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Flashcards

Chapter 3: Sensing

Matt Clancy Physics (AS) AS - Level Inglês 124 flashcards

Flashcards do conjunto (124)

  1. 13.1: CURRENT, P.D., AND ELECTRICAL POWER.
  2. 2What are ions?Electrically charged atoms/ molecules
  3. 3What gives metals their electrical properties?Presence of positive ions and the movement of negative electrons around them.
  4. 4What does a flow of charged particles produce?An electric current.
  5. 5Give an example of what this could be.Electrons in a wire.
  6. 6What is the current?The rate of flow of charge.
  7. 7What is the unit of current?amperes / amps, A.
  8. 8Why is the current the same at all points along a stream of charged particles?All the charged particles are moving in the same direction with the same mean speed.
  9. 9What must happens if some of the charges are diverted?The current must divide in such a way that the 2 divided currents add to give the original current.
  10. 10What is Kirchoff's first law?Currents at a junction must add up
    I = I1 + I2
  11. 11What is Kirchoff's first law a consequence of?The conservation of charge.
  12. 12Under what 2 circumstances will charges move?1) When they are attracted by charges of the opposite sign.
    2) When they are repelled by charges of the same sign.
  13. 13What happens inside a battery cell?Chemical reactions produce a potential energy difference between the 2 terminals- creates a +ve charged terminal and a -ve charged terminal.
  14. 14What will happen if the poles of the cell are joined by a conducting path?Both +ve and -ve charges will lose potential energy.
    Positive charges will flow from the + pole to the - pole
    Negative charges will flow from the - pole to the + pole.
  15. 15Why is the movement of negative charges more usual in a circuit?Metals contain free electrons, which are negatively charged.
  16. 16What happens when charges move between 2 points in a circuit?Their electrical potential energy changes by an amount ΔE.
  17. 17What is the work done in this movement given by?W = ΔE
  18. 18What is potential difference/ voltage, V?The potential energy difference ΔE per unit charge moving between 2 points.
  19. 19So what is the potential energy change?ΔE = W = VQ
  20. 20For negative charges moving 'uphill' from 0V to 1.5V, what is the potential difference, what about the potential energy change (and their potential energy)?The potential difference = +1.5V,
    Potential energy change is negative, so these charges have lost potential energy.
  21. 21What about positive charges moving 'downhill' through a p.d. of -1.5V?Potential energy change is negative, so they will also have lost potential energy.
  22. 22What will a p.d. between the ends of a wire do?It will accelerate the free electrons in it.
  23. 23Why do these electrons not gain kinetic energy?Their movement down the wire is obstructed by their interactions with the positive ion cores of metal atoms.
  24. 24What does the potential energy lost do?The potential energy lost does work on the wire, heating it.
  25. 25What is this 'wasted' energy called?Dissipation.
  26. 26What is the process called?The process is often called joule heating.
  27. 27What is power?Power is the rate at which energy is transferred.
  28. 28What is power measured in?The Watt, W.
  29. 29What is the ratio V/I for any conductor called?Its resistance.
  30. 30What is the equation for resistance?R = V/i
  31. 31What is the unit of resistance?Ohms, Ω
  32. 323.2: CONDUCTORS & RESISTORS.
  33. 33What happens when there is a constant current in a conductor?Forward push on the moving charges from the p.d. between the ends of the conductor is balanced by the obstructing effect of their interactions with the atoms in the conductor
  34. 34So what does electrical resistance do?Electrical resistance stops the charges accelerating.
  35. 35What is the resistance of a component?The ratio of p.d. across a component to the current the p.d. produces for that component
  36. 36What is resistance a measure of?A measure of how difficult it is to get current through the component- The number of volts of p.d. you need to get a current of 1 Ampere.
  37. 37What is the case for most metals at a constant temperature?The resistance is constant, so I∝V.
  38. 38What is this known as?Ohm's law.
  39. 39What is a conductor that obeys Ohm;s law described as?An ohmic conductor.
  40. 40What is the opposite of resistance?Conductance, G.
