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Charging – Revision Pack (P6)
Diode Characteristics:
You would set up the diode circuit
as you would for a resistor or bulb.
Most diodes need a voltage of
0.6V before they start to work.
When a diode is connected so
current can pass (voltage is high
enough), it is forward biased.
When a diode is connected so that current CANNOT pass (voltage is too low), it is
reverse biased.
An AC supply to a single
diode produces a halfwave rectified output.
REMEMBER – AC input
means DC output
A resistor is always used in series with a diode to protect the diode.
How a Diode Works:
A diode consists of an n-type semiconductor and a p-type semiconductor joined
together.
The n-type has an excess of electrons, while the p-type has a shortage of electrons –
the gaps are called ‘holes’.
The space either side of the junction (between the two semiconductors) has NO
electrons or holes.
If you were going to connect the positive terminal of a power supply to the n-type
semiconductor, then the space in the junction would widen and NO current passes.
If you were going to connect the positive terminal of a power supply to the p-type
semiconductor, them the space in the junction would narrow, eventually disappear
and a current passes.
Rectifier Circuits:
Four diodes can be arranged to make a bridge
circuit.
The addition of a capacitor makes the output
smoother.
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B
C
A
D
Rectification:
In the bridge circuit, during the positive half-cycle, the current passes from A to B to
the external circuit, to D then C, then back to the AC supply.
During the negative half-cycle, the current passes from C to B, to the external circuit,
to D then A then back to the AC supply.
This makes sure that the DC output is always positive at B and negative at D.
Storing Charge:
The chemicals in a battery continue to produce energy for a battery to use. When
the chemicals are all used up, the battery no longer works.
The capacitor stores electrical energy. There is NO continual energy source.
Smoothing:
A smoothing capacitor acts as a reservoir. When the DC voltage from the rectifier
circuit falls, the capacitor supplies current to the input. The capacitor charges near
the peak value of the varying DC (just as it’s running out of steam).
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Truth Tables:
When there are several logic gates combined together, truth tables can be used as
a simple way to work out what happens within the system; for example:
A
0
1
0
1
1
1
0
0
Input Signals
B
C
0
0
0
1
1
1
1
1
1
1
0
1
1
1
0
0
D
0
0
0
0
1
1
1
1
Output
Z
0
0
0
0
1
1
1
0
REMEMBER – work step-by-step!
Switching Logic Gates:
Switches, LDR’s and thermistors are all used in a potential divider circuit to give logic
gates a high input.
Using the switch as an example, when it is open, he input
to the logic gate is low (0).
However, when the switch is closed, the input is
connected directly to the 5V supply – here, the input is
high (1).
Thermistors and LDR’s work in the same way; just use
ideas about resistance rather than it being open or
closed.
NOTE – if a variable resistor was used in place of a fixed resistor, a threshold level can
be set.
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How a Relay Works:
STEP 1 - When a small current passes through a coil,
the iron armature is attracted
STEP 2 – the armature pivots and pushes an
insulating bar against the central contact
STEP 3 – the central contact moves, opening the
normally closed contacts and closing the normally
open contacts.
Normally open contacts
Common
Normally Closed contacts
This represents a coil
of wire
Current output from logic gates is low BUT can be passed through a
relay to switch on a larger current needed for motors, heaters etc.
Furthermore, the relay also isolates the logic circuit from the HIGH
current to avoid damage to the logic gates.
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Inducing Voltages:
When a wire is held stationary between the poles of a magnet, there is NO current in
the wire.
If the wire is moved
upwards, or the
magnet is moved
downwards, a
voltage is induced
in the wire and
current flows.
When the wire moves downwards, or the magnet moves upwards, the induced
voltage is reversed and the current flows in the opposite direction.
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If you reverse the direction of the magnetic field, you will also reverse the direction of
the induced voltage and current.
Whenever a magnetic field changes, a voltage is ALWAYS created.
You can control the size of the induced voltage; it is dependent on the rate at which
the magnetic field changes:
-
If the wire is moved really quickly, then the induced voltage will be higher
If the wire is moved slowly, the induced voltage will be lower
AC Generators:
In a power station generator, the magnetic field inside the coil is produced by
electromagnets. The electromagnetic is made from coils of wire (rotor coils) which
are turned by the turbine.
An alternating current is generated in the stator coils which surround the rotor coils.
If you increase the speed at which the electromagnet rotates:
-
The current induced in the stator coils will also increase
The frequency of the generated voltage is also increased
If you increase the number of turns on the electromagnet:
-
You increase the magnetic field
(and as such) induce a larger voltage in the stator coils
Some AC generators have a coil rotating between the poles of a magnet.
As the coil rotates, the direction of the current reverses every half turn to ensure that
the coil rotates in the same direction.
Slip rings are connected to the
ends of the coil to allow the coil to
spin without winding the wire
around itself.
