gcse science physics unit P6

HideShow resource information
  • 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 
1 of 17

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)
2 of 17

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)
3 of 17

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. 
4 of 17

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)
5 of 17

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 

6 of 17

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 
7 of 17

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
8 of 17

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 
9 of 17

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 

10 of 17

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 
11 of 17

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 
12 of 17

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 
13 of 17

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)

14 of 17

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 
15 of 17

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 
16 of 17

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 
17 of 17

Comments

Bushra Patel

Report

this is perfect!

Ebonyiley

Report

cheers mate, gotta test in about an hour

kellyroe1

Report

love that.

xXNUFCXx

Report

urr get a life man 

xXNUFCXx

Report

shout out j cookey

cylinderlobsided

Report

shout out to rayboy3000

Similar Physics resources:

See all Physics resources »