A) Know that electric current is the rate of flow of charge 1 What is current?

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1 PHYSICS AS UNIT 1 SECTION 3: CURRENT ELECTRiCiTY Question Answer A) Know that electric current is the rate of flow of charge 1 What is current? The rate of flow of charge Measured in amps (A) Symbol is I B) Understand and be able to use the equation I = ΔQ/ ΔT 2 How do you calculate current? 3 How is one coulomb defined? 4 How do you measure current in a The amount of charge that passes in 1 second when the current is 1 ampere. Using an ammeter Attach it in series (so that the current through the ammeter is the same as the current going through the component) C) Understand that potential difference is the work done per unit charge 5 What is The amount of work done per unit charge potential To make the charge flow around a circuit, you need to give the charge energy to difference? move This energy is supplied by the power source (e.g. a battery). When a unit of charge flows through the power source, it is raised through a potential And energy is transferred to the charge as potential energy 6 What is work? When energy is transferred, we say that work is done (work is done to raise it from one point to another) So the power source does work to move the charge around the circuit. Volts (V) Symbol is V or p.d The p.d across a component is 1 volt is when you convert 1 joule of energy moving 1 coulomb of energy through the component 7 What is p.d measured in? 8 What is one volt defined as? D) Understand and be able to use the equation V=W/Q 9 How do you calculate potential difference? 10 How is p.d. measured in a V = W/Q Where V is potential difference (in volts) Where W is work done (in joules) Where Q is charge (in coulombs) Using a voltmeter Which is placed in parallel (around a component) E) Know that resistance is defined by R = V/I 11 What is If you put a p.d across a component, a current will flow How much current flows for a specific p.d depends on the resistance of the component. Therefore resistance is the a material's opposition to the flow of electric current Measured in Ohms (Ω)

2 Symbol is R F) Know that Ohm s law is a special case where I V 12 What are Ohmic conductors? 13 What is Ohms law? 14 What would an I/V graph look like for an Ohmic conductor? Ohmic conductors are electrical components that obey the Ohm law. They are usually metal E.g. a fixed resistor A rule to predict how the current would change as the applied potential difference increases for a certain type of conductor (Ohmic conductors) Provided the temperature is constant, the current through an Ohmic conductor is directly proportional to the potential difference across it. It s a straight line going through the origin This means that the resistance is constant and I V (as I increases, V increases by the same amount) 15 How to find the resistance of the Ohmic conductor from the graph? 16 What does a steeper gradient mean? 17 What do the negative quadrants tell us? Y=mx+C I = mv R = V/I 1/R = V/I The steeper the gradient, the smaller the resistance The shallower the gradient, the larger the resistance The I/V line for an Ohmic conductor is symmetrical in the positive and negative quadrants. This means that the voltage has the same affect on current whether it flows one way or the other Therefore the voltage has the same affect on current whether it comes from an AC or DC source H) Recognise and understand I-V curves for semiconductor diodes and filament lamps 18 How does The steeper the gradient, the lower the resistance the gradient The gentler the gradient, the higher the resistance represent Because the gradient represents 1/R 19 How can you measure the I-V characteristic s of a component? 1) Use a variable resistor to increase/decrease resistance in small, equal steps. (Which increases/decreases the amount of current going through the circuit) 2) For each change in resistance, take a reading from the ammeter to find the current through the component and a reading from the ammeter to find the voltage across it. 3) Plot them on a graph 4) Reverse the direction of the current (by swapping the wires from the energy source) and collect negative I/V values. 5) Plot those on a graph too