  41. 41What is conductance defined as?G = I/V
  42. 42What is the unit of conductance?The Siemen, S.
  43. 43Since conductance is the inverse of resistance, what may it also be given by?G = 1/R
  44. 44What can you think of conductance as?How many amperes of current you get from a p.d. of 1 volt.
  45. 45For parallel circuits, what happens to the current and potential difference?The current shared, p.d. constant.
  46. 46Why is the current shared?Charges moving from the battery divide at junctions (Kirchoff's first law)
  47. 47Why is the potential difference constant?Potential 'hill' down which each charge falls is the same whichever path, so p.d. across each conductor is the same.
  48. 48For series circuits, what happens to the current and potential difference?The current is constant, while the p.d. is shared.
  49. 49Why is the current constant?Charges moving from the battery can only take one path
  50. 50Why is the potential difference shared?As a charge Q moves around the circuit, the potential energy is dissipated as heat in the 2 conductors and the total energy must be that obtained from the battery, so:
    VQ = V1Q + V2Q,
    dividing by Q gives V = V1 + V2,
  51. 51What must you do in circuits with combinations of both series and parallel components?First add parallel components, then add series components.
  52. 52When are metals ohmic conductors?If the temperature can be kept constant.
  53. 53In reality, why can't the temperature be kept constant? What does this do?Current in a wire dissipates the energy through joule heating, which will raise temperature and in turn increase the resistance of the wire.
  54. 54What is a component which does not obey ohm's law under ordinary working conditions known as?A non-ohmic conductor.
  55. 55What is a negative impact of metals having an increased resistance and decreased conductance when they get hotter?Problem if precise and constant resistance values are needed.
  56. 56What is a positive impact of metals having an increased resistance and decreased conductance when they get hotter?The known variation of the resistance of metals with temperature allows them to be used as temperature sensors.
  57. 57What happens when samples of semiconducting non-metals such as silicon are heated?The resistance decreases.
  58. 583.3: CONDUCTIVITY & RESISTIVITY.
  59. 59What are the resistance and conductance of a metal wire examples of? Why?Extensive properties. This is because they depend on the length and thickness of the wire, as well as the metal.
  60. 60What do intensive properties of a material not depend on? What does this mean?The dimensions of the sample used, constant for materials.
  61. 61Give 3 examples of intensive properties of materials.1) Density
    2) the Young Modulus
    3) the Refractive Index
  62. 62What are 2 intensive properties linked to resistance and conductance?Resistivity and conductivity.
  63. 63In terms of resistance, what does doubling the length of a wire do? Why?Doubling the length L doubles the resistance R.
    Because resistances in series add.
  64. 64What 2 things does this imply?That:
    R∝L
    G∝1/L
  65. 65In terms of conductance, what does doubling the cross-sectional area of a wire do? Why?Doubling the cross-sectional area A doubles the conductance G.
    Because conductances in parallel add.
  66. 66What 2 things does this imply?That:
    G∝A
    R∝1/A
  67. 67What does combining these 2 relationships give?R∝L/A
    and
    G∝A/L
  68. 68Why does each of these relationships need a constant of proportionality which is true for a material?To allow calculations for and comparisons between different materials.
  69. 69What does these 2 relationships become with constants of proportionality? What are these constants and what are their units?G=σA/L
    where σ(sigma) is the electrical conductivity (unit Sm^-1)
    and
    R=ρL/A
    Where ρ(rho) is the electrical resistivity (unit Ωm)
  70. 70What is the relationship between electrical conductivity and electrical resistivity?Electrical conductivity is the inverse of electrical resistivity and vice versa.
    σ = 1/ρ just as G=1/R
  71. 71What can we use conductivity and resistivity to do?We can use them to classify materials as conductors, insulators or semiconductors.
  72. 72Why must a logarithmic scale be used for the range of values of conductivity and resistivity?Because the range of values for conductivity and resistivity is immense.
  73. 73What are the conducting properties of: 1) Superconductors? 2) Semiconductors? 3) Insulators?1) Superconductors have no resistance at all
    2) Semiconductors conduct, but not very well.
