AS level physics unit one section 3

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ghoran
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AS level physics unit one section 3
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2013-12-28 05:06:36
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current electricity finished january test but not unit
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  1. an electric current is defined as
    the rate at which electrically charged particles pass through a point in a circuit
  2. the size of the current is measured in
    coulombs per second or amperes (amps)
  3. how many amperes is equal to 1 coulomb per second
    1
  4. in metallic conductors the charge carriers are
    electrons ,
  5. which way do electrons move in a circuit
    from the negative terminal of the dc supply towards the positive charge . confusion can arise because current is normally shown as moving from the positive terminal towards the negative terminal . this is referred to as conventional current
  6. all current arrows on wires and component symbols point in
    the conventional current direction
  7. the size of the current is defined mathematically by
    • I = ΔQ/Δt 
    • where :
    • I = current in amperes (A)
    • Q = charge in coulombs (C)
    • t = time in seconds 
    • Δ = change in ... (charge or current in this case)
  8. what must exist to make a current flow
    potential difference (p.d.)
  9. a potential difference is defined as
    the electrical energy transferred or converted per unit of charge passing between the two points
  10. potential difference is measured in
    joules per coulomb or volts
  11. how many coulombs per second is equal to one volt
    1
  12. the size of the potential difference (p.d.) is defined mathematically by
    • V = W/Q 
    • were :
    • V = p.d. in volts (V) 
    • W = work (energy) in joules (J)
    • Q = charge in coulombs (C)
  13. a charge gains energy when it
    passes through a cell
  14. a charge releases the energy its gained as it
    passes through the components in a circuit (e.g. a lamp or resistor) i.e. p.d. exists across the component . thus both a cell and component have a p.d. across them when charge flows in a circuit
  15. charges faces opposition when they
    flow around a circuit . this is called resistance and it is measured in ohms (Ω)
  16. the potential difference needed to make a current flow in a circuit is dependant on the
    resistance in the circuit . the bigger the resistance , the more p.d. is required to make a certain current flow
  17. resistance is defined by the equation
    • R = V/I 
    • where :
    • I = current in amps (A)
    • V = p.d. in volts (V)
    • R = resistance in ohms (Ω)
  18. milliamps 
    symbol - 
    quantity - 
    • mA
    • 1X10-3 amps 
  19. microamps 
    symbol - 
    quantity - 
    • A
    • 1x10-6Amps
  20. kilohm
    symbol - 
    quantity - 
    • K
    • 1x103 ohms 
  21. megohms
    symbol - 
    quantity -
    • M
    • 1x106 ohms 
  22. draw a diagram to show the metric prefix scale
  23. the number of charge carriers is equal to 
    total charge/charge on charge carrier 
  24. example : in a conductor the charge carriers each have a charge of 1.6x10-19 C
    a) calculate the number of charge carriers passing a point in the conductor per second if the current is 4 microamps 
    • Q = It 
    • Q = 4x10-6 x 1 = 4x10-6C
    • number of charge carriers is equal to the total charge divided by the charge on charge carrier
    • number of charge carriers = 4x10-6/1.6x10-19
    • number of charge carrier = 2.5x10x1013
  25. draw a diagram to show a circuit that can be be used to investigate how the potential difference across a component affects the current through it . 
    explain how to use the equipment to produce characteristic curves for the component
    • the component under test is placed in the circuit as shown so that the circuit is complete when the switch is closed (note the diagram should have a switch on it) 
    • by varying the resistance using the variable resistor a range of current and p.d. values can be recorded for each change in resistance 
    • the battery is reversed and the variable resistor varied over the same range to produce  a second set of readings 
    • a graph of current/voltage can be drawn using the results this is the characteristic curve for the component
  26. draw a graph to show the characteristic curve for a resistor or wire both of which are ohmic conductors
    • when current is plotted against the p.d. a straight line graph is obtained 
    • the positive part of the graph shows current flowing from positive to negative and the negative part of the graph shows the current reversed . 
    • the current and p.d. are directly proportional to each other (straight line through the origin) when the current flows in either direction . the conductor is said to follows ohms law 
  27. draw and explain a graph showing the characteristic curve for a diode which is a semiconductor
    • in the case of the semiconductor diode , the shape of the curve obtained depends on the direction in which the current is flowing . 
