Download electrical current

Electricity
What is it and how does it work?
Some definitions to start with
•An electrical current is caused by the flow of negatively charged electrons in
a conductor
•A conductor is a material that allows the electrical current to flow easily.
Examples include copper and iron.
•An insulator does not allow electrical current to pass through it easily.
Examples include air, wood, glass. Can you suggest a few others?
•Current is measured in Amps (I), also known as amperes, with an Ammeter
•The potential of the current is called the Voltage (V) and measured with a
voltmeter. Also known as potential difference.
•A resister (R) is something that reduces or slows the current and is measured
in Ohms (). Some conductors are more resistant that others. The greater the
diameter and the shorter its length, the smaller the resistance
•A circuit is the path along which the electrical current can flow
•A switch can be used to break a current in a circuit
•Electrical current can be one of two kinds, AC and DC…
The difference between AC & DC
AC stands for alternating current and means that
the flow of electrons can move in both directions.
The electrical circuits in our homes are AC and
they alternate, or move back and forth, about fifty
times every second. This type of current is
produced by generators similar to those in power
stations. It is more efficient than DC but more
dangerous if you get an electric shock. AC shock
causes a person’s muscles to contract and
remain that way so they may not be able to let go
of the source of the shock.
DC stands for direct current and
means that the current flows in one
direction only, from the negative
terminal to the positive terminal. This
type of current is produced in
batteries. Shocks from these currents
produce a sudden jolt and can throw
a person across the room.
Remember
‘AC holds &
DC throws’
Drawing a Circuit – the symbols
Wire
Battery
Resistor
+
Open
switch
Lamp
Ammeter
A
Closed
switch
Voltmeter
V
Motor
M
Drawing a Circuit in Series
In a series circuit, the current can follow only one
path. The following series circuits have two lamps
and two switches joined to a single battery.
If both switches are closed the
circuit is complete and both
the lamps glow
If one or both of the
switches are open then the
circuit is broken and the
current cannot flow around
to make a complete circuit.
Neither lamp glows
Drawing a Circuit in Parallel
In a parallel circuit, the current can follow more than
one path. The following parallel circuits have two
lamps and two switches joined to a single battery.
As with the series circuit, if both switches
are closed then both lamps glow
Unlike the series circuit, when in parallel
one switch open breaks only that one
circuit. With the other switch closed a
continuous circuit allows that lamp to
glow. Our homes are on parallel circuits
for this reason – as soon as we turn off
one light switch we don’t loose the
electricity to ALL our lights!!!!
As with the series circuit, if both switches
are open then neither lamps glow
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+
++
++
++
- - -- --
- - -
Flow of electrons
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-
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- -
So how does electricity
flow through a circuit?
The battery
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Flow of electrons
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- - -
- - -
- - -
- - -
- - -
-
- - -
- - Flow of electrons
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-
-
++
++
++
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- -
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-
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-
-
-
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- - -
-
- -
- -
- - -- Copper wire conductor
------The resistance in
the wire causes
the wire to heat
++
- + and produce light
-- +++ --- - -- --The narrow tube
represents tungsten in
-- a lamp. The tungsten
creates a resistance to
-the flow of electrons.
- - - - - - - - - - - - -
So how does electricity flow through a circuit?
•To understand how a current travels through a conductor,
and how it reacts or changes when it comes across a resistor
can be explained using a water model.
•The following model will help you imagine what is happening
to the electrical current by relating it to a water current.
•Remember that it is a model and does not fit in every way.
The water model
Pressure (P)
before the
resistor is
greater than…
Think of the
Volts being
the pressure
of the water
Think of the quantity of
water crossing a point
in the circuit as the
Amps or Intensity
…Pressure (P) after the resistor
A narrow
pipe can
represent
a Resistor
Therefore a
potential difference
is created across
the resistor
Understanding how volts and amps react
within a circuit – Kirchoff’s Laws
•One of Kirchoff’s Laws states that the current flowing into the fork in a
a parallel circuit is equal to the sum of the current in each of the
possible choices (considering that the resistance is equal in both of the
parallel circuits).
•This means that I1 = I2 + I3 as shown on the following diagram:
I2
I1
I1
I3
•If the current splits 4 times then the same rule applies.
•I1 = I2 + I3 + I4 + I5. What would this look like on a circuit diagram?
•Look at the following diagram and calculate the amps for I3.
Hopefully your
answer was 3A
Understanding how volts and amps react
within a circuit – Kirchoff’s Laws
•Another of Kirchoff’s Laws states that in a series circuit the
voltage drop across a set of resistors must equal the input
voltage of the battery.
•If there were three resistors this means that V0 = V1 + V2 + V3
V0 = 9V
V1
V2
V3
•If these three lamps were identical then the voltage would be
evenly shared across them. This means that V1 = V2 = V3. In this
case, what is the value of V1, V2 and V3?
Understanding how volts and amps react
within a circuit – Kirchoff’s Laws
How does this law work when the lamps are in parallel?
V0 = 9V
V1 =
9V
V2 =
9V
For each of the two circuits, the law states that the voltage across
the resistor(s) in the circuit must equal the input voltage of the
battery…therefore…
Understanding how volts, amps and resistors
interact with each other – Ohm’s Law
•Ohm’s Law states that the voltage in a circuit is equal to the product of
the current (amps) and the resistance
•This means that voltage = current or amps x resistance
•This is written as V = IR
•Here’s an example: if you are using a 9V battery in a lighting circuit and
read only 0.05A in the circuit, how much resistance must there be in the
circuit?
•The answer is: V = IR so, 9 = 0.05 x R or 9/0.05 = 180 Ohms
•Try this one: Given the following circuit, find the current that flows
through it.
•V = IR so, 3 = I x 6 = 3/6 = 0.5 A
The End