Electricity for Gadgets

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Electricity for Gadgets
OCR Gateway
W Richards The Weald School
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P6a Resisting
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Circuit Symbols
12/03/2017
Variable resistor
Diode
Switch
Bulb
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery
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Electric Current A Definition
12/03/2017
Electric current is a flow of negatively charged
particles (i.e. electrons).
By definition, current is the rate of flow of
charge
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Basic ideas
Words volts, amps, ohms, voltage, ammeter,
voltmeter
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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
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Using Variable Resistors (Rheostats)

• What will happen to the ammeter reading when the
  resistance is increased?
• How will this affect the brightness of the bulb?

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Resistance
The resistance of a component can be calculated
using Ohms Law
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An example question

1. What is the resistance across this bulb?
2. Assuming all the bulbs are the same what is the
   total resistance in this circuit?

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More examples
6V
12V
What is the resistance of these bulbs?
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Resistance
Resistance is anything that opposes an electric
current.

• What is the resistance of the following
• A bulb with a voltage of 3V and a current of 1A.
• A resistor with a voltage of 12V and a current of
  3A
• A diode with a voltage of 240V and a current of
  40A
• A thermistor with a current of 0.5A and a voltage
  of 10V

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Current-Voltage graphs for conductors
1) An Ohmic conductor
2) A non-Ohmic conductor
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 Resistance
When a voltage is applied it basically causes
electrons to move towards the positive end of the
battery
Negative
Positive
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?
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P6b Sharing
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Potential Dividers
VIN x
(R1 R2)
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Some example questions
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Potential Dividers

• Notice the following
• If R2 is much greater than R1 then the output
  voltage is more or less equal to VIN.
• If R2 is much less than R1 then the output
  voltage is more or less zero.
• 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 Thermistors
12/03/2017
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Two simple components
12/03/2017
2) Thermistor resistance DECREASES when
temperature INCREASES
1) Light dependant resistor resistance
DECREASES when light intensity INCREASES
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Using Thermistors and LDRs in circuits

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

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Practical applications
Heres 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
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Resistors in Series and Parallel
The following equations can be used to calculate
the total resistance in a circuit
RT R1 R2
For resistors in series
For resistors in parallel
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Example questions
Calculate the equivalent resistance
40O
1)
10O
2)
20O
10O
20O
100O
50O
100O
3)
4)
20O
100O
50O
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P6c Its Logical
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Computers
All of these computers (and modern ones) are
based on transistors, which have enabled
computers to get smaller over the years.
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Computers before Transistors
The Harwell Dekatron a relay-based computer
(first run in 1951)
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Transistors
A transistor acts like a switch
Collector
When a SMALL current flows through the
base-emitter part of the transistor a different
current is switched on through the
collector-emitter part.
Base
Emitter
Ie Ib Ic
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Drawing electronic circuits
1) Two resistors in series
2) Two resistors in series with one bulb in
parallel
4) An ammeter measuring the current through a
resistor
3) A voltmeter measuring the voltage across two
bulbs in series
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Using a transistor as a switch for an LED
9V
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.
0V
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Combining Transistors
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)
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Logic gate symbols
NOT gate
AND gate
OR gate
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Logic gates
NOT gate the output is NOT what the input is
Logic gates are the basics behind any kind of
processor. Here are the three basic ones
Input Output
0 1
1 0
Input A Input B Output
0 0 0
0 1 0
1 0 0
1 1 1
AND the output is on if A AND B are both on
Input A Input B Output
0 0 0
0 1 1
1 0 1
1 1 1
OR the output is on if A OR B are on
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NAND and NOR gates
NAND gate
Input A Input B Output
0 0 1
0 1 1
1 0 1
1 1 0
Input A Input B Output
0 0 1
0 1 0
1 0 0
1 1 0
NOR gate
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P6d Even More Logical
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Complex Logic Gate problems
Work out the truth tables for the following
arrangements
A B J O
0 0 1 1 0 1 0 1
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Complex Logic Gate problems
Work out the truth tables for the following
arrangements
A B J O
0 0 1 1 0 1 0 1 1 1 0 0 1 1 0 1
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Complex Logic Gate problems
Work out the truth tables for the following
arrangements
A B C J K O
0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1
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Complex Logic Gate problems
Work out the truth tables for the following
arrangements
A B C J K O
0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 1 1 0 1 0 1 0 1 1 1 1 0 1 0 1 0 0 0
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Complex Logic Gate problems
Work out the truth tables for the following
arrangements
A B C D J K L O
0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
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Complex Logic Gate problems
Work out the truth tables for the following
arrangements
A B C D J K L O
0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0
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Using LDRs and Thermistors as inputs
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
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Some 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 Gates
LEDs work on low power so they can be used to
indicate the status of the output from a logic
gate
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Relays
Circuits 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
Words motor, larger, electromagnet, small
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P6e Motoring
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Comparing magnets and solenoids
Magnet
Solenoid
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Magnetic Field around a current-carrying wire
Right hand corkscrew or right hand grip rule
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Magnetic field around a rectangular coil
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The Motor Effect

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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Current-carrying wire in a magnetic field
Q. Where will this wire go?
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Revision of DC and AC
V
DC stands for Direct Current the current only
flows in one direction
Time
1/50th s
AC stands for Alternating Current the current
changes direction 50 times every second
(frequency 50Hz)
230V
T
V
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Electric Motors
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Electric Motors
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P6f Generating
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Electromagnetic Induction
N

• The direction of the induced current is reversed
  if
• The wire is moved in the opposite direction
• The field is reversed

• The size of the induced current can be increased
  by
• Increasing the speed of movement
• Increasing the magnet strength

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Electromagnetic induction

• The direction of the induced current is reversed
  if
• The magnet is moved in the opposite direction
• The other pole is inserted first

• The size of the induced current can be increased
  by
• Increasing the speed of movement
• Increasing the magnet strength
• Increasing the number of turns on the coil

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AC Generators
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AC Generators
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Questions on the AC Generator

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 Transforming
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Transformers
A transformer is basically a device made of two
coils of wire around an iron core
The circuit diagram for a transformer
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Transformers
Current through primary
Time
Magnetic field
Time
Voltage induced in secondary
Time
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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
and the current changes using this formula
VPIP VSIS
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Simple 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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More example questions
Primary 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 Transformers
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 dont make
_____ with each other. This stops the user from
getting ___________ from the mains supply.
Words electrocuted, shaver, voltage, contact,
coils
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Transformers in the National Grid
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
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Power through the National Grid
The following equation can be used to calculate
the amount of power loss in the National Grids
cables
Power I2R

1. A 10O 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 2.2MO calculate the power
   dissipated by the motor.
3. A 2KW heater has a resistance of 20 O. Calculate
   the current through it.

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Transformers and Power
If 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 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 Charging
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Diodes
The current flows easily in the forwards
direction but there is very high resistance to
the reverse current.
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
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How 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 Rectification
A group of 4 diodes can be used to make a bridge
rectifier circuit to make full-wave
rectification
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The Capacitor
A capacitor is a device that can store charge (it
has a capacity). Heres how they work
e
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Charging and discharging a capacitor
Current in circuit
Time
P.d. across capacitor
Current in circuit
Time
Time
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Smoothing
Many devices need a constant voltage supplied to
them, rather than AC. Capacitors can be used in
smoothing circuits to smooth out a supply
Output without capacitor
Output with capacitor
Input