The brushes are contacts that the
touch the slip rings to complete the
circuit.
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Transistors:
In a transistor, a small base current can switch on a GREATER
current through the collector.
To work out the current coming out of the emitter:
Ie = I b + Ic
Logic Gates:
Type of Gate
AND
OR
NOT
Symbol
Explanation
To give a high output,
both A and B must have a
high output or be on.
AND gates are like two
switches in series; the
circuit can’t work if one
switch is off.
To give a high output,
one of A or B must have a
high output or be on. OR
gates are like two
switches in parallel; only
one switch needs to be
on for the circuit to work.
To give a high output, the
input must be low or
completely off. NOT
gates will give the
opposite of the (single)
input – the output is not
what the input is.
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Making an AND gate:
Two transistors can be combined to make an AND gate.
1) A small input at A will switch on the upper
transistor.
2) A small input at B will switch on the lower transistor.
3) A current at A and at B are needed to switch on
the 6V supply.
The other logic gates can be made by using different
combinations of transistors.
Transistor circuits ALWAYS have a high value resistor in the
base circuit to limit the current.
NAND and NOR gates:
Type of Gate
NAND
Symbol
NOR
Explanation
It behaves like an AND
gate followed by a NOT
gate.
It behaves like an OR
gate followed by a NOT
gate.
Truth Tables:
Truth tables show how logic gates behave.
-
When there is some input (or output), a 1 is entered in the table
When there is NO input (or output), a 0 is entered in the table
AND Gate:
Input A
0
1
0
1
OR Gate:
Input B
0
0
1
1
Output
0
0
0
1
NAND Gate:
Input A
0
1
0
1
Input B
0
0
1
1
Input A
0
1
0
1
Input B
0
0
1
1
NOR Gate:
Output
1
1
1
0
Output
0
1
1
1
Input A
0
1
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0
1
NOT Gate:
Input A
0
1
Output
1
0
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0
0
1
1
Output
1
0
0
0
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Magnetic Fields:
A magnetic field can be created if an electric current moves
through a coil of wire.
The direction of a field around a wire can be found using the
right hand grip rule. You should imagine you’re wrapping your
RIGHT hand around a wire. Your thumb should point towards
the direction of the current. Your other fingers will then give the
direction of the magnetic field (see left).
The field pattern of a long coil of wire is similar to that of a regular bar magnet;
opposite forces attract, and like-forces repel.
When a wire is placed between the poles of a magnet, the wire moves out of the
gap when the current is switched on.
The X shows us that the current is flowing
into the paper; it looks like the back of
an arrow that we’ve just fired.
The dot (.) shows us that the current is
flowing away from the paper; it looks
like an arrow is coming towards us.
Motor Direction:
Fleming’s left hand rule can be used to predict
the direction in which a motor turns.
Your Thumb = Motion
Your First Finger = Field
Your Second Finger = Current
The direction of current, magnetic field and
motion are all at right angles to each other.
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Turning Coils:
When a current passes through a coil, placed between the poles of a magnet, there
is a force on each side of the coil. If you use Fleming’s Left Hand Rule, you can see
that the motion (or forces) on each side of the coil are opposite in direction. As one
side is forced up, the other is forced down – this means that the coil starts to spin.
As you can see, because the direction
of current changes on each side, the
force or motion also changes, making
the coil spin.
You can make the coil spin faster by:
-
Increasing the number of turns
on the coil
Increasing the size of the current
Increasing the strength if the
magnetic field
Practical Motors:
The job of a commutator is to make the coil continue to spin.
The direction of current in the coil is revered every half turn; this ensures that the
force on the coil is ALWAYS in the same direction.
The magnets in practical motors have curved
poles to give a radial magnetic field.
A radial field increases the force on the coil
and keeps it constant as the motor turns. It
also ensures that the field lines are always 90o
to the coil.
Additional NOTES:
If the direction of the current is reversed, then the wire moves in the opposite
direction – this is the same for reversing the direction of the magnetic field.
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Resistance and Current:
Higher resistance means lower current (flow of electrons).
The higher the current, the brighter the bulb is in the circuit.
The higher the current, the faster a motor turns.
NOTE – voltage is needed to make the current flow through
a circuit – it pushes the current along (as seen to the left).
Resistance (R) = Voltage (V) / Current (I)
R = Ohms, V = Volts, I = Amps
What affects resistance?
The resistance of a wire is dependent upon its length.
A variable resistor (symbol to the left) works by
changing the lengths of wire in the circuit; the longer
the wire, the greater the resistance.
Ohmic and Non-Ohmic Devices:
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Ohm’s law is the idea that the current travelling through something is directly
proportional to the voltage across it; if you double the voltage, then the current will
also double.
Resistors, at a constant temperature, follow Ohm’s law – the
graph is a straight line passing through the origin.
The steeper the gradient of the graph (left), the greater the
resistance – this is because if the gradient is steeper, the
current will be less and the voltage more.
