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 

electronic components getting smaller (adv/disadv)

  • 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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