# gcse science physics unit P6

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• Created by: charlie
• Created on: 15-06-13 20:17

## circuits + resistors

learn symbols: cell, battery, power supply, switch (open/close) bulb, resistor (fixed/variable) ammeter, voltmeter, thermistor, LDR

resistance is caused by collisions in a conductor

• when e- flow throw metal resistance comes from atoms collided with
• collisions cause atoms to vibrate- more vibration- more they get in way- more collisions
• inc. in collisions = inc. in resistance + temps

variable resistors

• longer the wire the more atoms electrons collide with
• used for controlling speed of motors or brightness of bulb
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## voltage-current graphs + resistance

slope (1/gradient) of the voltage-curent graph shows resistance

• current through resistor is proportional to voltage - straight line for fixed resistors (ohmic)
• filament lamp- current inc. so does temp- curved line where resistance changes (non-ohmic)

resistors in parallel reduce resistance in circuit

• resistors in series total resistance is sum  (Rt = R1 + R2 + R3)
• resistors connected in parallel provide more paths - lower resistance so higher current
• 1/Rt = 1/R1 + 1/R2 + 1/R3 (learn)
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## potential dividers

larger the share of total resistance the larger the share in total voltage

potential dividers are useful

• run device that requires certain voltage from battery of diff. voltage:
• Vout = Vin x (R2/R1+R2) (equation is given)
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## LDRs + thermistors

LDRs

• darkers it gets the resistance goes up- useful for electronic circuits- nightlights, burglar alarms

thermistors

• colder it gets resistance goes up- useful temp. sensors-car engine temp gauges + thermostats

thermistor + fixed resistor is potential divider makes temp sensor

• when temp inc. = voltage is 1 when it dec. voltage =0 when thermistor is in R1 position
• switch thermistor to R2 position + output would switch too
• replace fixed resistor with variable- make device triggering output at chosen temp.
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## transistors

electric switches

• small current used to control flow of much larger current
• Base = 'switch' contolling current flow- small current to base + large current through transistor
• Collector = current flows in
• Emitter = current flows out

Ie = Ib + Ic

e,g switching on an LED

• current flows when switch is closed
• high resistor before transistor base- no damage
• current closes transistor allowing current to flow throught turning LED on
• e.g used to turn on LED when temp in room drops cold - use potential divider circuit with thermistor in (temp drops, Vout inc., tansistor switched on, LED comes on)
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## logic gates

digital systems either ON or OFF

logic gate is type of digital processor

• made up of resistors + transistors - process info + give out diff outputs

NOT gate (inverter)

• input=1 output= 0            input=0 output=1

AND gate

• input= 0 0 output= 0       input= 1 0 output=0       input= 0 1 output=0     input=1 1 output=1

OR gate

• input= 00 output= 0       input= 1 0 ouput=1        input=0 1 ouput= 1     input= 1 1 output=1

NAND - compining not + and gates giving opposite outputs

NOR - combining not + or gates giving opposite outputs

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## using logic gates

• resistance changes 'threshold voltage'- how hot or bright needs to be for signal
• variable resistor allows threshold voltage to be adjustable

AND longic gates made from two transistors

• if input is 0 in either- current still cant complete circuit so output is 0
• if both inputs 1 then both transistors closed + current can flow
• other logic gates made from diff. combinations of two transistors
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## LEDs + relays in logic circuits

LED's - light emitting diodes

• diode which gives out light + only lets current through in one direction
• can be used to show output of logic gate - uses less power + lasts longer
• ususally placed in series with resistor so no large current can damage it

relay switch connects two circuits

• log-gate would be damaged if current needed for the motor is passed throught it
• relay isolates low voltage circuit from high voltage circuit needed for output device
• makes it safer to use e.g turning key to start car motor

how it works

• switch in low current circuit closed- electromagnet turned on- attracting iron contact on rocker
• roker pivots + closes contacts in high current circuit - motor spins
• low-current switch open- electromagnet stops pulling-rocker returns- high-current circuit broken
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## magnetic fields

region where magnetic materials (iron+steel) + wires carrying electrical current experience a force acting on them

current-carrying wire creates magnetic field

• made of concentric circles, wire centre, straight, - use Righ-Hand thumb rule

rectangular coil reinforces magnetic field

• concentric circles around each side o loop reinforce at centre
• if coil has lots of turns- magnetic field from all individual loops reinforce eachother more

meagnetic field round solenoid

• magnetic field inside current carrying solenoid (coil of wire) is strong + uniform
• outside is normal bar magnet field so ends of solenoid act like north + south poles
• if direction of current reversed poles switch sides
• looking into one directly tells you whether N or S
• inc. strength adding soft iron core inside(elctromagnet) or including more turns
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## the motor effect

a current in magnetic field experiences a force

• two magnetic fields affect one another resulting in movement
• full force wire is at 90d + is stronger when current or magnetic field made stronger
• forces always act same direction relatived to magnetic field + direction of current in wire
• chinging direction of field o current affects direction of resultant force

flemmings left hand reule tells you which way force acts

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## simple electric motor

4 factors which speed it up:

• more current      -more coil turns     -stronger magnetic field     -soft iron core in coil
• forces act on two side arms of coil + usual forces acting on any current in magnetic field
• because coil on spindle + forces act one up + down    - it therefore rotates
• split-ring commutator swaps contacts every half turn- keeping motor spinning in same direction
• direction of motor reversed by swapping polarity of DC supply or swapping magnetic poles over

practical motors have pole pieces which are very curved

• link coil to axle + axle spins round - can make axle powerful enought to turn anything
• use pole pieces which are so curved that they form hollow cylinder- coil spins inside
• curved pole pieces have radial magnetic field- inc. strength + making motor more efficient
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## electromagnetic induction

creation of voltage (maybe current) in wire which is experiencing change in magnetic field

moving magnet in coil of wire induces a voltage or conductor through magnetic field

• move magnet in opposite direction or polarity reversed means voltage/current reversed
• backwards + forwards creating AC current
• done also by spinning magnet end to end in coil- how generators work
• turn magnet- magnetic field through coil changes- inducing voltage- this induces a current
• turn magnet through half  turn direction of magnetic field reverses- voltage reverses- current flows in other direction
• keep turning current same direction with produce AC current

four factors affect size of induced voltage:

• magnet strength     -area of coil     -no. of coil turns     -speed of movement
• turn magnet faster creates higher peak voltage + higher frequency
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## generators

turning a coil to create a current

• constructed like motor- as coil or magnet spins a current is induced producing AC current
• slip ring commutator + brushes so that contacts dont swap every half turn
• so porudce AC can be displayed in CRO display

dynamos + power stations- turn magnet instead of coil

• still causes field through coil to swap every half turn producing AC current
• same CRO tracers
• power stations rotate electromagnet in coil of wire- output voltage changed by coils
• size + frequency changed by rotating electromagnet coil fasters
• sometimes used on bikes- pedals turn cog wheel on bike wheel+ as it moves it moves the magnet creating AC current to power lights
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## transformers

three types:

• STEP-UP TRANSFORMERS- step V up + have more turns on secondary coil
• STEP-DOWN TRANSFORMER- step V down + have more turns in primary coil
• ISOLATING TRANSFORMERS- dont change V + have same no. of coils both sides

work by electromagnetice induction

• primary coil produces magnetic field which stays with in iron core
• nearly all passed through to secondary coil- hardly any lost
• AC current in primary means field in core constantly changing direction
• changing magnetic field felt by secondary inducing alternating voltage in secondary
• relative no. of turns on two coils- whether voltage induced in secondary is more or less
• wouldnt use DC- nothing out of secondary- still magnetic field in iron core but wouldnt be constantly changing so no induction on secondary because you need a changing field

nearly 100% efficient- power in = power out  (VpIp = VsIs) (equation is given)

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## transformers part 2

transformer equation used either way up:

Vp/Vs = Np/Ns        V= voltage       N=no. of turns (equation is given)

transformers used on national grid

• trasmit lot of power you want high voltage- less power loss due to heat due to resistance
•  (power loss=current^2 x resistance)
• requires transformers + huge insulated pylons - stup up T. at start + step down T. at end

isolating transformers used in bathrooms

• bathroom shaver socket- safety- mains circuit connected to earth so if touched you will complete the circuit resulting i electric shock
• allows being able to shave without phsically being connected to mains- minimising risks of live parts touching earth lead - reducing risks
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## diodes + rectification

diodes can only let current flow in one way

• high resistance the other way - triangle points the way the current flows

diodes made from semiconductors like silicon

• made from two diff. types of silicon joined together at 'p-n junction'
• one half made from silicon that has impurity added to provide free extra e- called 'n-type semiconductor' (n stands for negative e-)
• different impurity added to other half resulting in fewer free e- leaving lots of empty space (holes) this is called 'p-type semiconductor' (p stands for positive holes)
• when no V across diode- electrons + holes combine creating no free e- or holes- now insulator
• voltage across diode :
• in RIGHT direction means free holes + e- have energy to get across insulating region to other side so current flows
• in WRONG direction- free holes + e- pulled away from insulating region staying on same side- no current flows

diodes used to rectify AC

• single diode only lets through current in half of cyle - half wave rectification
• full wave rectification needs 'bridge circuit' 4 diodes so current flows through component in same direction
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## capacitors

capacitors store charge

• charge it by connecting to source of V- current flows round circuit- charged stored in cap.
• flow of current dec. the longer charge time
• more charge on cap. the more V across it
• when V acroos cap. equal to battery- current stops + capacitor fully charged
• when battery removed, cap. discharges - current flows in opposite direction

cap. used in smoothing circuits

• from rectified AC it can be smoothed adding capacitor in parallel to output device
• component gets current alternately from power supply + capacitor

• markers : use less raw materials + customers like/complex to produce + more expensive
• users: more portable + more powerful + feature filled/ more expensive + easier to lose
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this is perfect!

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cheers mate, gotta test in about an hour

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love that.

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