Download Electricity

23/05/2017
P6 Electricity for
Gadgets
AGAC
Circuit Symbols
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Variable
resistor
Diode
Switch
Bulb
A
V
Ammeter
Voltmeter
LDR
Resistor
Power Supply
Capacitor
Thermistor
Battery
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Equations for this unit (Foundation)
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Equations
For
This
Unit
(Higher)
Basic ideas…
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Electric current is when electrons start to flow around a
ammeter to measure it and it is
circuit. We use an _________
measured in ____.
amps
Potential difference (also called _______)
voltage is
how big the push on the electrons is. We use a
voltmeter to measure it and it is measured in
________
______,
volts a unit named after Volta.
Resistance is anything that resists an electric current. It is
measured in _____.
ohms
Words: volts, amps, ohms, voltage, ammeter, voltmeter
More basic ideas…
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If a battery is added
the current will
________
increase because
there is a greater
_____
push on the electrons
so they move ______
faster
If a bulb is added the
decrease
current will _______
because there is
resistance in
greater ________
the circuit, so the
slower
electrons move _____
Words – faster, decrease, slower, increase, push, resistance
Resistance
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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
(in )
=
Georg Simon Ohm
1789-1854
Voltage (in V)
V
Current (in A)
I is used for current because current
was originally referred to as “electrical
intensity”.
I
R
An example question:
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Ammeter
reads 2A
A
1)
R = V = 10 =
V
I
1)
Voltmeter
reads 10V
What is the resistance across this bulb?
5
2
Assuming all the bulbs are the same what is
the total resistance in this circuit?
5 x 3 = 15 
More examples…
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3A
6V
4
12V
3A
2
2A
4V
Difficult!!
2V
1A
Difficult!
What is the
resistance of
these bulbs?
2
Resistance
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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.
3
2) A resistor with a voltage of 12V and a current of 3A 4 
3) A diode with a voltage of 240V and a current of 40A 6 
4) A thermistor with a current of 0.5A and a voltage of
10V
20 
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F
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Current-Voltage graphs for conductors
1) An Ohmic conductor
V
High
resistance
2) A non-Ohmic conductor
V
Low
resistance
I
In Ohmic conductors the
resistance of the conductor
stays the same.
I
In non-Ohmic conductors
(like filament bulbs) the
resistance of the conductor
increases as the current
increases (when it gets hot).
LDRs and Thermistors
1) Light dependant
resistor – resistance
DECREASES when light
intensity INCREASES
Resistance
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2) Thermistor –
resistance DECREASES
when temperature
INCREASES
Resistance
Amount of light
Temperature
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F
Divides a value of voltage (1) by the
corresponding value of current(1)
d
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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
Potential Dividers
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VIN
R1
VOUT
R2
0V
0V
The Potential Divider equation:
VOUT
VIN x
(R2)
(R1 + R2)
Find the output voltage
VIN
R1 
Example
R2 
0V
VOUT
0V
VIN x
VOUT
(R2)
(R1 + R2)
50V
100 
Q1
100 
0V
VOUT
0V
25V
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(Vout)
1
Find the output voltage
VIN
R1 
Example
R2 
0V
VOUT
VOUT
0V
VIN x
(R2)
(R1 + R2)
3V
200 
Q2
100 
0V
VOUT
1V
0V
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(Vout)
2
Find the output voltage
VIN
R1 
Example
R2 
0V
VOUT
0V
VIN x
VOUT
(R2)
(R1 + R2)
12V
5
Q3
15 
0V
VOUT
9V
0V
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(Vout)
3
Practical applications
Here’s a potential
divider that is used to
control light-activated
switches…
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Vin
VOUT
0V
When the light intensity on the LDR decreases its
decrease
increase
resistance will ________.
This causes VOUT to _______
off
so the processor and output will probably turn _____.
The
sensitivity
variable resistor can be adjusted to change the ________
of the whole device.
Words – decrease, sensitivity, increase, off
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input
(2)
output
0V
Comparing magnets and solenoids
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Magnet:
Solenoid:
N
S
Magnetic Field around a currentcarrying wire
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“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
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Magnetic field around a rectangular coil
This looks like the solenoid
seen on a previous slide
Revision of DC and AC
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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
The Motor Effect
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N
S
1) What will happen to this wire?
move away
2) How can you make it move faster? stronger magnetic field
add battery/ more current
3) How can you make it move in a different direction?
reverse current OR reverse magnet
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Current-carrying wire in a magnetic field
N
Field goes
from N to S
F = Force
Current is coming
upwards towards you
B=
Magnetic
field
I = Current
S
Q. Where will this wire go?
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Current-carrying wire in a magnetic field
N
F = Force
B=
Magnetic
field
Q. Where will this wire go?
I = Current
S
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downwards
Electric Motors
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F
3
Electromagnetic Induction
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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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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
DC Generators
23/05/2017
Basically, 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
AC Generators
Magnetic
Field
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Slip rings and
brushes
S
N
AC Generators
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Voltage
Time
Questions on the AC Generator
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1) How does the generator
work?
Electromagnetic
induction
2) How would you increase its
output? Give two answers
More coils
Stronger magnet
3) How would you increase the
frequency?
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
Transformers
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Current through primary
Time
Magnetic field
Time
Voltage induced in secondary
Time
Transformers
• 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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Transformers
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step up
Transformers are used to _____
__ or step down _______.
voltage
They only work on AC because an alternating
________ current in the
magnetic______.
field
primary coil causes a constantly alternating _______
This will “_____”
induce 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 primary (Np)
Voltage across secondary (Vs)
No. of turns on secondary (Ns)
…and the current changes using this formula:
VPIP = VSIS
Some example questions
Primary
voltage
Vp
Secondary
voltage
Vs
No. of turns
on primary
Np
6V
24V
100
400V
200V
250V
? 23
? 25V
230V
? 2000
No. of turns
on secondary
Ns
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Step up or
step down?
