Series and parallel circuits

Book page 77 – 79

Syllabus 2.8, 2.9, 2.14

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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? ©cgrahamphysics.com 2016

Circuit components can be wired in series or parallel.

A parallel circuit contains

junctions and so there is more

than one path for the current.

A series circuit has all its

components wired in the

same loop.

These tree lights are

wired in series.

Car headlights are wired in

parallel. What would happen

if they were wired in series?

What are series and parallel circuits?

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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,

the current in the circuit will _______ and the potential difference will_________. This is only true if the cells are connected with the positive and negative terminals in ___________.If you add two more identical bulbs into a circuit in series , the resistance will ______the current through each bulb will ____________ and the potential difference across each bulb will be _________ the potential difference across the cell

treble

treble

the same direction

decrease

a third of

Stay the same, decrease, halve, double, treble, drop by two thirds, the same direction, different directions, a third of, the same as

treble

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Fill in the blanks • If you add two more identical cells into a circuit in parallel,

the current in the circuit will _______ and the potential difference will_________. This is only true if the cells are connected with the positive and negative terminals in ___________.

• If you add two more identical cells into a circuit in parallel, the current through each bulb will ____________ and the potential difference across each bulb will be _________ the potential difference across the cell

treble

stay the same

the same direction

drop by two third

the same as

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

• 𝑅 =𝑉

𝐼

• 𝑅 =12

0.1= 120Ω

Different size resistors

A 1 Ω

2 Ω

1.5 Ω

If bulbs with different resistances are connected in parallel

to a 3 V battery, what will the current be through each?

A3

A2

A4

6.5 A current

A2

A3

A4

3 A

1.5 A

2 A

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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