Download 8.3 Electrical Current 2

Electric
Current
Part 2
What is an Electrical Current?
• An electrical current is defined as a flow of positive
charges from a positive electrode to a negative electrode.
What is an Electrical Circuit?
• An electrical circuit is a
complete pathway that
allows positive charges
(actually electrons) to
flow.
Parts of Electrical Circuits
• Most electrical circuits have four main components: a
power source to make a current flow, a path(s) for current
to move through, a device (load) that runs off the energy
that the current gives it and a switch to start and stop
current flow.
Symbols for Electrical Circuits
• When drawing electrical circuits,
symbols are used to represent parts
of the circuit. These symbols need to
be memorized.
Sketch a Circuit
• From the picture
make a circuit
diagram.
Sketch a Circuit
• From the picture make
a circuit diagram.
Sketch a Circuit
• From the picture make
a circuit diagram.
Measuring an Electrical Current
• An electrical current is measured with a device called an
ammeter. This device records the number of charges
passing through it per second. Specifically, a current of 1
ampere (amp) is a flow past a point of 1 coulomb of charge
per second (1 A = 1C/s). The ampere is named in honor of
Andre-Marie Ampere who investigated electrical current.
What is a Coulomb of Charge?
• A coulomb is the amount of charge carried by 6.242 x 1018
electrons or protons. It is a – 6.242 x 1018 electrons or a +
6.242 x 1018 protons. The unit of coulomb is named in
honor of Charles-Augustin de Coulomb who investigated
many laws of electrical charges.
Electrical Current Formula
• I = Q/t (I is current, Q is charge and t is time)
• 1 A = 1 C/s (A is amps of current, C is coulombs of charge
and s seconds of time)
Electrical Current is Measured by an Ammeter
• An ammeter measure electrical current. It is placed in line
so that all the electrical current passes through it.
Voltage, The “Push” Behind or Energy Carried by
the Charge (Current) Moving
• Voltage is defined as the energy carried per unit of charge.
It is the energy in Joules (J) per Coulomb (C) of charge.
• V = E/Q, 1 Volt = 1 Joule/Coulomb
Measuring Voltage Rise and Drop (Voltmeter)
• Since voltage is a potential difference between two
regions, voltage is measured with a voltmeter at two points
in an electrical circuit to see the difference in voltage at
these two points. If the voltage is measured across the
power source, it will be a voltage (energy) rise. If the
voltage is measured across a load, it will be a voltage
(energy) drop.
Measuring Voltage Rise and Drop (Voltmeter)
• Since voltage is a potential difference between two
regions, voltage is measured with a voltmeter at two points
in an electrical circuit to see the difference in voltage at
these two points. If the voltage is measured across the
power source, it will be a voltage (energy) rise. If the
voltage is measured across a load, it will be a voltage
(energy) drop.
Ammeter and Voltmeter : Different Connections
• An ammeter is placed directly in the current flow to
measure it while the voltmeter is placed with connections
at two different locations to measure the voltage difference
at these two points.
Electrical Resistance : Ohms
• Electrical
Resistance is the
property of any
material that slows
the flow of current
through it. There is
resistance to
current flow
because current
bumps up against
atoms and bounces
back before
resuming its flow.
Electrical Resistance : Different for Different
Substances
• Some substances have high
resistance (insulators).
• Some metals have higher
resistance than others
(nichrome is higher than
aluminum which is higher
than copper)
• Copper is commonly used
in electrical circuits
because it has very low
electrical resistance.
Electrical Resistance : Affected by a Conductor’s
Thickness
• The thicker a wire, the lower its electrical resistance, just
like it is easier to suck up a milkshake through a thicker
straw than a thinner straw.
Electrical Resistance : Greater With Longer Wires
• The longer a wire, the greater its electrical resistance since
the current bumps into more atoms along its way, slowing
its progress.
Resistance of Wires and Loads
• In a circuit, the electrical resistance of wires is usually very
low (so that energy is not lost in the wires) but the
electrical resistance of the load(s) is high because they use
up the most of the energy that the charge is carrying.
Electrical Resistance Produces Heat
• The more electrical resistance a wire has, the more heat it
produces. Devices that produce heat (like toaster
elements or stove elements) use conductors with high
electrical resistance. Resistance takes energy from the
current and turns it into heat.
Measuring Electrical Resistance : Ohms
• Electrical resistance is measured in ohms (Ω), a unit
named in honor for Georg Ohm who made many
discoveries about electrical circuits.
Resistors
• A resistor is a device placed in an electrical circuit to
control the flow (amperes) of current. Its symbol is shown
below right.
Resistor Colour
Code
• Resistors come with
colour bands and these
indicate what resistance
the manufacturer made
these with.
Find The Resistor’s Resistance
What is its Resistance?
What is its Resistance?
Ohm’s Law : Relation Between Volts Amps and Resistance
• In an electrical circuit, the more voltage (“push”) applied to
a current, the more current will flow and the more
resistance in the conductor, the less current will flow.
• R=V/I
Ohm’s Law : R = V / I
• In an electrical circuit, the ratio of the voltage to the current
is a constant (the resistance). The more voltage (“push”)
applied to a current, the more current will flow. R = V / I
Ohm’s Law : R = V / I
• In the voltage (“push”) applied to a current doubles (like
the circuit to the right), the current will also double if the
resistance is the same. R = 2V / 2I or R = V / I
Using the Ohm’s Law Equation, R = V / I
• Put the letters of the equation into a triangle as shown with
V over top of I (V/I). To find a resistance, divide V by I. To
find current I, divide V by R. To find voltage V, multiply I
times R. In the pyramid below, the horizontal lines (---)
stand for division and the vertical line (|) stands for
multiplication.
Using the Ohm’s Law Formula for Problems
• In problems, determine what is given and what is to be
determined (the unknown). Rewrite Ohm’s Law to
determine what operation (multiplication or division) needs
to be performed.
• If I is unknown, I = V/R
• If R is unknown, R = V/I
• If V is unknown, V = I x R