05/09/2015 Electricity for Gadgets W Richards The Weald School OCR Gateway

19/04/23

Electricity for GadgetsElectricity for Gadgets

W Richards

The Weald School

OCR GatewayOCR Gateway

mailto:[email protected]

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P6a ResistingP6a Resisting

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Circuit SymbolsCircuit Symbols

VA

Battery

Cell

Fuse

Resistor

LDR

Voltmeter

Ammeter

Variable resistor

Diode

Switch

Bulb

Thermistor

19/04/2319/04/23Electric Current – A Electric Current – A DefinitionDefinitionElectric 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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More basic ideas…More basic ideas…If a battery is added the current will ________ because there is a greater _____ on the electrons so they move ______

If a bulb is added the current will _______ because there is greater ________ in the circuit, so the electrons move _____

Words – faster, decrease, slower, increase, push, resistance

19/04/23Using Variable Resistors Using Variable Resistors (Rheostats)(Rheostats)

V

A

1) What will happen to the ammeter reading when the resistance is increased?

2) How will this affect the brightness of the bulb?

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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:

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

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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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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

19/04/23Current-Voltage graphs for Current-Voltage graphs for conductorsconductors

1) An Ohmic conductor 2) A non-Ohmic conductor

High resistanc

e

Low resistanc

e

V

I

V

I

In Ohmic conductors the resistance of the

conductor stays the same. The gradient gives

us the resistance.

In non-Ohmic conductors (like filament bulbs) the

resistance of the conductor increases as the current increases (when it

gets hot).

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Understanding ResistanceUnderstanding ResistanceWhen a voltage is applied it basically causes electrons to move towards the positive end of the battery:

Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. What would happen if we increased the temperature of the metal?

IonsElectronsNegative Positive

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P6b SharingP6b Sharing

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Potential DividersPotential Dividers

0V

VIN

VOUT

0V

R1

R2

(R1 + R2)

VIN x(R2)VOUT

The Potential Divider equation:

19/04/23Some example questionsSome example questions

0V

12V

VOUT

0V

100

100

0V

1.5V

VOUT

0V

50

45

0V

50V

VOUT

0V

10

75

0V

3V

VOUT

0V

75

25

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Potential DividersPotential Dividers

0V

VIN

VOUT

0V

R1

R2

Notice the following:

1) If R2 is much greater than R1 then the output voltage is more or less equal to VIN.

2) If R2 is much less than R1 then the output voltage is more or less zero.

3) If two variable resistors are used in place of R1 and R2 then the output voltage will have an adjustable threshold.

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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 Using Thermistors and LDRs in circuitscircuits

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) Try designing your own circuit that warns you when an intruder enters your home, maybe by standing between a source of light and an LDR

19/04/23Practical applicationsPractical applications

0V

Vin

VOUT

Here’s a potential divider that is used to control light-activated switches…

When the light intensity on the LDR decreases its resistance will ________. This causes VOUT to _______ so the processor and output will probably turn _____. The variable resistor can be adjusted to change the ________ of the whole device.

Words – decrease, sensitivity, increase, off

19/04/23Resistors in Series and Resistors in Series and ParallelParallel

The following equations can be used to calculate the total resistance in a circuit:

1 = 1 + 1

RT R1 R2

RT = R1 + R2For resistors in series:

For resistors in parallel:

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Example questionsExample questionsCalculate the equivalent resistance:

1)

3)

2)

4)

10Ω

10Ω

40Ω

20Ω

20Ω

100Ω

100Ω

20Ω

100Ω 50Ω

50Ω

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P6c It’s LogicalP6c It’s Logical

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ComputersComputers

IBM 5100 portable PC (1975), 25kg, 1.9MHz processor, 64K RAM, 200K internal tape storage, cost up to $20,000

Apple 1 (1976) – 1Mhz processor, 4K RAM, cost $670

Commodore 64 (1982) – 1Mhz processor, 64K RAM, 16 colours, cost $600, 17m sold

Apple Macintosh (1984) – 8Mhz processor, 512K RAM, cost $2500

All of these computers (and modern ones) are based on transistors, which have enabled

computers to get smaller over the years.

19/04/23Computers before Computers before TransistorsTransistors

The “Harwell Dekatron” a relay-based computer (first run in 1951):

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TransistorsTransistors

A transistor acts like a switch:

Base

Collector

Emitter

When a SMALL current flows through the base-emitter part of the transistor a different current is switched on through the collector-emitter part.