    • when the diode is forward biased (arrow facing the direction of the conventional current) :
    • between 0V and about 0.7V , the diode offers a large resistance to current 
    • between about 0.7V and 1V the resistance of the diode falls rapidly and a large current flows - this is shown by the steep rise in the graph 
    • when the diode is reversed biased (arrow facing opposite direction to conventional current) :
    • the diode offers high resistance , so very little or no current flows 
    • at the breakdown voltage typically between 50 and 500 V , a large current flows 
    • most diodes cannot recover and are destroyed by the heating effect of the large current
  28. draw and explain a graph showing the characteristic curve for a filament lamp which is a non ohmic conductor
    when a filament lamp is connected in a circuit and the voltage is steadily increased the graph becomes less steep . the p.d. and voltage don't increase proportionally because the current heats the filament and so increases the resistance and therefor decreases the rate of increase of current with p.d. . the curve is symmetrical on either side of the origin showing that the lamp behaves in the same way for current flowing through it in either way .
  29. all the characteristic curves can be produced automatically using a voltage sensor (V) and a current sensor (A) . these together with a data logger (D) capture data which is then fed into the computer for analysis . draw A typical set up and explain how characteristic graphs can be produced from the equipment
    • please note that the filament lamp may be another component . 
    • the potential difference is varied across the component under investigation (wire , resistor , lamp , diode) using a potential divider and the current is recorded . the data logger software is then used to display the collected data in a tabular and graphical form
  30. ohms law is a special case and only applies to certain components in certain conditions . ohms law states that
    • the current in a conductor is directly proportional to the p.d. across it 
    • I is proportional to V 
    • provided that the temperature and other physical conditions remain the same 
  31. the current voltage graphs show clearly whether or not a component obeys ohms law .
    therefor name the ohmic conductors we have seen 
    the resistor/wire is the only ohmic conductor while the semiconductor diode and the filament lamp are non ohmic conductors 
  32. two factors which affect the resistance of a conductor are its 
    length and cross sectional area 
  33. resistance is ..... to length so doubling length ..... the resistance 
    • proportional 
    • doubles
  34. resistance is ..... proportional to area so doubling the cross sectional area ..... the resistance 
    • inversely 
    • halves 
  35. don't confuse cross sectional area wit diameter 
    cross sectional area of a wire is equal to
    pi x (d/2)
  36. doubling the diameter of the wire will ..... the resistance by .........
    • reduce 
    • a factor of 4 
  37. the resistivity of a material is given by :
    • resistivity = AR/l
    • where : 
    • A is the cross sectional area of the conductor in m2
    • R is the resistance of the conductor in ohms 
    • l is the length in m 
    • the resistivity is a constant of the material from which the conductor is made and measured in ohm metres 
  38. when converting mm2 to m2 divide by
    106
  39. draw a diagram to show how the resistivity of a material in the shape of a wire can be measured . and explain how to find the resistivity using the apparatus
    • please note that the wire under test is taped to a metre rule . 
    • start by measuring the 100cm of the wire under test . tape the wire on to a metre rule , to avoid any kinks or twists . connect the wire to the circuit using crocodile clips . 
    • record , in a table , the p.d. displayed on the voltmeter and the current displayed on the ammeter for this length of wire . 
    • move the voltmeter connection along the wire in the range 100cm to 30cm and record the p.d. and current for each length 
    • calculate the resistance of wire for each recorded length using R = V/I 
    • measure the diameter of the wire several times over its length , using a micrometer , to determine a mean value for the diameter 
    • use the mean diameter to calculate the cross sectional area using A=pix(d/2)2
    • plot resistance (y axis) against length (x axis) 
    • since R = resitivity x l /A 
    • the graph is a straight line through the origin and resistivity can be found from the gradient . the gradient = R/l = resistivity/area so to find resistivity we have to find the gradient go the graph and times it by the cross sectional area 
    • essential notes // avoid large currents which will heat the wire and increase the resistance
    • essential notes // a multimeter set on the ohms range could be used to measure the resistance directly , instead of using a battery , ammeter and voltmeter . however the ohms range usually has an uncertainty of + or - 1 ohms 
  40. temperature always affects conduction , no matter whether the material is a conductor , and insulator or a semiconductor . in conductors the resistance increases as 
    the temperature increases 
  41. metal wires and resistors have 
    free electrons that move when a p.d. is applied , causing a current to flow . the metal also has vibrating positive ions . electrons collide with these ions . causing the wire to have resistance to current 
  42. explain what happens as the temperature of the wire increases 
    the positive ions and electrons asorb heat energy , causing the ions to vibrate with greater amplitude and the electrons to move faster . both of these effects result in a greater number of collisions between electrons and ions i.e. the resistance of the conductor increases . However the gradient of the graph isn't very steep showing that resistance doesn't change greatly with temperature 
  43. draw a graph to show the increase in resistance of a conductor with increased temperature 
  44. a thermistor is a device used for
    temperature measurement and control
  45. in the case of thermistors the resistance decreases significantly as 
    temperature increases 
  46. draw a graph to show how the resistance of thermistors varys with temperature
  47. small increases in temperature produce large changes in resistance of the thermistor . explain why
    • the thermistor is made from semiconductor material and therefor has few electrons to produce a current . as the temperature of the thermistor increases , the thermal energy is enough to release further electrons from the ions to make the material conductive , this means resistance decreases .
