note that the energy delivered to a circuit must be
used in the circuit, or the wires will burn
Nothing is perfect
If an electrical device were perfect, all the electrical
energy that it uses would be converted into the desired
form of energy.
 However some energy is always converted into heat.
 You can determine the efficiency of an electrical device
by using the following relationship:
Percent efficiency = Useful energy output X 100%
Total energy input
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