Edexcel GCSE Physics

Transcription

1 Edexcel GCSE Physics Tpic 10: Electricity and circuits Ntes (Cntent in bld is fr Higher Tier nly)

2 The Structure f the Atm Psitively charged nucleus surrunded by negatively charged electrns Subatmic Particle Relative Mass Relative Charge Prtn 1 +1 Nucleus 1 0 Electrn 0 (0.0005) -1 Electricity Dide, +ve side --> -ve side Ammeter LDR Thermistr Battery Vltmeter Fixed Resistr Variable Resistr Filament Bulb Cell Mtr LED Series and Parallel - Series Circuits Clsed circuit The current is the same everywhere - Parallel Circuits Branched circuit Current splits int multiple paths Ttal current int a junctin = ttal current in each f the branches Vltage is the same acrss each branch Ptential Difference represented by V - Ptential difference is measured in Vlts - Energy transferred per unit charge, Jule per Culmb Measured acrss tw pints, as it is the amunt f energy per unit charge t mve frm ne pint t the next - Measured with a vltmeter, placed in parallel acrss a cmpnent - There can be a vltage acrss a cmpnent, in a clsed r pen circuit - When it is in a clsed circuit, and there is a ptential difference (vltage), current will always flw E = QV Energy transferred (jule, J) = charge mved (culmb, C) ptential difference (vlt, V) physicsnet.c.uk Here the resistrs are nt all the same, hence different amunts f current flw thrugh each branch

3 Current, represented by I - Current is measured in amps - Rate f flw f charge (the flw f electrns in the wires) Measured at any single pint n the circuit - Measured with ammeter which is placed in series - V = IR - Ptential difference (vlt, V) = current (ampere, A) resistance (hm, Ω) Resistance - Greater resistance, the harder it is fr charge t flw thrugh the cmpnent, therefre the current is smaller - Variable resistr changes the amunt f resistance f the cmpnent, changing the amunt f current that flws in the circuit Series Parallel Cmpnents are cnnected end t end All the current flws thrugh all the cmpnents Can nly switch them all ff at nce PD (ptential difference) is shared acrss the whle circuit PD f pwer supply = sum f PD acrss each cmpnent Current is the same thrugh all parts f the circuit Current at ne pint = current at any ther pint Ttal Resistance is the sum f the resistance in each cmpnent R 1 + R 2 = R Resistance f tw cmpnents is bigger than just ne f them, because the charge has t push thrugh bth f them when flwing rund the circuit Cmpnents are cnnected separately t the pwer supply Current flws thrugh each ne separately Yu can switch each cmpnent ff individually PD is the same acrss all branches PD f pwer supply = PD f each branch Because charge can nly pass thrugh any ne branch Current is shared between each f the branches Current thrugh surce = sum f current thrugh each branch Ttal resistance is less than the branch with the smallest resistance Tw resistrs in parallel will have a smaller verall resistance than just ne 1/R1 + 1/R2 = 1/R Because charge has mre than ne branch t take, s nly sme charge will flw alng each branch

4 Device Characteristics inteleducatinresurces.intel.c.uk gradient f each is 1/resistance, s a sharper gradient means a lwer resistance Hw resistance changes - With current As current increases, electrns (charge) has mre energy When electrns flw thrugh a resistr, they cllide with the ins in the resistr The current here is ding wrk against the resistance This transfers energy t the ins, causing them t vibrate mre (heating resistr) This makes it mre difficult fr electrns t flw thrugh the resistr S resistance increases, and current decreases This may be a benefit, as sme appliances like a taster use heating filaments that have a high resistance t get ht easily. - With temperature Nrmal wires - See abve, the same prcess ccurs as atms vibrate when ht THERMISTOR ONLY Htter temperatures, resistance is lwer Used in temperature detectrs/thermstats - With Length Greater length, the mre resistance, and the lwer the current Electrns make their way thrugh mre resistr atms, s it is harder t get thrugh than if yu were using a shrter wire - With Crss Sectinal Area Thinner wires give greater resistance Because less verall rm fr electrns t pass thrugh between atms - With Light LDR (Light Dependent Resistr) ONLY Greater the intensity f light, the lwer the resistance S resistance greatest when dark Used in autmatic night lights - With Vltage DIODE ONLY Dide allws current t flw freely in ne directin In the ppsite directin, it has a very high resistance, s n current can flw Efficiency lw resistance wires means less energy lss as current flws thrugh the circuit

