# Physics P6

Ocr Gateway, Physics P6

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## Circuits- Basics

Current= the flow of electrons around the circuit, it will only flow through a component if there is a voltage across that component- measured in amps, A

Voltage= driving force that pushes the current around- measured in volts V

Resistance= anything that slows the flow down, measured in ohms

If you increase the voltage- then more current will flow

if you increase the resistance- then less current will flow- or more voltage will be needed to keep the same current flowing

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## Standard Test Circuit

The Ammeter:

• measures the current flowing through the component
• must be placed in series
• can be put anywhere in series but never parallel

The Voltmeter:

• measures the voltage across the component
• must be placed in parallel but not around the variable resistor or the battery
• the proper name is potential difference

basic circuit gives you V-I graphs

the component, ammeter and variable resistor are in series, but the variable resistor is in parallel, as you vary the variable resistor it alters the current flowing through the current

allows you to get pairs of readings from the ammeter and voltmeter

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## Variable Resistor

• good for altering the current flowing through a circuit
• turn the resistance up, current drops
• turn the resistance down, current goes up
• used to control things like volume of CD player or speed of food processor
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## Voltage-Current Graphs

Different resistors- current through a resistor is proportional to voltage. Different resistors have different resistances so have different slopes

Filament lamp- as the temperature of the filament increase the resistance increases hence the curve

calculating resistance:

R=V/I

for straight line graphs the resistance is 1/gradient

for curved lines- R=V/I (resistance= voltage / current)

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## Potential Dividers

The higher the resistance the greater the voltage drop

The larger the share of the total resistance, the larger the share of the total voltage

• if resistances are equal, each resistor takes half the voltage
• if top resistor has 80% of the total resistance it will take 80%
• if top resistance has 60% will take 60% of the total voltage
• the point between the two resistors is the 'output' of the potential divider
• the 'output' voltage can be varied by swapping one of the resistors for a variable resistor

potential dividers are useful as they allow you to run a device that requires a certain voltage from a battery of a different voltage

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## LDR's

Light- dependant Resistor:

• bright conditions, resistance falls
• in darkness, resistance is highest
• makes useful for various electronic circuits e.g. automatic night lights and burglar detectors

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## Thermistor

=temperature dependant resistor

• in hot conditions, resistance drops
• in cool conditions, resistance goes up
• thermistors make useful temperature sensors e.g. car engine temperature gauges and electronic thermostats

Using a thermistor and a variable resistor in a potential divider you can make a temperature sensor that triggers an output device at a temperature you choose

can make the a temperature sensor that gives a high voltage output when its hot and a low voltage output when its cold (resistance high)

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## Magnetic Fields

A magnetic field is a region where magnetic materials (e.g. iron and steel) and also wires carrying currents experience a force acting on them

magnetic fields can be shown by field diagrams, the arrow on the field lines always point from the north pole to the south pole

A current carrying wire creates a magnetic field:

• there is a magnetic field around a straight, current-carrying wire
• the field is made up of concentric circles with the wire in the centre

A rectangular coil reinforces the magnetic field:

• the circular magnetic field around the sides of the loop reinforce each other at the centre
• if the coil has lots of turns, the magnetic fields from all the individual loops reinforce each other even more
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## The magnetic field round a Solenoid

• the magnetic field inside a current carrying solenoid is strong and uniform
• outside the coil the field is just like the one around a bar magnet
• this means that the ends of the solenoid act like the north and south pole of a bar magnet
• if the direction of the current is reversed the N and S poles swap ends
• the direction of current flow tells you whether its the N or S pole you are looking at
• you can increase the strength of the magnetic field around the solenoid by adding a magnetically 'soft' iron core through the middle of the coil- then becomes an electromagnet
• a magnetically soft material magnetises and demagnetises very easily, as soon as you turn off the current through a solenoid the magnetic field disappears
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## The motor effect

a current in a magnetic field experiences a force:

• when a current carrying wire is put between 2 magnetic poles, the 2 magnetic fields affect each other
• to experience the full force, the wire has to be at 90' to the magnetic field, if the wire runs along the magnetic field you won't experience any force at all
• the force gets bigger if either the current or magnet field is made bigger
• the force always acts in the same direction relative to the magnetic field of the magnets and the direction of the current in the wire
• a good way of showing the force is to apply a current to a set of rails inside a horseshoe magnet, a bar magnet is placed on the rails which completes the circuit this generates a force that rolls the bar along the rails
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## Flemings left hand rule

Left hand:

First finger points in the direction of the Field

seCond finger in the direction of the Current

thuMb will then point in direction of the force (Motion)

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## The simple electric motor

factors which speed it up: more current, more turns in coil, stronger magnetic field, a soft iron ore