Ie = Ib + Ic

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Drawing electronic circuitsDrawing electronic circuits1) Two resistors in series

4) An ammeter measuring the current through a resistor

3) A voltmeter measuring the voltage across two bulbs in series

2) Two resistors in series with one bulb in parallel

VIN

0V

VIN

0V

VIN

0V

V

VIN

0V

A

19/04/23Using a transistor as a switch for an Using a transistor as a switch for an LEDLED

9V

0V

Basically, a small current through the base-emitter part of the transistor will “switch on” a larger current through the LED and cause it to emit light.

A high resistor is placed here – why?

19/04/23Combining Combining TransistorsTransistors

Consider this circuit:

Input A Input B Output

Off Off

Off On

On Off

On On

Off

Off

Off

On

The output is only on if A AND B are on. This is called an “AND” gate is a basic logic gate. Other logic gates can be made from transistor

combinations and they usually take voltages of 5V (“on”) or 0V (“off”)

A

B

Out

6V

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Logic gate symbolsLogic gate symbols

NOT gate:

AND gate:

OR gate:

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Logic gatesLogic gatesLogic gates are the basics behind any kind of processor. Here are the three basic ones:

NOT gate – “the output is NOT what the input is”

AND – “the output is on if A AND B are both on”

OR – “the output is on if A OR B are on”

Input Output

0 1

1 0


0 0 0

0 1 0

1 0 0

1 1 1


0 0 0

0 1 1

1 0 1

1 1 1

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NAND and NOR gatesNAND and NOR gates

NAND gate:

NOR gate:


0 0 1

0 1 1

1 0 1

1 1 0


0 0 1

0 1 0

1 0 0

1 1 0

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P6d Even More LogicalP6d Even More Logical

19/04/23Complex Logic Gate Complex Logic Gate problemsproblems

Work out the truth tables for the following arrangements:

A

B

JO

A B J O

0011

0101



A

B

JO

A B J O

0011

0101

1100

1101



A

B

C

J

K O

A B C J K O

00001111

00110011

01010101



A

B

C

J

K O

A B C J K O

00001111

00110011

01010101

00000011

01010111

10101000



A B C D J K L O

0000000011111111

0000111100001111

0011001100110011

0101010101010101

A

B

CD

J

K L

O



A B C D J K L O

0000000011111111

0000111100001111

0011001100110011

0101010101010101

0000000000001111

0111011101110111

1000100010001000

0000000000001000

A

B

CD

J

K L

O

19/04/23Using LDRs and Thermistors as Using LDRs and Thermistors as inputsinputs

Thermistors and LDRs can be used as the input to a logic gate:

When the light intensity on the LDR decreases its resistance will ________. This causes the input to the ____ gate to turn ___ so the output will turn on. The variable resistor can be adjusted to change the ________ of the whole device.

Words – AND, sensitivity, increase, on

5V6V

Output

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Some problems to solveSome problems to solve• The pump on a central heating system is switched

on at room temperature if the system is switched on (with the slide switch). When the temperature rises the pump needs to be switched off.

• Design a circuit that will sound a buzzer if the temperature of a hot radiator falls during the day ONLY. Include a test switch to check the operation of the buzzer.

• Design a circuit for a gardener that will warn them of cold conditions at night. The alarm should be able to be switched off.

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Using LEDs with Logic GatesUsing LEDs with Logic GatesLEDs work on low power so they can be used to indicate the status of the output from a logic gate:

A

B

JO

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RelaysRelaysCircuits such as those containing logic gates only take very ______ currents. These circuits may be needed to operate a device that takes a much _____ current, e.g. a _________. To do this the circuit would need a RELAY switch, a device made of an _______ that can operate a switch. A relay switch is activated by the small current and the switch part is placed in the circuit needing a large current:

Symbol for relay:

Words – motor, larger, electromagnet, small

M

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P6e MotoringP6e Motoring

19/04/23Comparing magnets and Comparing magnets and solenoidssolenoids

N SMagnet:

Solenoid:

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Magnetic Field around a Magnetic Field around a current-carrying wirecurrent-carrying wire

“Right hand corkscrew” or “right hand grip” rule

19/04/23Magnetic field around a Magnetic field around a rectangular coilrectangular coil

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The Motor EffectThe Motor Effect

S

N

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?

19/04/23Current-carrying wire in a magnetic Current-carrying wire in a magnetic fieldfield

S

N

F = Force

B = Magnetic

field

I = Current

Q. Where will this wire go?

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Revision of DC and ACRevision of DC and AC

DC stands for “Direct Current” – the current only flows in one direction:

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)

1/50th s

230V

V

V

Time

T

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Electric MotorsElectric Motors

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Electric MotorsElectric Motors

The “split ring commutator” changes the current every half

rotation (hence the red-blue colour change)

Notice the curved poles – these will give a greater magnetic field strength near the coil

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P6f GeneratingP6f Generating

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Electromagnetic InductionElectromagnetic Induction

N

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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ElectromagnetElectromagnetic inductionic 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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AC GeneratorsAC Generators

Magnetic Field

Slip rings and brushes

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AC GeneratorsAC Generators

SN

Voltage

Time

19/04/23Questions on the AC Questions on the AC GeneratorGenerator

1) How does the generator work?

2) How would you increase its output? Give two answers

3) How would you increase the frequency?

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P6g TransformingP6g Transforming

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TransformersTransformers

A transformer is basically a device made of two coils of wire around an iron core:

The circuit diagram for a transformer:

19/04/23TransformerTransformerss

Time

Time

Time

Current through primary

Magnetic field

Voltage induced in secondary

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TransformersTransformersTransformers 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)

…and the current changes using this formula:

VPIP = VSIS

19/04/23Simple transformer Simple transformer questionsquestions

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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More example questionsMore 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 current of 0.5A is supplied to a transformer that steps down a voltage from 230V to 12V. What is the current from the secondary coil?

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Isolating TransformersIsolating Transformers

Words – electrocuted, shaver, voltage, contact, coils

An isolating transformer is used in some mains circuits (for example, a bathroom _____ socket). Isolating transformers do not change the _____; they simply consist of two ______ that don’t make _____ with each other. This stops the user from getting ___________ from the mains supply.

19/04/23Transformers in the National Transformers in the National GridGrid

Electricity reaches our homes from power stations through the National Grid:

If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much ______ loss by the time electricity reaches our homes. This is because the current is ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage is _____ and the current and power loss are both ____.

Words – step down, high, power, low, high

Power stationStep up

transformerStep down

transformerHomes

19/04/23Power through the National Power through the National GridGrid

The following equation can be used to calculate the amount of power loss in the National Grid’s cables:

Power = I2R

1) A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor.

2) A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor.

3) A 2KW heater has a resistance of 20 Ω. Calculate the current through it.

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Transformers and PowerTransformers and PowerIf this transformer is 100% efficient then the power output from the secondary coil is the same as the power input to the primary coil.

Power = voltage x current

in W in V in A

In previous units we came across this equation:

If we apply this equation to transformers and assume 100% efficiency we conclude:

Vp x Ip = Vs x Is

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Example questionsExample questions

1) A transformer connected to the mains supply in the UK runs on a voltage of 230V and a current of 0.1A. If its output voltage is 12V what is its output current?

2) Another step down transformer outputs 120V and 2A. If it runs from the mains what is its input current?

3) A step up transformer near a power station would convert 20,000V and 10A into 300,000V and what current?

1.9A

1.04A

0.07A

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P6h ChargingP6h Charging

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DiodesDiodesI

V

A diode is a device that only allows current to flow in one direction:

If alternating current is passed through a diode it becomes “half-wave rectified”:

V

T

V

T

The current flows easily in the forwards direction but there is very high resistance to the reverse current.

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-+

-+

+

++

+-

- -

-

Holes Electrons

How Diodes workHow Diodes work

Diodes work by having areas of extra electrons and areas

of extra “holes”:

When the battery is connected like this the electrons move towards the holes and jump from one hole to another, which makes it seem like the holes are moving.

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Full-Wave RectificationFull-Wave RectificationA group of 4 diodes can be used to make a “bridge rectifier” circuit to make full-wave rectification:

AC supply

Load

V

T

V

T

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The CapacitorThe CapacitorA capacitor is a device that can store charge (it has a “capacity”). Here’s how they work:

e

19/04/23Charging and discharging a Charging and discharging a capacitorcapacitor

P.d. across capacitor

Time

Current in circuit

TimeP.d. across capacitor

Time Time

Current in circuit

19/04/23

SmoothingSmoothingMany devices need a constant voltage supplied to them, rather than AC. Capacitors can be used in “smoothing” circuits to “smooth” out a supply:

AC supply

InputOutput without

capacitorOutput with capacitor

Documents

05/09/2015 Electricity for Gadgets W Richards The Weald School OCR Gateway