Transcript Electricity - The Friary School

18/07/2015
Electricity for Gadgets
OCR Gateway
W Richards
The Weald School
P6a Resisting
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Circuit Symbols
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Variable
resistor
Diode
Switch
Bulb
A
V
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery
Electric Current – A Definition
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Electric current is a flow
of negatively charged
particles (i.e. electrons).
+
-
e-
Note that
electrons go
from negative
to positive
By definition, current is “the
rate of flow of charge”
e-
Basic ideas…
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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
More basic ideas…
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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)
A
V
1) What will happen to the ammeter reading when
the resistance is increased?
2) How will this affect the brightness of the
bulb?
Resistance
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Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
Georg Simon Ohm
1789-1854
The resistance of a component can be
calculated using Ohm’s Law:
Resistance
(in )
=
V
Voltage (in V)
Current (in A)
I
R
An example question:
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Ammeter
reads 2A
A
V
Voltmeter
reads 10V
1) What is the resistance across
this bulb?
2) Assuming all the bulbs are the
same what is the total resistance
in this circuit?
More examples…
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3A
6V
12V
3A
2A
4V
2V
1A
What is the
resistance of
these bulbs?
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.
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
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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. The
gradient gives us the
resistance.
I
In non-Ohmic conductors
(like filament bulbs) the
resistance of the conductor
increases as the current
increases (when it gets hot).
Understanding Resistance
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When a voltage is applied it basically causes electrons to move
towards the positive end of the battery:
Negative
Electrons
Ions
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?
P6b Sharing
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Potential Dividers
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VIN
R1
VOUT
R2
0V
0V
The Potential Divider equation:
VOUT
VIN x
(R2)
(R1 + R2)
Some example questions
12V
50V
100 
100 
0V
10 
VOUT
0V
3V
75 
0V
VOUT
0V
1.5V
75 
25 
0V
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50 
VOUT
0V
45 
0V
VOUT
0V
Potential Dividers
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VIN
R1
VOUT
R2
0V
0V
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.
LDRs and Thermistors
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Two simple components:
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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Using Thermistors and LDRs in circuits
A
V
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
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
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
Resistors in Series and Parallel
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The following equations can be used to calculate the total
resistance in a circuit:
For resistors in series:
For resistors in parallel:
RT = R1 + R 2
1 =
1 +
1
RT
R1
R2
Example questions
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Calculate the equivalent resistance:
1)
40Ω
10Ω
2)
20Ω
10Ω
3)
100Ω
100Ω
20Ω
20Ω
4)
100Ω
50Ω
50Ω
P6c It’s Logical
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Computers
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Apple 1 (1976) – 1Mhz
IBM 5100 portable PC (1975), 25kg,
processor, 4K RAM, cost $670
1.9MHz processor, 64K RAM, 200K
internal tape storage, cost up to $20,000
All of these computers (and
modern ones) are based on
transistors, which have
enabled computers to get
smaller over the years.
Commodore 64 (1982) – 1Mhz
processor, 64K RAM, 16
colours, cost $600, 17m sold
Apple Macintosh (1984) – 8Mhz
processor, 512K RAM, cost $2500
Computers before Transistors
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The “Harwell Dekatron” a relay-based computer (first run in 1951):
Transistors
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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.
Ie = Ib + Ic
Drawing electronic circuits
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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
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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
A high resistor is placed
here – why?
Combining Transistors
Consider this
circuit:
6V
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A
B
Input A
Input B
Output
Off
Off
Off
Off
On
Off
On
Off
Off
On
On
On
Out
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”)
Logic gate symbols
NOT gate:
AND gate:
OR gate:
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Logic gates
Logic gates are the basics
behind any kind of
processor. Here are the
three basic ones:
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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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
NAND and NOR gates
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NAND gate:
Input A
Input B
Output
0
0
1
0
1
1
1
0
1
1
1
0
NOR gate:
Input A
Input B
Output
0
0
1
0
1
0
1
0
0
1
1
0
P6d Even More Logical
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Complex Logic Gate problems
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Work out the truth tables for the following arrangements:
A
J
O
B
A
0
0
1
1
B
0
1
0
1
J
O
Complex Logic Gate problems
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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
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Work out the truth tables for the following arrangements:
A
J
B
K
O
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
K
O
Complex Logic Gate problems
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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
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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
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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
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
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 LEDs with Logic Gates
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LEDs work on low power so they can be used to indicate the
status of the output from a logic gate:
A
B
J
O
Relays
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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:
P6e Motoring
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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
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Magnetic field around a rectangular coil
The Motor Effect
N
S
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
N
F = Force
B=
Magnetic
field
I = Current
S
Q. Where will this wire go?
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
Electric Motors
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Electric Motors
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The “split ring
commutator” changes the
current every half
rotation (hence the redblue colour change)
Notice the curved poles – these will give a
greater magnetic field strength near the coil
P6f Generating
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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
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?
2) How would you increase its
output? Give two answers
3) How would you increase the
frequency?
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
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Current through primary
Time
Magnetic field
Time
Voltage induced in secondary
Time
Transformers
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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:
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
Simple transformer questions
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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
?
?
More example questions
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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?
Isolating Transformers
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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.
Words – electrocuted, shaver, voltage, contact, coils
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 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 through the National Grid
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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.
Transformers and Power
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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.
In previous units we came across
this equation:
Power = voltage x current
in W
in V
in A
If we apply this equation to transformers and assume 100%
efficiency we conclude:
Vp x Ip = Vs x Is
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?
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1.9A
2) Another step down transformer outputs 120V
and 2A. If it runs from the mains what is its
input current?
1.04A
3) A step up transformer near a power station
would convert 20,000V and 10A into 300,000V
and what current?
0.07A
P6h Charging
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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
How Diodes work
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Diodes work by having areas of
extra electrons and areas of
extra “holes”:
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.
+
+
+
Electrons
-
+
-
+
-
+
-
-
Full-Wave Rectification
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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
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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Many 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
Input
Output without
capacitor
Output with
capacitor