• there are forces acting on the two sides arms of the coil
• these forces are just the usual forces which act on any current in a magnetic field
• the coil is on a spindle and the forces act one up and one down, it rotates
• the split ring commutator is a clever way of 'swapping the contacts every half tern to keep the motor rotating in the same direction'
• the direction of the motor can be reversed either by swapping the polarity of the DC supply or swapping the magnetic poles over

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## practical motors

practical motors have pole pieces which are very curved

• link the coil to an axle and the axle spins around
• if you can make your motor powerful enough the axle can turn anything
• the problem is that the simple motor is too inefficient to power big and heavy stuff
• instead practical motors use pole pieces which are so curved they form a hollow cylinder, the coil spins round inside the cylinder
• you can attach almost anything onto a motor axle e.g. a food mixer or a fan
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## Electromagnetic Induction

Electromagnetic Induction: the creation of a voltage in a wire which is experiencing a change in magnetic field

Moving a magnet in a coil of wire induces a voltage:

• moving the magnet from side to side creates a little 'blip' of current
• if you move the magnet in the opposite direction the voltage/current is reversed
• if the polarity of the magnet is reversed then voltage/current is reversed too
• if you keep the magnet (or coil) moving backwards and forwards you produce a voltage that keeps swapping direction- AC current
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## Electromagnetic Induction

Get the same effect by turning a magnet end to end in a coil you create a current that lasts as long as you spin the magnet:

• as you turn a magnet the magnetic field through the coil changes- this change in the magnetic field induces a voltage which can make current flow
• when you've turned the magnet through half a turn the direction of the magnetic field through the coil reverses, when this happens the voltage reverses so the current flows in the opposite direction
• if you keep turning the magnet in the same direction then you'll get AC current
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## Factors affecting size of induced voltage

If you want a bigger peak voltage you must increase at least one of these:

• the strength of the magnet
• the area of the coil
• the number of turns on the coil
• the speed of movement

to reduce the voltage you would reduce one of those factors

if you turn the magnet faster you will get a higher peak voltage and a higher frequency

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## AC generators

• generators rotate a coil in a magnetic field
• their construction is much like a motor
• as the coil spins a current is induced in the coil, this current changes every half turn
• instead of a split-ring commutator, AC generators have slip rings and brushes so the contacts don't swap every half turn
• this means they produce an AC voltage
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## Dynamos

• Dynamos are a slightly different type of generator, they rotate the magnet instead of the coil
• this still causes the field through the coil to swap every half turn so the output is just the same as for a generator
• this means you get the same CRO traces
• Dynamos are sometimes used to bikes to power the lights
• the cog at the top is moved so it touches one of the bikes wheels, as the wheel moves around it turns the cog which is attached to the magnet creating an AC current powering the light
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## Transformers

3 types:

Step up:

• step the voltage up, have more turns on the secondary coil than the primary

step down:

• step the voltage down, more turns on primary coil than the secondary coil

Isolating transformer:

• don't change the voltage at all and have the same number of turns on the primary and secondary coils
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## Transformers and electromagnetic induction

• the primary coil produces a magnetic field which stays within the iron core- so nearly all of it passes through the secondary coil
• because there is alternating current in the primary coil, the field in the iron core is constantly changing direction
• this rapidly changing magnetic field is felt by the secondary coil
• the changing field induces an alternating voltage in the secondary coil-electromagnetic induction
• the relative number of turns on the two coils determines whether the voltage induced in the secondary coil is greater or less than the voltage in the primary
• if you supplied DC current to the primary you'd get nothing out of the secondary, it wouldn't be changing so there would be no induction
• Transformers only work with AC don't work with DC at all
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## Transformers- the core

The iron core carries magnetic field- not current

• the iron core is used only for transferring the changing magnetic field from the primary to the secondary coil
• no electricity flows round the iron core
• the iron core is laminated with layer of insulation to reduce eddy currents in the iron
• Eddy currents are little 'whirlpools' of charges that build up in the iron, heating it up and therefore wasting energy

transformers are nearly 100% efficient so power in = power out

VpIp = VsIs

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## The transformer equation

primary coil / secondary coil = number of turns on primary / number of turns on secondary

Vp / Vs = Np / Ns

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## The national grid

You get both step up and step down transformers on the national grid:

• to transmit a lot of power you either need high voltage or high current
• high current= loss as heat due to resistance of the cables
• the formula for power loss due to resistance= P=I2R
• because of the I squared if the current is 10 times bigger the losses will be 100 times bigger
• it is much cheaper to boost the voltage and keep the current very low
• this requires a transformer as well as big pylons with huge insulators, but this is still cheaper
• the transformers have to step the voltage up at one end for efficient transmission and then bring it down to safe usable levels
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## Isolating Transformers

