Download Series and Parallel Circuits

ELECTRICAL CIRCUITS
The CELL
The cell stores chemical energy and transfers it to
electrical energy when a circuit is connected.
When two or more cells are
connected together we call this
a Battery.
The cells chemical energy is
used up pushing a current round
a circuit.
What is an electric current?
An electric current is a flow of microscopic particles
called electrons flowing through wires and
components.
+
-
simple circuits
Here is a simple electric circuit. It has a cell, a
lamp and a switch.
cell
wires
switch
lamp
To make the circuit, these components are connected
together with metal connecting wires.
simple circuits
When the switch is closed, the lamp lights up. This is
because there is a continuous path of metal for the
electric current to flow around.
If there were any breaks in the circuit, the current
could not flow.
circuit diagram
Scientists usually draw electric circuits using symbols;
cell
lamp
switch
wires
circuit diagrams
In circuit diagrams components are represented by
the following symbols;
cell
ammeter
battery
voltmeter
switch
motor
lamp
buzzer
resistor
variable
resistor
types of circuit
There are two types of electrical circuits;
SERIES CIRCUITS
PARALLEL CIRCUITS
SERIES CIRCUITS
The components are connected end-to-end, one
after the other.
They make a simple loop for the current to flow
round.
If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out.
Light Bulbs in Series
• Lights connected in a row, one after the other are
IN SERIES
• REMOVE one bulb and the circuit is BROKEN
• If Circuit is BROKEN, they all go out.
PARALLEL CIRCUITS
The components are connected side by side.
The current has a choice of routes.
If one bulb ‘blows’ there will still be a complete circuit
to the other bulb so it stays alight.
measuring current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
Light Bulbs in Parallel
• Lights connected on different wires that share the
same current flow as the original wire provide
multiple paths for electricity to flow.
• REMOVE one bulb and the circuit is STILL ACTIVE
IN OTHER AREAS.
• Other remaining bulbs still have current and glow
measuring current
This is how we draw an ammeter in a circuit.
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
measuring current
SERIES CIRCUIT
• current is the same
at all points in the
circuit.
2A
2A
2A
PARALLEL CIRCUIT
• current is shared
between the
components
2A
2A
1A
1A
copy the following circuits and fill in the
missing ammeter readings.
3A
?
4A
?
3A
1A
?
4A
?
4A
1A
1A
?
measuring voltage
The ‘electrical push’ which the cell gives to the current
is called the voltage. It is measured in volts (V) on a
voltmeter
V
measuring voltage
Different cells produce different voltages. The
bigger the voltage supplied by the cell, the bigger the
current.
Unlike an ammeter a voltmeter is connected across
the components
Scientist usually use the term Potential Difference
(pd) when they talk about voltage.
measuring voltage
This is how we draw a voltmeter in a circuit.
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
series circuit
• voltage is shared between the components
3V
1.5V
1.5V
parallel circuit
• voltage is the same in all parts of the circuit.
3V
3V
3V
measuring current & voltage
copy the following circuits on the next two
slides.
complete the missing current and voltage
readings.
remember the rules for current and voltage
in series and parallel circuits.
measuring current & voltage
a)
6V
4A
A
V
V
A
measuring current & voltage
b)
4A
6V
A
V
A
V
A
answers
a)
b)
4A
6V
6V
4A
6V
4A
4A
3V
2A
3V
4A
6V
2A
SERIES CIRCUITS - RESISTANCE
Resistance means to “resist” or slow down the flow of
current. RESISTORS are devices, like light bulbs, that
make use of this slowing down of current.
Total Resistance in Series
•Resistance is measured
with an Ohmmeter, named
after the unit of
Resistance, the Ohm
.
•In a series circuit, you
ADD all the resistance in
that wire section together.
3
3  + 3  = 6  total
3
PARALLEL CIRCUITS - RESISTANCE
Resistance along a parallel circuit is reduced because you
give the current another way to go, multiple paths!!
BUT THINGS GET WEEEEEIRD when you add these up...
Total Resistance in Parallel
(1/R1 + 1/R2) = 1/Rtotal
In a parallel circuit, you ADD
the INVERSE of all the
resistance in that wire section
together and then take the
inverse of the answer…
You are making the pathway
BIGGER for the current to flow
through by putting resistors
side by side – parallel.
3
3
1/3  + 1/3  = 2/3  now flip and divide
1/Rtotal = 2/3 (then press 1/x button) = 1.5 
Results
When you added these
two together inversely,
the total resistance
was LOWER than
either of the resistors.
3
3
That is because you
are providing more
area for the charge to
flow, so you are
reducing resistance.
1.5  Total
WHICH IS BRIGHTER?
So… you know that resistance is different in these
circuits. Which type of circuit would allow current
to flow faster, making the brightest bulbs?
SERIES CIRCUITS
?
?
PARALLEL CIRCUITS
?
?
WHICH IS BRIGHTER?
The bulbs in PARALLEL will be brighter because the
circuit allows current to flow more easily
SERIES CIRCUITS
PARALLEL CIRCUITS
Combination Circuits
• Combo circuits contain both parallel and
series portions in one single circuit.
Solving Combo Circuits – STEP 1
• Solve for the Current going through the total circuit.
• STEP 1
• Calculate total
resistance of Parallel
path first.
• (1/3 + 1/9) = 1/0.44
• = 2.25 
Solving Combo Circuits – STEP 2
• Solve for the Current going through the total circuit.
• STEP 2
• Now it is as if the circuit
is just a bunch of
resistors in series,
EASY!
• Add the resistance
together to get the total:
• 2.25 + 12 + 6 = 20.25 
2.25 
Solving Combo Circuits – STEP 3
• Solve for the Current going through the total circuit.
• STEP 3
• Total R = 20.25 
• I = V / R = 120/20.25
• I=6
A
2.25 
Solving Combo Circuits
CHALLENGE
• Solve for the Current going over the 3  resistor ONLY
Solving Combo Circuits – STEP 1
• Solve for the Current going over the 3  resistor
• STEP 1
• Remember that current
along a series circuit is
always the same at all
points.
• Current in a parallel
circuit is shared
according to I = V / R
6A
?
6A
?
6A
6A
Solving Combo Circuits – STEP 2
• Solve for the Current going over the 3  resistor
STEP 2
• Must find the voltage going
across the parallel path!!!
• Voltage at parallel branch
= the total I X total R of
the branch
• Total I = 6 A
• Total R = 2.25 
• V = 13.5 V
6A
13.5 V
13.5 V
6A
?
6A
?
6A
Solving Combo Circuits – STEP 3
• Solve for the Current going over the 3  resistor
STEP 3
• V = 13.5 V
• Now just calculate current
at 3  resistor
• I = V / R = 13.5 / 3 = 4.5 A
• And at the 9  resistor
• I = V / R = 13.5 / 9 = 1.5 A
6A
13.5 V
13.5 V
6A
4.5 A
6A
1.5 A
6A
Power in Circuits
• Remember that power is just a measure of
how fast you do work.
• In circuits Power is the same. Work is
voltage and the rate is Current
• Unit of Power is a Watt
• P = VI
Calculate Current with Power
• A 40 Watt light bulb is connected to a
standard 120 V outlet.
• What is the current being drawn through
the bulb?
• P = VI… I = P / V = 40 / 120 = 0.33 A
• A brighter 100 W bulb?
• 0.83 A
• More current = More power usually