3 20 What are the I-V characteristic s of a filament lamp? Curve that starts steep but gets shallower as the voltage rises Has rotational symmetry (same for AC/DC) Non Ohmic (not a straight line) 21 Why is the IV graph of a filament lamp curved? 22 Why do filament lamps blow? 23 What are semiconductors? Because, although it s a metallic conductor, it s not ohmic. Which means that the conditions don t stay constant the wire filament heats up which increases resistance. This is because as the particles in the wire heat up, they get more energy, which makes them move more. This makes it harder for the charge to get through the wire As the resistance decreases, the current increases This means that the filament will heat up more But there is a limit to the amount of current that can flow through them More current means an increase in temperature Which means an increase in resistance, which means current decreases again So they level off at a high current. More likely to blow when you first switch them on Because the filament has a lower resistance when it s cold Which means that the initial current flowing through the filament will be larger than the normal current So the filament is more likely to burn out at this time It also heats up very quickly from cold to it s operating temp when switched on This rapid temp change can cause the filament to blow too. Group of materials that aren t as good as conducting electricity as metals, because they have fewer charge carriers (e.g. electrons) However, if electricity is supplied to them (e.g. by increasing temperature) more charge carriers (e.g. electrons) can be released and resistance decreases This means that they can become sensors for detecting changes in environment I) To be able to describe the qualitative effect of temperature on the resistance of metal conductors and thermistors J) Know that semiconductors can be used as sensors (e.g. temperature sensors) 34 What is a thermistor? 35 How to find IV graph of a thermistor? Component with resistance that depends on it s temperature NTC thermistors are Negative Temperature Coefficient Which means that the resistance decreases as the temperature goes up

4 Measure it s current and voltage at then plot current against V 36 How to find Resistance/ temp graph of a thermistor? Measure it s resistance at different temperatures and plot them on a graph Control temp by using water bath and digital thermometer 37 What does the IV graph tell us about a thermistor? 38 What is a diode? 39 What are the IV characteristic s of a diode? 40 What are LDRs? 41 What are the IV characteristic s of an LDR? The gradient gets steeper as the voltage increases Which means that as the voltage increases, the resistance is getting smaller. Therefore warming a thermistor frees up the electrons of the atom, meaning more charged particles can flow And therefore the current can flow more easily It also has rotational symmetry so can be fitted to an AC or DC power supply A diode is designed to let current flow only in one direction Forward bias is the direction in which the current flows from negative to positive Diodes require 0.6 volts in the forward bias before they will conduct This is called the threshold voltage In reverse bias, the resistance of a diode is very high and the current that flows is very tiny. At about -50 volts in reverse bias, the diode breaks down and little/no current will flow. Light dependent resistor Similar to a thermistor As the voltage (light intensity) increases, the gradient get s steeper so resistance is less and the current is greater However, rather than heat providing this extra potential difference (voltage) it s light

5 42 What would a resistance/ light intensity graph of an LDR look like? As the light intensity increases, the resistance decreases Because the light energy frees up the electrons in atoms Allowing more current to flow easier K) Know that the resitivity (ρ) of a material is defined as ρ = RA/L 43 What is A measure of how much a particular material resists current flow. resistivity? 44 What does Resistivity depends on the structure of the material, the environmental factors resistivity such as temperature and light intensity. depend on? It is a property of the material 45 What is resistivity defined as? 46 What does resistance depend on? 47 What is the equation linking these? 48 How does area affect 49 How does length affect 50 How does resistivity affect The resistance of a 1m length with a 1m 2 cross sectional area. It is measured in ohm-meters (Ωm) Length (L) Area (A) cross sectional area Resisitivity (ρ) ρ = RA / L which can be rearranged to give: R = ρl / A from the equation ρ = RA / L, we get R = ρl / A So area is inversely proportional to resistance ie, as area increases the resistance decreases from the equation ρ = RA / L, we get R = ρl / A So length is directly proportional to resistance ie, as length of wire increases the resistance increases from the equation ρ = RA / L, we get R = ρl / A So resistivity is directly proportional to resistance ie, if resistivity of different metals increases the resistance increases L) Know that siperconductivity is a property of certain materials which have zero resistivity at a critical temperature which depends on the material 51 What is normal materials have some resisitivity even really good conductors like silver wrong with that resistance means that whenever electricity flows through them, they heat up, normal and some of the electrical energy is wasted as heat materials? 52 What are superconductors? 53 What is the difficulty with superconductors? You can lower the resistivty of many materials like metals by cooling them down. If you cool some materials down to below transition/critical temperature, their resisitivty dissapears entirely and they become a superconductor Most metals have critical temperatures below 10 Kelvin (-263 C) Getting things that cold is difficult and expensive The closest we ve got is a metal oxide at 140K But we still have a long way to go M) Know some applications of superconductors (e.g. very strong electromagnetics and power cables) 54 What are the They have zero resistance, so none of the electrical energy is turned into heat, benefits of none is wasted super- this makes them extrememly efficient

