Book page 77 – 79
Syllabus 2.8, 2.9, 2.14
Series and parallel circuits
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Find the Fib!
(1) The symbol for a bulb is
(2) In a parallel circuit potential difference is the
same as the supply voltage on all branches.
(3) A voltmeter is used to measure the current
through a device
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(1) The symbol for a resistor is
(2) In a series circuit potential across each
component adds up to the supply voltage.
(3) A voltmeter must be set up in series.
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(1) A switch must be closed for current to flow.
(2) Potential difference is measured using an
ammeter.
(3) A ammeter must be set up in series.
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(1) Potential difference is measured in amps.
(2) The symbol for a switch is
(3) A cell is necessary to push electrons around
a complete circuit.
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Starter
• Are they parallel or in series?
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Aim
• Know how series and parallel circuits are used
• Know the relationship between current, voltage and
resistance
Key words
• Ohm
- Battery
• Ohm’s Law
- Cell
• Resistance
- resistor
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Have you ever wondered
The bigger picture
Where do we need circuits in real life?
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What are series and parallel circuits?
Circuit components can be wired in series or parallel.
A series circuit has all its
components wired in the
same loop.
These tree lights are
wired in series.
A parallel circuit contains
junctions and so there is more
than one path for the current.
Car headlights are wired in
parallel. What would happen
if they were wired in series?
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In a series circuit
• One switch can turn all the components on and off together
• If one bulb or any other component breaks, it causes a gap in the
circuit and all other bulbs will go off
• The voltage supplied by the cell is shared between all components
• The more bulbs are added, the dimmer all light bulbs get
• The larger the resistance of the component, the bigger the share
of voltage
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In a parallel circuit
• Switches can be placed in different parts of the circuit to switch each
bulb on and off individually or all together
• If one bulb or any other component breaks, only the bulbs on the
same branch of the circuit will be affected
• Each branch of the circuit receives the same voltage, so if more
bulbs are added to a circuit in parallel they all stay bright
Total voltage = Voltage branch 1 = Voltage branch 2
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Current in a series circuit
•
•
•
•
•
•
In a series circuit the current is the same in all parts
Current is not used up as it passes around the circuit
Size of current depends on voltage supplied
The current will double if another cell is added in series
The extra energy allows more charge to flow per second
If more bulbs are added, the current gets less, because bulbs have
resistance
• The more bulbs the more resistance in the
circuit
• When the resistance is higher, less current
will flow
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Current in parallel circuits
• In a parallel circuit the current is split
• At each branch the current will divide
• What goes in, must come out
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Fill in the blanks
• If you add two more identical cells into a circuit in series,
treble
the current in the circuit will _______
and the potential
treble
difference will_________.
This is only true if the cells are
connected with the positive and negative terminals in
the same direction
___________.If
you add two more identical bulbs into a
treble
circuit in series , the resistance will ______the
current
decrease
through each bulb will ____________
and the potential
a third of
difference across each bulb will be _________
the potential
difference across the cell
Stay the same, decrease, halve, double, treble, drop by two thirds, the same direction, different directions, a third of, the same as
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Fill in the blanks
• If you add two more identical cells into a circuit in parallel,
treble
the current in the circuit will _______
and the potential
stay the same This is only true if the cells are
difference will_________.
connected with the positive and negative terminals in
the same direction
___________.
• If you add two more identical cells into a circuit in parallel,
drop by two third
the current through each bulb will ____________
and the
the same as
potential difference across each bulb will be _________
the
potential difference across the cell
Stay the same, halve, double, treble, drop by two thirds, the same direction, different directions, a third of, the same as
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Resistance
•
•
•
•
All circuit components offer resistance to the flow of charge
Connecting wires allow charges to flow easily
Wires have a low resistance
Some components need a lot of energy to push the charges
through.
• This energy is usually
converted to heat
• These components have
a high resistance
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Definition of Resistance.
• Resistance is a property of a conductive material that affects how
much electrical current can flow through the material.
• A conductor with a high resistance, is very good at limiting the flow
of electrons through a circuit.
• The units of resistance is the Ohm (Ω).
• Voltage, current and resistance can be linked by Ohm’s law:
Voltage V (V) = current I (A) x Resistance R (Ω)
• 𝑉 =𝐼×𝑅
Example
• When a voltage of 12V is applied across a buzzer a current of 0.1A
flows. Calculate the resistance of the buzzer
• Solution
𝑉
• 𝑅=
• 𝑅=
𝐼
12
0.1
= 120Ω
Different size resistors
6.5 A
A
current
1Ω
A2
2Ω
A3
1.5 Ω
A2
3A
A3
1.5 A
A4
2A
A4
If bulbs with different resistances are connected in parallel
to a 3 V battery, what will the current be through each?
The same voltage will cause a larger current to flow
through a small resistance than a big one, so the bulb with
the lowest resistance will have the biggest current.
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Plenary: Comparing circuits
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Multiple-choice quiz
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Key words
• Ohm – the unit of resistance
• Ohm’s Law - A law relating the voltage difference between
two points, the electric current flowing between them, and
the resistance of the path of the current. V = IR,
• Resistance – the opposition to the flow of charge
• Resistor – a component that opposes the flow of charge
• Battery – two or more cells joined together
• Cell – A chemical source of voltage
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