Transcript Physics in Action - Droitwich Spa High School

Physics in Action
W Richards
Worthing High School
Electronic systems
Electronic systems are made up of 3 parts:
1) An INPUT SENSOR – these detect changes in the
environment
Examples:
2) A PROCESSOR – these decide what action is needed
Examples:
3) An OUTPUT DEVICE – these are controlled by the
processor
Examples:
Logic gates
Logic gates are the basics
behind any kind of
processor. There are 3
that you need to know for
GCSE:
AND – “the output is on if A
AND B are both on”
OR – “the output is on if A
OR B are on”
NOT gate – “the output is
NOT what the input is”
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 gate symbols
NOT gate:
AND gate:
OR gate:
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.
The Relay
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:
M
Words – motor, larger,
electromagnet, small
Symbol
for relay:
The Resistor Colour Code
We would use this to determine
the value (in ) of a resistor:
Significant figures
E.g this
resistor would
have a value of
42 x 10 = 420
and a tolerance
of 5%
0.01
0.1
5%
0
0
1
1
1
10
1%
2
2
100
2%
3
3
1k
4
4
10k
5
5
100k
6
6
1M
7
7
10M
8
8
9
9
Tolerance
(no band =
20%)
Multiplier
Drawing electronic circuits
12V
V
This voltmeter
will read 12V
0V
VIN
This bulb and
resistor are in
parallel
0V
Drawing electronic circuits
1) Two resistors in series
VIN
2) Two resistors in series with
one bulb in parallel
VIN
0V
3) A voltmeter measuring the
voltage across two bulbs in
series
0V
4) An ammeter measuring the
current through a resistor
VIN
V
VIN
A
0V
0V
Potential Dividers
VIN
R1
VOUT
R2
0V
0V
The Potential Divider equation:
VOUT
VIN x
(R2)
(R1 + R2)
Some example questions
12V
50V
100 
100 
0V
0V
3V
75 
0V
VOUT
0V
1.5V
75 
25 
0V
10 
VOUT
50 
VOUT
0V
45 
0V
VOUT
0V
Practical applications
Here’s a potential
divider that is used to
control light-activated
switches…
Vin
VOUT
0V
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
Transistors
A transistor acts like a switch:
Collector
Base
Emitter
When a SMALL current
flows through the baseemitter part of the
transistor a different
current is switched on
through the collectoremitter part.
A light dependent switch
6V
Power supply
Output
device
0V
A light dependent switch
1) When the light on the LDR decreases its resistance _________,
which will decrease the ________ across the variable resistor
2) This will cause VOUT to ____. The____ gate will recognise this as a
“0” and convert it into a “1”, i.e. a current will flow into the resistor
3) The resistor limits the amount of current flowing into the
transistor, to avoid __________ it
4) When the transistor detects the current at its _____ it will
“switch __” the collector-emitter current
5) A small current will then flow through the _______
6) The relay will then switch on a _____ current in the output circuit
7) The “reversed biased” diode is also placed in the circuit to act as a
“_______” to prevent current flowing back into the transistor
when the relay is switched _____
Words – base, buffer, on, increases, damaging,
relay, off, larger, voltage, drop, NOT
A light dependent switch
We could modify this circuit (if we wanted to…)
1) Swap these two
around and the
output will now
switch on when it
becomes LIGHT, not
when it becomes dark
2) Adjust this
resistor to vary the
sensitivity
6V
0V
The Capacitor
A capacitor is a device that can store charge (it has a
“capacity”). It is basically made of two plates:
…or…
Charge builds up on these plates and the voltage
between them increases until it reaches the supply
voltage.
Charging and discharging a capacitor
P.d. across
capacitor
P.d.
Increase
resistance or
capacitance
Time
Time
P.d.
Increase
resistance or
capacitance
Time
Time
Time delay circuits
6V
Power supply
R
Output
device
0V
Time delay circuits
6V
Power supply
R
Output
device
“1”
0V
Time delay circuits
1) When the switch is closed the capacitor is being short circuited so
no charge builds up on it
2) This means that the input to the NOT gate is __, so the output is
1 and the output device is ___
3) When the switch is released the capacitor starts to ________ up
4) When the voltage across the capacitor reaches a certain level the
input to the NOT gate becomes __ so its output is 0
5) This means that the output device is now switched ___
6) To INCREASE the amount of time taken to switch the device off
you could:
1) Increase the _________ of the capacitor
2) _________ the resistance of the resistor R
Words – charge, 1, capacitance, increase, 0, off, on
Electronic systems
System
CCTV
Internet
Mobile Phones
Advantages
Disadvantages
Lenses
Lenses use the idea of refraction:
When light enters a
MORE DENSE medium
it slows down…
A prism uses this idea
to split light. This
happens because purple
light is refracted more
than red light
Another example:
The lens in the eye is used to focus what we see:
Converging and diverging lenses
CONVERGING (Convex)
Thickest at the centre
DIVERGING (Concave)
Thinnest at the centre
Ray diagrams
The rays of light are
refracted INWARDS and
meet at the focus, F.
F
F
The image formed is REAL –
in other words, it can be
seen on a screen
The rays of light are
refracted OUTWARDS.
A VIRTUAL image is formed
– in other words, the image
doesn’t actually exist
Using lenses to correct vision
Short
Long
Ray diagrams
To draw ray diagrams follow these two rules:
1) Draw a ray from the TOP of the object PARALLEL to
the axis and then going through F:
F
2) Draw a ray from the TOP
of the object going through
the CENTRE of the lens
(which will be undeviated)
This image is
REAL, UPSIDE
DOWN and
SMALLER than
the object
Practical uses of lenses
Cameras use the lens arrangement where the
object is beyond 2F – this is why they can’t be
used at very short range.
Magnifying glasses use the arrangement where
the object is between F and the lens – this is
why they don’t work when you move them away
from the object.