5 Testing Relatinships: - Pints fr All Make sure cmpnent(s) d nt verheat, leave t cldwn between each reading Repeats and take average - Varied wire resistance Wires ranging in resistance frm 1Ω t 10Ω Cnnected t DC f 2, 4, 6,, 10, 12V Cnnected in series t an Ammeter, parallel t Vltmeter Make sure all the ther wires used have negligible resistance Measure the current fr each vltage fr each wire Plt a graph t shw the relatinship between the pd and current - Filament Lamps Cnnected t DC f 2, 4, 6,, 10, 12V Cnnect the filament lamp t Ammeter in series and Vltmeter in parallel, Measure the current fr each vltage Plt a graph t shw relatinship between the pd and current Nn-linear shws R varies - Dides Cnnected t DC f 1, 1.5, 2, 4, 6,, 10, 12V Cnnect t an Ammeter in series and Vltmeter in parallel, Measure the current fr each vltage Switch the dide the ther way rund t recrd current fr -1, -1.5, -2, -4V Plt graph fr the psitive and negative ptential differences t shw the relatinship - LDR Cnstant vltage f 12V Cnnect t ammeter Shine lamp immediately nt thermistr and measure current Mve the lamp ~10cm away and measure current Keep ding this until 50cm Calculate resistance at each light intensity Plt graph f resistance against light intensity - Thermistr Cnstant vltage f 12V Cnnect t an Ammeter Place in ice water with thermmeter Measure current at 0 degrees. Add ht water and stir, measuring current at 10, 20,, 60 degrees Calculate the resistance Plt a graph f resistance against temperature

6 Pwer - Energy transferred (jule, J) = current (ampere, A) ptential difference (vlt, V) time (secnd, s) E = I V t - Pwer is energy transferred per secnd P = E t - Pwer is directly prprtinal t current and vltage, s dubling current dubles pwer P = IV - Electrical pwer (watt, W) = current (ampere, A) ptential difference (vlt, V) P = I 2 R - Electrical pwer (watt, W) = current squared (ampere 2, A 2 ) resistance (hm, Ω) - Pwer lss is prprtinal t resistance, and t current squared - Energy is transferred frm chemical ptential in batteries t electrical energy in wires t any frm f useful energy in the devices they pwer AC/DC - AC is alternating current, which cmes frm the mains Current cntinuusly varies, frm psitive t negative (charge changes directin) - DC, direct current, is the mvement f charge in ne directin nly Cells and batteries supply direct current - In the UK, mains supply is at 50Hz and 230V Plug - In a plug there are 3 different wires - Live wire This is a brwn clur It carries vltage frm mains t appliance This may be dangerus even if mains circuit is ff, as current may still be flwing thrugh it - Neutral Wire This is a blue clur Cmpletes the circuit - Earth wire This has green and yellw stripes It is the safety wire used t stp the appliance becming live It is cnnected t the earth and t the casing If the live wire tuches the metal casing f the appliance, it will becme live (yu ll get a serius electric shck if yu tuch it, as current flws thrugh yu t the grund) - Fuse The earth wire is cnnected t the metal casing, and its lw resistance means the current will g frm the casing thrugh the earth wire and t the grund Cnnected t the live wire If a large current passes thrugh live wire, fuse heats up and melts, breaking the circuit preventing a fire r damage Pwer Rating the pwer f the appliance when in use - Greater pwer rating, greater energy cnsumptin per secnd S uses mre energy in a given time