Download Electric Current and Circuits

Electric Current and Circuits
Review
Current
CURRENT: a flow of charged
particles (electrons) through
a conductor
Current, I, is measured in
amperes, A, or “amps”.
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I
time
Andre Amperefirst defined current
Circuits
ELECTRIC CIRCUIT: Charges
moving in a closed loop
• A circuit requires a both a
conductor, usually metal
wires, and a “charge pump”.
• CHARGE PUMP: a device
that provides a potential
difference so that charges
keep moving.
• Potential Difference = Voltage
= “pressure”
Alexander Volta- found that
electrical current could be
harnessed and made to flow
Made a “voltaic pile”- the
first battery
Resistance
all conductors offer some resistance to the
flow of charges, even metal wires.
RESISTANCE =
voltage V
R

current I
The unit for resistance is the OHM, W.
This equation is often called OHM’S LAW
voltage V
R

current I
Henry Cavendish, who found a value
for “G”, also experimented with
electricity. In playing with “Leyden
Jars”, he stated that the “velocity” (the
current that flowed out of the jar)
varied directly with the “degree of
electrification” (the voltage). His work
laid the groundwork for George Ohm to
write Ohm’s Law.
George Ohm- first
determined the
math relationship
now called Ohm’s
Law
Resistance in Wires
Thick wires have less resistance
than thin wires.
The filaments in light bulbs are
very thin and offer a lot of
resistance. The thinner the
filament, the higher the
resistance. AND, the higher the
resistance, the brighter the
bulb!!
The filament in a 100W bulb is
thinner than the filament in a
60W bulb!
Certain metals offer less resistance to the
flow of charges than others.
Example: Copper is a better conductor than
iron
The resistance of a wire of length L and
cross sectional area A is given by
RESISTANCE, R =
L
r
A
where r is the resistivity of that particular
metal.
Electric Power
Power is the rate that work is done or
energy is transferred, that is
Energy ( Joules)
Power =
Time (sec onds)
Power is measured in Watts, W
Electric power delivered to a circuit by a
power supply is given by
Power = Current x Voltage
P = IV
This equation can be combined with Ohm’s
Law, R = V / I in its different forms:
V = IR, I = V / R
Examples
How much energy does a 75 W light bulb give off in
five minutes?
Power = Energy / time
Energy = Power x time
Energy = 75 W x 5 x 60 seconds
Energy = 22500 J
What is the power output of a 3 A motor running on
regular house voltage?
P = IV
P = 3A x 120V
P = 360 W
“Power” lines
• The higher the current, the more
the wires in the circuit heat up,
thereby “wasting” energy. This
is a big problem when electric
companies must provide
electricity at great distances
away from the power plants.
• The solution: Electric lines that
carry current great distances
are at very high voltage, so the
current can be small.
P = IV
P = IV
High Voltage / Low Voltage
TRANSFORMERS:
devices that “stepup” the voltage at
the power plant and
then “step-down”
the voltage at the
customers’ location.
V
P
I
V
I
R
SERIES CIRCUITS
The same current through each
device.
EQUIVALENT RESISTANCE:
What is the net resistance?
What one resistor could
replace a group of resistors?
For resistors wired in series, the
equivalent resistance is given
by:
Req = R1 + R2 + R3 + …
PARALLEL CIRCUIT
Same voltage drop across each
device.
For resistors wired in parallel, the
equivalent resistance is given
by:
1
1
1
1



 ...
R eq
R1 R 2 R 3
Use the 1/x or x-1 button on your calculator! But don’t
forget to hit that same button AFTER you hit “EQUAL” in
order to get the equivalent Resistance for parallel circuits!
Example: What is the equivalent resistance
of a 10 W, 20 W, and 30 W resistor wired in
series? In parallel?
Series: Req = 10 + 20 + 30 = 60 W
Parallel 1 / Req = 1/10 + 1/20 + 1/30
Req = 5.45 W
There’s much less resistance if resistors are
wired in parallel than if they’re wired in
series.
With less resistance, the charge pump will
able to push much more current around the
circuit.
As you add more and more resistors in
series, the equivalent resistance
INCREASES. For maximum resistance
and minimum current- use series
wiring.
As you add more and more resistors in
parallel, the equivalent resistance
DECREASES. For minimum resistance
and maximum- use parallel wiring.
Characteristics of Series and
Parallel Wiring
Series: If one component goes out,
They all go out!
As more resistors are added, the equivalent resistance
Increases!
which means that the current in that part of the circuit
Decreases!
Parallel: If one component goes out,
They rest still work!
As more resistors are added, the equivalent resistance
Decreases!
which means that the current in that part of the circuit
Increases!
Schematic diagram of series circuit
with three reisstors
Schematic diagram of parallel
circuit with three resistors
V = IR
2 resistors of 8W and 10W are wired in
series with a 12 V battery. What is the
total current?
To find the current flowing through the
circuit, you first must find the total
(equivalent) resistance of the circuit!
Total Resistance = 8 + 10 = 18W
Total Current = V / R = 12 / 18 =
0.667amps
What is the voltage drop across the 10W
resistor?
Since the resistors are in series, the total
current runs through each resistor, so
DV = IR = 0.67amps x 10W = 6.67V
What is the voltage drop across the 8W
resistor?
DV = IR = 0.67amps x 8W = 5.33V
2 resistors of 8W and 10W are wired in
parallel and the combination is wired to a
12V power supply. What is the total
current?
1 / Total resistance = 1 / 8 + 1 / 10
Total resistance = 4.44W
Total current = V / R = 12 / 4.44 = 2.7amps
Electrical Safety
• Fuse: a short piece of
metal that melts if
current exceeds a set
value (to protect device)
• Circuit breaker: an
automatic switch that
opens the circuit when
current exceeds a set
value. (uses a bimetallic
strip)
• Ground-fault circuit
interrupter: opens a
circuit if the current
going into a device is
not the same as the
current coming out of
the device.
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• Short Circuit- a circuit
that is formed when wires
touch. This effectively
shortens the path of the
circuit because the current
no longer passes through
the resistor. The
extremely low resistance
in the circuit produces very
high current, which could
cause melt down and fire.
•Ground wire: provides
a path for high current to
go in case of a short
circuit- it is attached to
metal bars that literally go
into the ground.
• Ammeter- device to measure amps
(current), measure “in series”
• Voltmeter- device to measure voltage,
measure “in parallel”
• Ohmmeter- device to measure resistance,
measure with NO power supplied to the
circuit!!
• Multimeter- does all three