6 conductors? electrical circuits will work really quickly because theyre s no resistance to slow current down 55 What are some uses of superconductors? 1) Power cables that transmit electricity without any loss of power 2) Really strong electromagnets that have many applications e.g. in medicine (MRI scanners) and the Maglev trains N) be able to find the power of a component using P=IV, P=I 2 R or P=V 2 /R 56 What is The rate of transfer of energy power? Measured in Watts (W) Where 1 Watt is equivalent to 1 Joule per second 57 How to calculate power using Energy? 58 How to calculate power in electrical circuits? 59 How can you calculate power from P = E/t where P is power in Watts E is energy in Joules t is time in seconds P = VI where P is power in Watts V is p.d. in Volts I is currents in Amps Because p.d. is defined as the energy transferred per coloumb Current is the number of coulombs per second So I x V is the energy transferred per second if P = IV and V = IR then I = P/V substitute I into V = IR making V=PR/V Simplifying gives V 2 =PR Rearranging gives P=V 2 /R if P = IV and V = IR rearrange P = IV to give V = P/I Subsitute that into V=IR to give P/I=IR simplify to give P=I 2 R O) Know how to find the energy transferred in a curcuit: E=V I T 60 How to use P=E/t and P=VI to find Energy? 61 How to use P=E/t and P=V 2 /R to find Energy? 62 How to use P=E/t and P=I 2 R to find Energy? If P=E/t and P=VI then E/t=VI Therefore E=VIt If P=E/t and P=V 2 /R Then E/t=V 2 /R Therefore E=(V 2 /R)t If P=E/t and P=I 2 R Then E/t=I 2 R Therefore E=I 2 Rt P) Understand the high current required for a starter motor in a motor car 63 Why is a high current required to Modern cars have a starter motor which provides energy needed to start the engine The power produced by the starter motor needs to be very high because it needs

7 start the motor in a motorcar? to produce a lot of energy in a short amount of time (P=E/t) However, the p.d. is usually very small (about 12 Volts) P=VI so in order to provide a high Power with a low voltage, the current needs to be really high Q) Know what is meant by the internal resistance of a power source, r 64 What is Resistance comes from electrons colliding with atoms and losing energy 65 How do batteries produce energy? 66 What is internal Chemical reactions provide energy to make the electrons move In a battery, the electrons gain energy from the chemical reaction and collide with the atoms inside the battery this makes them lose energy and therefore the battery has resistance Internal resistance is the resistance within the power source the load resistance is the resistance of all the other components in the circuit (NOT the power source) 67 What is load R) Know what is meant by the electromotive force (e.m.f.) ε 68 What is the electromotiv e force? S) Be able to use: ε=e/q and ε=i(r+r) 69 How to calculate the ε? 70 What is the terminal p.d. (voltage) 71 What is the energy wasted by the internal 72 How to work out the e.m.f.? 73 What is the equation given for calculating ε using The amount of electrical energy porudced by the battery and transferred to each coulomb of charge is the e.m.f ε=e/q where ε is the e.m.f. in Volts E is Energy in Joules Q is charge in Coulombs The p.d. across the load resistance (R) is the energy transferred when one coulomb of charge flows through the load resistance This p.d. is called the terminal p.d. (voltage) If there was no internal resistance, the terminal p.d. would be the same as the e.m.f. However, in reality there s always some energy lost overcoming the internal resistance, so the terminal voltage is always slightly lower than the e.m.f. It s called the lost volts energy per coulomb supplied by the source (e.m.f.) = energy per coulomb used in load resistance (terminal voltage) + energy per coulomb wasted in internal resistance (lost volts) ε=i(r+r) where ε is the e.m.f. in Volts I is the current in Amps R is the load resistance r is the internal resistance 74 How can you use this to ε=i(r+r) expanding this gives ε=ir+ir