Are used in bathrooms

• most household transformers reduce the mains voltage for use in low-voltage devices
• isolating transformers have equal primary and secondary voltages, this means that they have an equal number of turns on the primary and secondary coils this is because the only purpose of an isolating transformer is safety
• the danger of the mains circuit is that it's connected to the earth so if you touch the live parts and also the ground you will complete the circuit
• the isolating transformer inside the shaver socket allows you to use a shaver without being physically connected to the mains, so minimises the risk of the live parts touching the earth so minimises the risk of being electrocuted
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## Diodes

Only let current flow in one direction

• flow freely in one direction and there's a very high resistance in the other direction
• is useful in electronic circuits

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## Diodes

are made from semiconductors like silicon

• silicon diodes are made from 2 types of silicon joined at a p-n junction, one half of the diode is made from silicon that has an impurity added to provide extra free electrons
• a different impurity is added to the other half so there are fewer free electrons- there are lots of empty spaces called holes
• when there is no potential difference across a diode electrons and hole recombine across creating a region where there no holes or free electrons- electrical insulators
• applying Potential difference in the right direction means the free holes and electrons have enough energy to get across the insulating region to the other side- current flows
• applying potential difference in the wrong direction means the free holes and electrons are being pulled the wrong way, so they stay on the same side and no current flows
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## Half wave rectification

A single diode only lets through current in half of the cycle

< the circuit

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## Full wave rectification

you need a bridge circuit, with 4 diodes, in a circuit bridge the current always flows through the component in the same direction

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## Capacitors

Capacitors store charge

• you charge a capacitor by connecting it to a source of voltage, a current flows around the circuit and charge gets stored
• the more charge that's stored on a capacitor, the larger the potential difference (voltage) across it
• when the voltage across the capacitor is equal to that of the battery, the current stops and the capacitor is fully charged
• the voltage across the capacitor won't rise above the voltage of the battery if the battery is removed the capacitor discharges
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## Capacitors and smoothing circuits

The output voltage from a rectified AC power supply can be 'smoothed' by adding a capacitor in parallel with the output device. a component gets current alternately from the power supply and the capacitor

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## Capacitors and time delay

Capacitors are used in timing circuits and in input sensors that need a delay, like on a camera when doing auto timer

• the switch is closed. initially the capacitor has no charge stored, and so the voltage drops across it is small, this means the voltage drop of the resistor must be big
• as the capacitor charges, the voltage drop across it increases, so the output voltage increases
• the shutter on the camera will open
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## Digital Systems and Logic Gates

Digital systems are either on or off either high or low either 0 or 1

Logic gates a type of digital processor, they are small but are made of really small components like transistors and resistors

each type of logic gate has it own sets of rules

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## NOT gate

Sometimes called an inverter

has one input which can be 1 or 0

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## AND gate

AND gate only gives an output of 1 if both the first and the second input are 1

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## OR gate

an OR gate just needs to be either 1 or 0

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## NAND gate

a NAND gate is like combining an AND and a NOT gate, if a AND gate would give and output of 0 a NAND gate would give 1

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## NOR gate

a NOR is like combining a NOT with an OR

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## Latch Circuit

• R= Input F
• S= input R
• Q= u
• Q- = Output T
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## Latch Circuit

1. Input F is 0 and output T is 0

• the top NOR gates output is 1, so the bottom NOR gates output is 0
• which means output T is 0

2. when the door is opened or the temperature falls:

• Input F becomes 1 so output U becomes 0
• so the bottom NOR gate gives 1

3. when the door is closed/ temperature rises

• Input F becomes 0 again but output T is 1 still
• so output U stays 0 and output T stays 1

4. to reset, briefly make input R equal 1 and since output u is 0, output T becomes 0

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## LEDs- light emitting diodes

An LED is a diode which gives out light

It only lets current go through in one direction

you can use a light-emitting diode to show the output of a logic gate, if the output is 1 the LED will light up

An LED is a better choice to show output that an an ordinary incandescent bulb because it uses less power and lasts longer

It is often connected in series with a resistor to prevent it being damaged by too large a current flowing through it

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## A relay switch

the output of a logic fate usually only allows a small current to flow through the circuit

an output device like a motor requires a large current

the solution is to have two circuits connected by a relay

the relay isolates the low voltage electronic system from the high voltage mains often needed for the output device

also means that it can be safer for the person using the device- can make sure any parts that could come into contact with a person are in a low-current circuit

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## How a relay works

• When the switch in the low current circuit is closed it turns on the electromagnet which attracts the iron contact on the rocker
• the rocker pivots and closes the contacts in the high current circuit and the motor spins
• when the low current switch is opened the electromagnet stops pulling, the rocker returns and the high current circuit is broken again
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