03/07/2015 p5 electric circuits ocr 21 st century georg simon ohm 1789-1854

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P5 Electric P5 Electric CircuitsCircuitsOCR 21OCR 21stst Century Century

Georg Simon Ohm 1789-1854

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P5.1 Electric Current – a Flow of What?

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Static ElectricityStatic Electricity

An introduction – click here

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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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Static ElectricityStatic Electricity

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Short Static ExperimentsShort Static Experiments19/04/23

Try the following quick static electricity experiments:

1) Rubbing a balloon on your jumper and “sticking” it to the wall

2) Charging a plastic rod by rubbing it with a cloth and then holding it near the water from a smooth-running tap

3) Charging a plastic rod and trying to pick up small pieces of paper (or someone else’s hair!) with it

4) Rubbing a balloon on someone else’s head – you might want to ask their permission first…

Can you explain what you saw in each of these experiments?

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Gold Leaf ElectroscopesGold Leaf ElectroscopesConsider a gold-leaf electroscope…

Now charge the top:

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This effect was seen because charges can easily move through

conductors

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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”

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Basic ideas…Basic ideas…Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.

Resistance is anything that resists an electric current. It is measured in _____.

Words: volts, amps, ohms, voltage, ammeter, voltmeter

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Understanding CurrentWhen a voltage is applied it basically causes the electrons in a conductor to move towards the positive end of the battery:

The main difference between conductors and insulators is that insulators have less of these free electrons.

IonsElectronsNegative Positive

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P5.2 Current and Energy

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Circuit SymbolsCircuit Symbols

VA

Battery

Cell

Fuse

Resistor

LDR

Voltmeter

Ammeter

Variable resistor

Diode

Switch

Bulb

Thermistor

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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

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Electric Current revisitedElectric Current revisitedWork is done by the battery to move the electrons around the circuit.

-+ e-

e-

When the electons arrive at the bulb energy is

transferred to it.

Note that we usually assume the resistance of these leads is very small.

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Electrical Power revisionElectrical Power revision

The amount of power being transferred in an electrical device is given by: P

IVPower = voltage x

current

in W in V in A1) How much power is transferred by a 230V fire that

runs on a current of 10A?

2) An electric motor has a power rating of 24W. If it runs on a 12V battery what current does it draw?

3) An average light bulb in a home has a power rating of 60W and works on 230V. What current does it draw?

Power is defined as “the rate of transferring energy” and is measured in units called “Watts” (W).

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Understanding ResistanceRecall our previous model of electric current but this time we’ll use it to explain resistance:

Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. This effect causes the metal to heat up.

IonsElectronsNegative Positive

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Using this heating effectUsing this heating effect19/04/23

This heating effect can have its advantages and its disadvantages. For example, consider an old-fashioned light bulb:

This heating effect causes the filament to emit light…

…but it also causes a lot of energy to be wasted to the environment

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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

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An example question:An example question:

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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

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More examples…More examples…

12V

3A

3A

6V

4V

2A

1A

2V

What is the resistance of these bulbs?

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ResistanceResistanceResistance 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

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LDRs and ThermistorsLDRs and Thermistors

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Two simple components:Two simple components:

2) Thermistor – resistance DECREASES when temperature INCREASES

1) Light dependant resistor – resistance DECREASES when light intensity INCREASES

Resistance

Amount of light

Resistance

Temperature

19/04/23Using Thermistors and LDRs in circuits

V

A

1) What will happen to the resistance of the thermistor when it gets hotter?

2) How will this affect the brightness of the bulb and the reading on the ammeter?

3) How will this affect the readings on the ammeter and voltmeter?

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Resistors in Series and ParallelConsider the total resistance of these combinations:

10Ω 10Ω10Ω

10Ω

The total resistance of this combination is 20Ω as the

battery has to move charges through both

resistors

The total resistance of this combination is only 5Ω as

the charges have more paths to move through

19/04/2319/04/23Current-voltage graph for a Current-voltage graph for a ResistorResistorI

V

Resistor

Notice that a current-voltage graph for a resistor of fixed value shows that current increases in proportion to voltage.

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P5.3 How do Parallel and Series Circuits work?

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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

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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)…

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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…

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Some example questions…Some example questions…

3A

6A

1A each

4A

2A

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Voltage and Work doneWhat does “voltage” mean?

The voltage (or potential difference) between two points is a measure of the work done per “bit” of charge moving between these points.

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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

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Voltage in a series circuitVoltage in a series circuit

V

V V

Notice that the voltages add up to the voltage across the battery – this is because the work done on each unit of charge by the battery must equal the work done by it to the bulbs.

19/04/23What if the Resistances are different?

60V

V1 V2

10Ω 20Ω

Q. What would each of these voltmeters read?

Total resistance = 30Ω

Therefore current = 2A

Therefore V1 = 2x10 = 20V and and V2 = 2x20

= 40V

The voltage across the bigger resistance is higher as more work is done by the battery to get the current

through it.

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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

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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

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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”

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An example question:An example question:

V1

V2

6V

3A

A2

A3

V3

A1

3A

2A

1A

6V

3V 3V

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Another example question:Another example question:

V1

V2

10V3A

A2

A3

V3

A1

3A

1.2A

1.8A

6.7V

5V 5V

19/04/23P5.4 How is Mains Electricity Produced?

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Electromagnetic Induction

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The direction of the induced current is reversed if…

1) The wire is moved in the opposite direction

2) The field is reversed

The size of the induced current can be increased by:

1) Increasing the speed of movement

2) Increasing the magnet strength

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Electromagnetic induction

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

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Transformers

Time

Time

Time

Current through primary

Magnetic field

Voltage induced in secondary

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Transformers

Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.

Words – alternating, magnetic field, induce, step up, voltage

We can work out how much a transformer will step up or step down a voltage:

Voltage across primary (Vp)

No. of turns on secondary (Ns)

Voltage across secondary (Vs)

No. of turns on primary (Np)

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Some transformer questions

Primary voltage

Vp

Secondary voltage

Vs

No. of turns on

primary Np

No. of turns on

secondary Ns

Step up or step down?

12V 24V 100 ? ?

400V 200V 20 ? ?

25,000V 50,000V 1,000 ? ?

23V 230V 150 ? ?

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Some example questionsPrimary voltage

Vp

Secondary voltage

Vs

No. of turns on

primary Np

No. of turns on

secondary Ns

Step up or step down?

6V 24V 100 ? ?

400,000V 200V ? 1,000 ?

25,000V ? 20,000 20 ?

? 230V 150 1,500 ?

1) A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil?

2) A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V?

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AC Generators

Magnetic Field

Slip rings and brushes

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Generators (dynamos)Generators (dynamos)The coil already has an iron

core, but the induced current could still be increased by:

1) Increasing the speed of movement

2) Increasing the magnetic field strength

3) Increasing the number of turns on the coil

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Large-scale production of ElectricityLarge-scale production of Electricity19/04/23

A generator at Drax power station in England

19/04/23AC Generator

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

N S N S

Current

Time

19/04/23DC and AC

DC stands for “Direct Current” – the current only flows in one direction and a common example is a battery:

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz). In the UK mains electricity is 230V AC, not DC, as AC is easier to generate and transmit over long distances.

1/50th s

230V

V

V

Time

T

19/04/23P5.5 How do Electric Motors work?

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The Motor Effect

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1) What will happen to this wire?

2) How can you make it move faster?

3) How can you make it move in a different direction?

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Electric Motors

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Using MotorsThe following devices are just some examples of devices that use electric motors: