25/08/2015 P6 Electricity for Gadgets AGAC. 25/08/2015 Circuit Symbols VA Battery Power Supply Capacitor Resistor LDR Voltmeter Ammeter Variable resistor

19/04/23

P6 Electricity for P6 Electricity for GadgetsGadgets

AGAC

19/04/23

Circuit SymbolsCircuit Symbols

VA

Battery

Power SupplyCapacitor

Resistor

LDR

Voltmeter

Ammeter

Variable resistor

Diode

Switch

Bulb

Thermistor

Equations for this unit Equations for this unit (Foundation)(Foundation)

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EquationsForThisUnit

(Higher)

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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

ammeteramps

voltage

voltmetervolts

ohms

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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

increase

pushfaster

decrease

resistance

slower

19/04/23

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

RII is used for current because current

was originally referred to as “electrical intensity”.

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An example question:An example question:

V

A

1) What is the resistance across this bulb?

R = V = 10 = 5

I 2

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

5 x 3 = 15

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?

4

2 2

Difficult!!

Difficult!

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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

3

4

6

20

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F

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.

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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LDRs and ThermistorsLDRs and Thermistors

2) Thermistor – resistance DECREASES when temperature INCREASES

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

Resistance

Amount of light

Resistance

Temperature

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F

d

Divides a value of voltage (1) by the corresponding value of current(1)

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Divides a value of voltage (1) by the corresponding value of current(1)

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F

LDR

thermistor

capacitor

diode

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Potential DividersPotential Dividers

0V

VIN

VOUT

0V

R1

R2

(R1 + R2)

VIN x(R2)VOUT

The Potential Divider equation:

19/04/23Find the output voltage Find the output voltage (Vout)(Vout) 11

0V

VIN

VOUT

0V

R1

R2

0V

50V

VOUT

0V

100

100

(R1 + R2)

VIN x(R2)

VOUT

25VQ1

Example


0V

VIN

VOUT

0V

R1

R2

0V

3V

VOUT

0V

200

100

(R1 + R2)

VIN x(R2)

VOUT

1VQ2

Example


0V

VIN

VOUT

0V

R1

R2

0V

12V

VOUT

0V

5

15

(R1 + R2)

VIN x(R2)

VOUT

9VQ3

Example

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

increase decreaseoffsensitivity

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(2)

input

0V

output

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

The X is like looking at a dart moving away from you

i.e. into the page. This is the direction of the

current in the wire. This produces a clockwise

magnetic field

The dot is like looking at a dart coming towards you i.e. Out of the page.

This is the direction of the current in the wire.

This produces an anticlockwise magnetic field

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

This looks like the solenoid seen on a previous slide

19/04/23

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

19/04/23

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?

move away

stronger magnetic fieldmore currentadd battery/

reverse current OR reverse magnet

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?

Field goes from N to S

Current is comingupwards towards you

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

S

NF =

Force

B =

M

ag

netic

field

I = C

urre

nt

Q. Where will this wire go?

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downwards

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Electric MotorsElectric Motors

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F

3

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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

19/04/23

DC GeneratorsDC GeneratorsBasically, a DC generator is an electric motor in reverse:

1) Remove the battery

2) Turn the coil manually instead. A DC current will be generated in the wires

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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?

Electromagneticinduction

More coilsStronger magnet

Turn it faster

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F

increases

increases

increases

stays the same

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increases

increases

increases

stays the same

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Move magnet away from coil or

move coil away from magnet

19/04/23TransformerTransformerss

Time

Time

Time

Current through primary

Magnetic field

Voltage induced in secondary

TransformersTransformers

• Look back at the previous slide. • The 2 points to realise are• 1 You only get a voltage if the

magnetic field is CHANGING• 2 You get a greater voltage if

the magnetic field is changing more quickly.

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19/04/23

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

step up voltagealternating

magnetic fieldinduce

19/04/23

Some example questionsSome 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 ? ?

400V 200V ? 1000 ?

250V ? 200 20 ?

? 230V 150 1500 ?

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 23V. What is the current from the secondary coil?

400 Step up

Step down

Step down

Step up

2000

25V

23

1:3

5A

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 230 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

powerhigh

step down

high low

19/04/23F

No output/zero

step-down

phone-chargers radio laptops low voltage lightingAny from:

Vout 240

= 200 4000

Vout =12

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Vout 240

= 200 4000

Vout = 12

An output voltage is induced in the secondary coil when a changing magnetic field passes through the coil

The changing magnetic field is produced by the primary coil

DC produces a steady field

AC produces a changing field

Any three from:

19/04/23

Isolating TransformersIsolating Transformers

Words – electrocuted, shaver, voltage, contact, coils, number

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 ____ which have the same _______ of turns that don’t make _____ with each other. This stops the user from getting ___________ from the mains supply.

shavervoltage coils

contactelectrocuted

number

19/04/23

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.

19/04/23

Full-Wave Rectification 1Full-Wave Rectification 1A 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

Full Wave Rectification 2Full Wave Rectification 219/04/23

AC supply

Load

AC supply

Load

+

-

-

+

The current can only flow through the diode in the direction of the arrow

Current flows from positive to negative

First half of the a.c. wave

Second half of the a.c. wave

Notice that the current flows the same way through the load resistor i.e. DC

19/04/23

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

SmoothingSmoothingCapacitors can be used in “smoothing” circuits to “smooth” out a supply:

AC supply

InputOutput without

capacitorOutput with capacitor

19/04/23

Logic GatesLogic GatesLogic gates are the basics behind any kind of processor. There are 5 that you need to know for GCSE:

NOT gate:

AND gate:

OR gate:

19/04/23

Logic GatesLogic Gates

NAND gate:

NOR gate:

19/04/23

Logic gatesLogic gates

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

Input A Input B Output

0 0 0

0 1 0

1 0 0

1 1 1


0 0 0

0 1 1

1 0 1

1 1 1

19/04/23


NAND – “an AND gate and a NOT gate in series”


0 0 1

0 1 1

1 0 1

1 1 0

TIP:You don’t need to learn this separately. Just learn the AND gate and this gate

just has the opposite outputs

19/04/23


NOR – “a NOR gate and a NOT gate in series”


0 0 1

0 1 0

1 0 0

1 1 0

TIP:You don’t need to learn this

separately. Just learn the OR gate and

this gate just has the opposite

outputs

19/04/23

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.

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

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

110

0

1101

J depends on A

O dependsOn B and J



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

J depends on A and B

K depends on C and J

00

0

0

0

0

11

010101

11



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

19/04/23

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

small

larger motorelectromagnet

19/04/23

Latch CircuitsLatch CircuitsA Latch is a switch that stays on after is has been switched on (a burglar alarm, for example).

Bistable Latch circuits can be made using NOR gates:

Bistable latch circuits work by:

1) A brief “on” signal at one ____ results in a permanent “___” signal at the latch ______

2) A brief “on” signal at the other input causes an “___” signal at the latch output

3) An “off” signal at both inputs leaves the latch signal _________.

Words – on, off, unchanged, output, input

inputon

output

off

unchanged

19/04/23

F

lowhigh

once the alarm starts it stays on (1)even if the door is shut orit is reset

s

19/04/23

0

0

0

1

fan needs a large current

logic gate only produces a small current

relay can use low input to switch large fan current

Any two from:

19/04/23

decreases

Input increases

Goes to 1/high/on

or

Allows the temperature at which the fancomes on to be adjusted

Documents

25/08/2015 P6 Electricity for Gadgets AGAC. 25/08/2015 Circuit Symbols VA Battery Power Supply Capacitor Resistor LDR Voltmeter Ammeter Variable resistor