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12/06/22 OCR Additional Science OCR Additional Science Electric Circuits Electric Circuits

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Page 1: P5 Electric Circuits

12/04/23

OCR Additional ScienceOCR Additional Science

Electric CircuitsElectric Circuits

Page 2: P5 Electric Circuits

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Static ElectricityStatic ElectricityStatic electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used:

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Page 3: P5 Electric Circuits

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Static ElectricityStatic Electricity

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Page 4: P5 Electric Circuits

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Circuit SymbolsCircuit Symbols

VA

Battery

Cell

Fuse

Voltmeter

Ammeter

Switch

Bulb

Page 5: P5 Electric Circuits

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Electric CurrentElectric CurrentElectric current is a flow of negatively charged particles (i.e. electrons).

Note that electrons go from negative to positive-+ e-

e-

By definition, current is “the rate of flow of

charge”

Page 6: P5 Electric Circuits

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More basic ideas…More basic ideas…

If a battery is added the current will ________ because there is a greater _____ on the electrons

If a bulb is added the current will _______ because there is greater ________ in the circuit

Page 7: P5 Electric Circuits

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Current in a series circuitCurrent in a series circuit

If the current here is 2 amps…

The current here will be…

The current here will be…

And the current here will be…

In other words, the current in a series circuit is THE SAME at any

point

Page 8: P5 Electric Circuits

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Current in a parallel circuitCurrent in a parallel circuit

A PARALLEL circuit is one where the current has a “choice of routes”

Here comes the current…

And the rest will go down here…

Half of the current will go down here (assuming the bulbs are the same)…

Page 9: P5 Electric Circuits

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Current in a parallel circuitCurrent in a parallel circuit

If the current here is 6 amps

The current here will be…

The current here will be…

The current here will be…

And the current here will be…

Page 10: P5 Electric Circuits

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Some example questions…Some example questions…

3A

6A

Page 11: P5 Electric Circuits

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Voltage in a series circuitVoltage in a series circuit

V

V V

If the voltage across the battery is 6V…

…and these bulbs are all identical…

…what will the voltage across each bulb be? 2V

Page 12: P5 Electric Circuits

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Voltage in a series circuitVoltage in a series circuit

V

V

If the voltage across the battery is 6V…

…what will the voltage across two bulbs be?

4V

Page 13: P5 Electric Circuits

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Voltage in a parallel circuitVoltage in a parallel circuit

If the voltage across the batteries is 4V…

What is the voltage here?

And here?

V

V4V

4V

Page 14: P5 Electric Circuits

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SummarySummary

In a SERIES circuit:

Current is THE SAME at any point

Voltage SPLITS UP over each component

In a PARALLEL circuit:

Current SPLITS UP down each “strand”

Voltage is THE SAME across each”strand”

Page 15: P5 Electric Circuits

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An example question:An example question:

V1

V2

6V

3A

A1

A2

V3

A3

Page 16: P5 Electric Circuits

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Another example question:Another example question:

V1

V2

10V3A

A1

A2

V3

A3

Page 17: P5 Electric Circuits

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Georg Simon Ohm 1789-1854

ResistanceResistance

Resistance is anything that will RESIST a current. It is

measured in Ohms, a unit named after me.

The resistance of a component can be calculated using Ohm’s Law:

Resistance = Voltage (in V)

(in ) Current (in A)

V

RI

Page 18: P5 Electric Circuits

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An example question:An example question:

V

A

1) What is the resistance across this bulb?

2) Assuming all the bulbs are the same what is the total resistance in this circuit?

Voltmeter reads 10V

Ammeter reads 2A

Page 19: P5 Electric Circuits

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More examples…More examples…

12V

3A

3A

6V

4V

2A

1A

2V

What is the resistance of these bulbs?

Page 20: P5 Electric Circuits

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ResistanceResistance

Resistance is anything that opposes an electric current.Resistance (Ohms, ) = Potential Difference (volts, V)

Current (amps, A)

What is the resistance of the following:

1) A bulb with a voltage of 3V and a current of 1A.

2) A resistor with a voltage of 12V and a current of 3A

3) A diode with a voltage of 240V and a current of 40A

4) A thermistor with a current of 0.5A and a voltage of 10V

Page 21: P5 Electric Circuits

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Electromagnetic Electromagnetic inductioninduction

The direction of the induced current is reversed if…

1) The magnet is moved in the opposite direction

2) The other pole is inserted first

The size of the induced current can be increased by:

1) Increasing the speed of movement

2) Increasing the magnet strength

3) Increasing the number of turns on the coil

Page 22: P5 Electric Circuits

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Generators (dynamos)Generators (dynamos)

Induced current can be increased in 4 ways:

1) Increasing the speed of movement

2) Increasing the magnetic field strength

3) Increasing the number of turns on the coil

4) Increasing the area of the coil

Page 23: P5 Electric Circuits

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Power and fusesPower and fuses

Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by:

P

IVPower = voltage x current

in W in V in A

Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).

Page 24: P5 Electric Circuits

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Power and fusesPower and fuses

Copy and complete the following table:

Appliance Power rating (W)

Voltage (V) Current needed (A)

Fuse needed (3, 5

or 13A)

Toaster 960 240

Fire 2000 240

Hairdryer 300 240

Hoover 1000 240

Computer 100 240

Stereo 80 240

Page 25: P5 Electric Circuits

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Energy and PowerEnergy and PowerThe POWER RATING of an appliance is simply how much energy it uses every second.

In other words, 1 Watt = 1 Joule per second

E

TP

E = Energy (in joules)

P = Power (in watts)

T = Time (in seconds)

Page 26: P5 Electric Circuits

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Some example questionsSome example questions

1) What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds?

2) What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?

3) Farhun runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating?

4) How much energy does a 150W light bulb transfer in a) one second, b) one minute?

5) Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson?

6) Damien’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?

Page 27: P5 Electric Circuits

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The Cost of ElectricityThe Cost of ElectricityElectricity is measured in units called “kilowatt hours” (kWh). For example…

A 3kW fire left on for 1 hour uses 3kWh of energy

A 1kW toaster left on for 2 hours uses 2kWh

A 0.5kW hoover left on for 4 hours uses __kWh

A 200W TV left on for 5 hours uses __kWh

A 2kW kettle left on for 15 minutes uses __kWh

Page 28: P5 Electric Circuits

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The Cost of ElectricityThe Cost of ElectricityTo work out how much a device costs we do the following:

Cost of electricity = Power (kW) x time (h) x cost per kWh (p)

For example, if electricity costs 8p per unit calculate the cost of the following…

1) A 2kW fire left on for 3 hours

2) A 0.2kW TV left on for 5 hours

3) A 0.1kW light bulb left on for 10 hours

4) A 0.5kW hoover left on for 1 hour

48p

8p

8p

4p

Page 29: P5 Electric Circuits

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EfficiencyEfficiencyEfficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it.For example, consider a TV:

Electrical Energy (200J)

Light (80J)

Sound (40J)

Heat (?)

Efficiency = Useful energy out

Energy in

x100%

Page 30: P5 Electric Circuits

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Some examples of efficiency…Some examples of efficiency…1) 5000J of electrical energy are put into a motor. The

motor converts this into 100J of movement energy. How efficient is it?

2) A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go?

3) A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it?