? 400 Step?up
1000
Step ?down
200
20
Step ?down
150
1500
Step?up
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?
1:3
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
5A
coil?
Transformers in the National Grid
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Electricity reaches our homes from power stations through the National
Grid:
Power station
Step up
transformer
Step down
transformer
Homes
If electricity companies transmitted electricity at 230 volts through
power loss by the time
overhead power lines there would be too much ______
electricity reaches our homes. This is because the current is high
___. To
overcome this they use devices called transformers to “step up” the
voltage onto the power lines. They then “____
step down
____” the voltage at the
end of the power lines before it reaches our homes. This way the voltage
high and the current and power loss are both ____.
low
is _____
Words – step down, high, power, low, high
F
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No output/zero
step-down
phone-chargers
Vout
240
Vout
radio
laptops
= 200
4000
=12
Any from:
low voltage lighting
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Vout
240
= 200
4000
Vout
= 12
Any three from:
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
Isolating Transformers
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An isolating transformer is used in some mains circuits (for
example, a bathroom shaver
_____ socket). Isolating transformers
do not change the voltage
_____; they simply consist of two ____
coils
which have the same _______
number of turns that don’t make
contact
_____ with each other. This stops the user from getting
___________
electrocuted from the mains supply.
Words – electrocuted, shaver, voltage, contact, coils, number
Diodes
A diode is a
device that only
allows current to
flow in one
direction:
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I
V
The current flows
easily in the forwards
direction but there is
very high resistance
to the reverse
current.
If alternating current is passed through a diode it becomes
“half-wave rectified”:
V
V
T
T
Full-Wave Rectification 1
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A group of 4 diodes can be used to make a “bridge rectifier”
circuit to make full-wave rectification:
AC supply
Load
V
V
T
T
Full Wave Rectification 2
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The current can only flow through
the diode in the direction of the arrow
First half of
the a.c. wave
Second half of
the a.c. wave
Current flows from
positive to negative
+
AC supply
-
Load
AC supply
+
Load
Notice that the current flows the same way through the load resistor i.e. DC
The Capacitor
A capacitor is a device that can store charge (it has a
“capacity”). Here’s how they work:
e
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Charging and discharging a capacitor
P.d. across
capacitor
P.d. across
capacitor
Current in circuit
Time
Time
Current in circuit
Time
Time
Smoothing
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Capacitors can be used in “smoothing” circuits to “smooth” out
a supply:
AC supply
Input
Output without
capacitor
Output with
capacitor
Logic Gates
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Logic 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:
Logic Gates
NAND gate:
NOR gate:
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Logic 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”
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Input
Output
0
1
1
0
Input A
Input B
Output
0
0
0
0
1
0
1
0
0
1
1
1
Input A
Input B
Output
0
0
0
0
1
1
1
0
1
1
1
1
Logic gates
NAND – “an AND gate and a
NOT gate in series”
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Input A
Input B
Output
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
Logic gates
NOR – “a NOR gate and a
NOT gate in series”
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Input A
Input B
Output
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
Some problems to solve
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•
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.
Using LDRs and Thermistors as inputs
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Thermistors and LDRs
can be used as the
input to a logic gate:
6V
5V
Output
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
Complex Logic Gate problems
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Work out the truth tables for the following arrangements:
A
J
B
A
0
0
1
1
B
0
1
0
1
O
J depends
on A
O depends
On B and J
J
O
1
1
0
0
1
1
0
1
Complex Logic Gate problems
23/05/2017
Work out the truth tables for the following arrangements:
A
J
O
B
A
0
0
1
1
B
0
1
0
1
J
1
1
0
0
O
1
1
0
1
Complex Logic Gate problems
23/05/2017
Work out the truth tables for the following arrangements:
A
J
B
K
C
A
B
C
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
0
1
0
1
0
1
0
1
J depends
on A and B
J
0
0
0
0
0
0
1
1
O
K depends
on C and J
K
0
1
0
1
0
1
1
1
O
Complex Logic Gate problems
23/05/2017
Work out the truth tables for the following arrangements:
A
J
B
K
O
C
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
Complex Logic Gate problems
23/05/2017
Work out the truth tables for the following arrangements:
A
J
O
B
C
D
K
L
A
B
C
D
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
J
K
L
O
Complex Logic Gate problems
23/05/2017
Work out the truth tables for the following arrangements:
A
J
O
B
C
D
K
L
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
Relays
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Circuits such as those containing logic gates only take very ______
small
currents. These circuits may be needed to operate a device that takes a
much larger
_____ current, e.g. a _________.
To do this the circuit would need
motor
a RELAY switch, a device made of an ____________
electromagnet 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:
M
Words – motor, larger,
electromagnet, small
Symbol
for relay:
Latch Circuits
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A 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:
input
1) A brief “on” signal at one ____
on signal
results in a permanent “___”
output
at the latch ______
2) A brief “on” signal at the other
off signal at the
input causes an “___”
latch output
3) An “off” signal at both inputs
leaves the latch signal _________.
unchanged
Words – on, off, unchanged, output, input
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F
low
high
once the alarm starts it stays on (1)
or
even if the door is shut
it is reset
s
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0
0
0
1
Any two from:
fan needs a large current
logic gate only produces a small current
relay can use low input
to switch large fan current
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decreases
Input increases
or
Goes to 1/high/on
Allows the temperature at which the fan
comes on to be adjusted