8 find voltage? and remember that V=IR So ε=v+v where V is the terminal voltage and v is the lost volts 75 How can you find terminal voltage? If ε=v+v then V= ε v T) Understand applications of e.m.f. and internal resistance, e.g. the low internal resistance of a car battery 76 How does a A starter motor needs a large current in order to supply large amounts of power car use a low considering the voltage is fairly low (12V) internal the battery therefore needs a low internal resistance to supply the large current 77 What are other uses of low internal 78 How to calculate the energy dissipated due to internal 79 How can you measure the internal 80 How to use the graph to find internal 81 How to use the graph to find e.m.f? Batteries (e.g. for torches or personal stereo (what?)) are the same they need a low internal resistance, generally batterues have an internal resistance of less than 1Ω P=I 2 R replace the R with the internal resistance and you have P=I 2 r in which the P is the energy lost as heat 1) set up the circuit as shown: 2) Vary the value of R (load resistance) and record the voltage and current at each different resistance 3) Plot V against I (should be a straight line graph) if you rearrange V= ε-ir to get V=-rI + ε then it s in the form of y=mx+c Therefore the internal resistance is the same as the gradient Which is calculated by finding the Δy/ Δx Note the gradient is negative because it s sloping downwards if you rearrange V= ε-ir to get V=-rI + ε then it s in the form of y=mx+c So the Y intercept (c) is ε, which is the e.m.f. of the source U) Understand that energy and charge are conserved in circuits 82 What As charge flows through a circuit, is doesn t get used up or lost this means that whatever flows into a junction will flow out again charge at a Current is the rate of flow of charge, so current too must be conserved at a junction? junction. 83 What is The total current entering a junction = the total current leaving it Kirchhoff s first law? I t = I 1 + I 2 + I 3 84 What Energy is always conserved In electrical circuits, energy is transferred around the circuit

9 energy in a Energy transferred to charge is the e.m.f. and energy transferred from a charge is the potential difference (p.d.) In a closed loop, these two quatities must be equal if the energy is conserved (which btw it is) 86 What is The total e.m.f. around a series circuit = the sum of the p.d.s across each Kirchhoff s component second law? aka ε= ΣIR (sum of the current multiplies by the load resistance) V) Know the relationship between current, voltages and resistance in series and parallel circuits, inclusing cells in series and identical cells in parallel 87 What current in a series 88 What e.m.f in a series The current will be the same at all points in the circuit because there are no junctions and current is always conserved The e.m.f. is split between components Kirchhoff s 2 nd law states that ε= ΣIR so ε= V 1 + V 2 + V 3 W) Be able to use R total = R 1 + R 2 + R 3 for resistors in series 89 What The voltage splits proportionally according to the resistance e.g. V=IR, so if R is higher, then V is higher voltage in a So if 1Ω resistor and 3Ω resistor, the 1Ω resistor would have ¼ of the p.d and the series 3Ω resistor would have ¾ of the p.d. 90 What resistance in a series 91 What current parallel 92 What voltage in a parallel V=IR IR total = IR 1 + IR 2 + IR 3 The current (I) is constant (always the same) so R total = R 1 + R 2 + R 3 The total resistance is equal to the sum of the resistors The current splits at junctions Kirchhoff s 1 st law states that total current entering a junction = the total current leaving it so I = I 1 + I 2 = I 3 There is the same p.d. across all components If there are separate loops, the e.m.f. equals the sum of the individual p.d.s so V/R total = V/R 1 + V/R 2 + V/R 3 X) Be able to use 1/R total = 1/R 1 + 1/R 2 + 1/R 3 for resistors in parallel 93 What resistance in a parallel 94 What is the total value of cells in series? 95 What is the total value of identical cells There is the same p.d. across all components So the Vs cancel out in this equation V/R total = V/R 1 + V/R 2 + V/R 3 giving: 1/R total = 1/R 1 + 1/R 2 + 1/R 3 For cells in series, you calaculate their total e.m.f. by adding their individual e.m.f.s this makes sense because each charge (electron) goes through each of the cells so gains every e.m.f. (electrical energy) from each one ε total = ε 1 + ε 2 + ε 3 Their total e.m.f. is the same size as the e.m.f. of each of the individual cells This is because the amount of charge flowing in the circuit is still the same, and it only goes through one of the e.m.f.s (batteries for example) so it only gains one lot