The same graph for a bulb will NOT be a straight line passing
through the origin. The gradient of the line increases as
current increases (it curves positively) – it does NOT follow
Ohm’s Law.
The resistance of a filament lamp increases as current increases.
When electrons collide with the filament (a thread-like fibre), it makes the atoms
vibrate more. The increased vibration means two things:
-
An increased number of collisions, which leads to more resistance
An increase in the temperature of the filament
In a fixed resistor, the gradient of the graph is equal to the resistance.
The gradient of a voltage-current graph for a filament bulb will increase as
resistance increases. To find the resistance, instantaneous results from the graph must
be used.
A non-Ohmic device shows a curve with an
increasing gradient, which means an increased
resistance.
If the voltage supplied across a non-Ohmic device is
increased, the electrons carry more energy. When
these electrons collide with the metal atoms in the
conductor, more energy is transferred and this
increases the resistance and the temperature (this
can be seen to the left).
NOTE – the cause of electrical resistance is the
electrons (charge carriers) colliding with the metal
atoms as they flow through a metal conductor, like
a wire.
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Potential Divider Circuits:
A potential divider is a simple circuit which uses resistors (as
well as LDR’s and thermistors) to supply a variable potential
difference (or output voltage).
Some of these circuits have just fixed resistors – in this instance,
the output voltage is a fixed proportion to the supply voltage;
it’s a bit like ratios. If your R1 and R2 values are the same, then
the output value will be half of the input voltage – you’ll
understand why when you see the equation below.
Calculating Output Voltage:
Some of these circuits have one variable resistor and one
fixed resistor – the output voltage can be altered using this.
VOUT = VIN x (R2 / R1 + R2)
When your R2 value is much greater than your R1 value, the output voltage will be
approximately the same as the input voltage.
When your R2 value is much less than your R1 value, the output voltage will be
approximately zero.
Some electrical components will only start working at a threshold voltage – when
you use a variable resistor as part of a potential divider, you can set this threshold
voltage.
Resistors in parallel:
We can arrange resistors in parallel and this will decrease the total resistance; the
equation to work out total resistance is shown below:
1/RT = 1/ R1 + 1/ R2 + 1 / R3
LDR’s and Thermistors:
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The resistance of an
LDR decreases as light
intensity increases.
The resistance of a
thermistor decreases
as temperature
increases.
Uses of Potential Divider Circuits:
Street lights use LDR’s in potential divider circuits this is placed in the R2 position (as seen to the left).
When it’s bright, the resistance of the LDR (R2) is
very low. The R1 fixed resistor is made to have a
higher resistance than the LDR – this will mean that
the output voltage will be very low and the street
light will NOT be on.
When it gets dark, the resistance of the LDR will
increase. It will increase so that it is more than the
R1 value. This will mean that the output voltage will
increase, and the street light will turn on.
A thermistor can be used to switch on a heater when the temperature falls too
much in a home – it uses the above principal.
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Transformer Design:
A transformer consists of two coils of
wire wound round an iron core.
The input AC voltage is connected
to the primary coil, while the output
AC voltage is obtained from the
secondary coil (see diagram).
Step-down transformers have MORE
turns on the primary coil.
Step-up transformers have MORE turns on the secondary coil.
How transformers work:
The changing current in the primary coil creates a changing magnetic field in the
iron core.
The changing magnetic field in the iron core induces a changing voltage in the
secondary coil:
Voltage across primary (Vp) = Number of turns on primary coil (Np)
Voltage across secondary (Vs)
Number of turns on secondary coil (Ns)
An isolating transformer has THE SAME number of turns on the primary and
secondary coil.
The mains supply is hidden in an isolating transformer. The output terminals are NOT
live so there is no danger of you getting electrocuted if you touch them with wet
hands. An ordinary circuit is not used because it would be live and could
electrocute you if you touched it with a wet hand AND the water and steam in
bathrooms could cause damage to the house’s wiring system.
Energy Loss:
When a current passes through a wire, the wire gets hot.
The transformer coils and overhead power lines of the national grid get hot and lose
energy to the surroundings.
Reducing Transmission Loses:
Power is a measure of how fast energy is used up.
Power Loss = (current2 x resistance)
The electrical power supplied to the primary coil of a transformer is dependent on
the input voltage and current:
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Pp = Vp x Ip
Similarly, the output power of the secondary coil is given by:
Ps = Vs x Is
If the transformer is 100% efficient, then the input voltage = the output voltage.
In a step-up transformer, increasing the voltage leads to a decrease in current by
the same factor. This assumes that the transformer is 100% efficient; e.g.
If the voltage was stepped up by a factor of 16 to 400kV, then the current in the
overhead cables would reduce to 1/16 of what it would be if electricity was
transmitted at 25,000V. (25,000 x 16 = 400,000V or 400kV).
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