    • essential notes// at higher temperatures the ions of the semiconductor vibrate more . This would normally cause the resistance to rise . However the release of conduction electrons is the dominant effect . this also explains the shape of the graph .
  48. why is care needed when passing currents through thermistors
    currents produce heat and this decreases the resistance of the thermistor , allowing more currents to flow . this further heats the thermistor producing further resistance changes and the process can continue until the component overheats and burns out or melts
  49. if the temperature of a conductor is reduced so that it approaches absolute zero (0k or -2730C) what happens to the electrical conductivity
    it disappears completely . The material is said to have become a superconductor . Its resitivity has dropped to zero and an electric current can pass through without transferring any energy to the conductor . the temperature at which the material becomes superconducting is known as the critical temperature T
  50. the critical temperatures for metal superconductors are typically
    close to absolute zero , 1 to 4k . ceramic superconductors now exist that have critical temperatures as high as 125K (-1480C)
  51. superconductors have important uses for example
    carrying electrical power without losses and constructing very strong electromagnets
  52. draw a graph to show resistivity against temperature for a high temperature superconductor
  53. superconductors are materials 
    that acquire zero resistance when they are cooled below a critical temperature 
  54. what are the rules for series circuits
    • potential difference is shared between various components . so the voltages round a series circuit always add up to the equal the source voltage 
    • current is the same everywhere 
    • the total resistance is the sum of all the resistances 
    • cell voltages add up
  55. what are the rules for parallel circuits
    • p.d. is the same across all components 
    • current is shared between branches
    • the total resistance of any number of resistors is given by 1/R= 1/R+ 1/R+ 1/R3
  56. Draw diagrams to illustrate how three 10 ohm resistors can be connected in four different ways . calculate the total resistance of each network of resistors
  57. to make current flow , a 
    p.d. must exist 
  58. the p.d. is the 
    amount of electrical energy that must be transferred to the charge and is measured in joules per coulomb , or volts 
  59. the charge releases the gained energy as it 
    passes through components in the circuit (e.g. lamp , motor , resistor etc) . all the potential energy lost by the charge is ultimately changed into heat 
  60. energy is measured in 
    joules 
  61. the energy converted to heat is given by  :
    • energy change (work done) W = VIt 
    • where :
    • W = energy change in joules (J)
    • V = p.d. in volts (V)
    • I = current in amperes (A)
    • t = time in seconds (s)
  62. power is the
    rate of change of change of energy and is measured in joules per second (Js-1) or watts (W) 
  63. power is given by :
    • P = IV 
    • where :
    • P = power in watts (W) or (Js-1)
  64. by substituting V=IR into P=VI We can arrive at an alternative equation :
    • P=I2R
  65. by substituting I = V/R into P=VI we can arrive at alternative equation 
    P=V2/R
  66. the equation P=I2R is important because
    it shows the heating effect is proportional to the square of the current . Therefor doubling the current will produce four times the rating of heating
  67. the power dissipated in a resistor R carrying current I is P . if the resistance is doubled and the current halved , what power is now dissipated 
    original power is given by P=I2R but I1 = (I/2)2  R1= 2R . Now power is given by P=I2/4X2R = I2XR/2 = 1/2 P
  68. in all circuits , what is conserved 
    electric charge i.e. all charge which arrives at a point must leave it 
  69. current is a flow of charge , so this can be stated as follows . At any point in a circuit where conductors join , the total current towards the point must equal
    • the total current flowing away from the point . or the algebraic sum of currents at a junction is zero . this is known as Kirchhoff's first law .
  70. in circuits , energy differences are expressed as
    potential differences and measured in volts .
  71. why do filament lamps blow
    • filament lamps are more likely to blow (the filament breaks) when you first switch them on . 
    • when you first switch a bulb on , the filament has a lower resistance because its cold . This 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 . 
    • the filament also heats up very quickly from cold to its operating temperature when it's switched on . the rapid temperature change could cause the filament wire to break to

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