10 in parallel? of the e.m.f. (electrical energy) Y) Understand how the potential divider can be used to supply variable p.d. 96 What is a A circuit with a voltage source and a couple of resistors in series potential the p.d. across the voltage soure is divided across the resistors in the ratio of their divider? resistances 97 What does a potential divider supply? 98 What does a potential divider circuit look like? You can use a potential divider to supply a potential difference (V out ) in the range of zero (minimum) up to the value of the e.m.f. (maximum) This can be useful if you want a varying p.d. source or one that is lower than the e.m.f. of the power source The V in is the e.m.f the V out is the voltage across the R 2 resistor 99 How can you find the value of the input voltage? 100 How can you find the value of the current? 101 How to find the V out? The total resistance of the external circuit (not internal) is R total =R 1 +R 2 The total voltage across the resistors is V s (total) and the current through them is I, which is constant So, V s = V 1 + V 2 which equals V s = IR 1 + IR 2 Which equals V s = I(R 1 + R 2 ) rearranged to find current is V s = I(R 1 + R 2 ) rearranged to find current is: I = V s / (R 1 + R 2 ) V=IR and I = V s / (R 1 + R 2 ) V out =IR 2 (output voltage is the current x the resistance of the second resistor) so when I = V out / R 2 I = V s / (R 1 + R 2 ) becomes: V out / R 2 = V s / (R 1 + R 2 ) which can be rearranged to give V out = R 2 / (R 1 + R 2 ) V s Z) Understand how you can use a thermistor or LDR in a potential divider to create a sensor 102 What is a A resistor that changes depending on the temperature thermistor? A NTC thermistor has a high resistance at low temperatures, but a low resistance at high temperatures

11 103 How may a thermistor work in a potential divider? Here, R 1 has been replaced with a thermistor If hot, the thermistor has a low resistance This means that R 2 has a high resistance when hot According to V=IR, a high resistance means a high voltage, so the V out will be high in the heat (and therefore low in the cold) 104 What is an LDR? 105 What are uses of Thermistor and LDR potential dividers? A resistor that changes depending on the light intensity An LDR has a high resistance in the dark and a low resistance in the light this means that in the dark, an LDR will have high resistance meaning the R 2 will have a low resistance and (according to V=IR) a low voltage But in the light, the LDR has a low resistance, so R 2 has a high resistance so the V out will be high Heat sensors Fire alarms control switches heating systems AA) Know that potential dividers have many applications e.g. as audio volume controls 106 What is a A circuit with a variable resistor replacing R 1 and R 2. potentiometer? It uses the sme idea as a potential divider 107 How does a potentiometer work? 108 What can a potentiometer be used as? You move a slider or turn a knob to adjust the relative sizes of R 1 and R 2 This way you can vary V out from 0V up to the source voltage this means when R 1 is at it s maximum resistance, V out will be at it s lowest (minumum volume) and when R 2 is at it s minumum resistance, V out will be at it s highest (maximum volume) A volume controller audio control in a loudspeaker