    3) Insulators conduct very little; almost not at all.
  74. 74What is the conducting difference between pure metals and alloys?Alloys generally conduct less well than pure metals.
  75. 75Give 3 examples of good conductors.1) Silver
    2) Copper
    3) Gold
  76. 76Give an example of a poor conductor.Polystyrene.
  77. 77What 3 things are needed to measure resistivity or conductivity for an insulator?1) Large value of A, Tiny value of L
    2) Large p.d.
    3) Very sensitive ammeter.
  78. 78What 3 things are needed to measure resistivity or conductivity for metals?1) Large value of L, tiny value of A
    2) Must be aware of complications caused by Joule heating of the wire.
    3) Must also be aware of systematic errors such as p.d. drops in places other than across the conductor in question
  79. 793.4: CONDUCTION UNDER THE MICROSCOPE.
  80. 80How do the structure and properties of metals show that the attractive forces in metals are large?The regular lattice structure of metallic crystals, with the high melting point and stiffness of metals.
  81. 81Describe the atomic structure of a metal.Lattice of positive ions surrounded by a delocalised cloud of mobile electrons electrons that move around in the lattice.
    This delocalised cloud of electrons provides the strong metallic bond through the force of attraction between the oppositely charged particles.
  82. 82What are these electrons represented as?Individual charged points moving with random velocities.
  83. 83What happens when a potential difference is applied across the lattice?Electrons each experience a force pulling them to the positive side. This produces a slow general drift in one direction superimposed on the rapid random motion.
  84. 84What is the rate of flow of charge due to this net movement?The current produced by the p.d.
  85. 85What happens to the electrons at room temperature?Electrons have kinetic energy, but their motion is obstructed by positive ions which have kinetic energy but vibrate in fixed positions.
  86. 86What do these vibrations do?The vibrations increase the temperature.
  87. 87What does this mean in terms of the flow of electrons in a metal?The electron paths in the lattice are obstructed more in a hotter metal than in a cooler one.
  88. 88What is the number density of a material?The number density is the number of free charges per cubic metre of material.
  89. 89What is the comparative number density for insulators and conductors?Insulators have a much lower number density than conductors.
  90. 90What can be calculated if the number density of the electrons is known?The mean drift velocity of the electrons can be calculated.
  91. 91What is different about electrical insulators compared to metals?They do not have mobile charges; there may be ions and electrons in them but they are not free to move.
  92. 92What is different about semiconductors compared to metals?Semiconductors do have free charges, but very few.
  93. 93What happens when semiconductors are heated?The proportion of ionised atoms increases (electrons are liberated)- increase in the number density of charge carriers so increased conductivity
  94. 94What are thermistors?Thermistors are cheap sensors made from semiconducting material.
  95. 95What does increasing the temperature of a thermistor do?Liberates electrons, causing the conductivity to increase, so resistance decreases
  96. 96What is another example of a semiconductor device?Another example is light-dependent resistors (LDRs).
  97. 97What does light falling on the exposed semiconductor (LDR) do?Light falling on the LDR liberates electrons, increasing the conductivity and thus decreasing the resistance.
  98. 98What can both thermistors and LDRs be used as?They can both be incorporated as sensors in potential divider circuits.
  99. 993.5: POTENTIAL DIVIDERS.
  100. 100Why are some circuits called potential dividers?Because the applied p.d. is divided according to the relative resistance values of V1 and V2.
  101. 101What may be used as R1 in a temperature-sensing circuit? Why?A thermistor may be used, because its resistance, and hence the p.d. across it, falls as the temperature increases.
  102. 102What does this mean for R2?Output voltage across R2 will therefore increase with increasing temperature.
  103. 103What must be noted about the resistance of the thermistor?Does not vary in a linear way with temperature, so the output voltage will not vary uniformly over the temperature range.
  104. 104What is Vout across?The value Vout is across the resistor R2.
  105. 105What does loading a potential divider refer to?Placing a component in parallel with R2.
  106. 106What does loading a potential divider do?Lowers the resistance of the part of the circuit with R2, lowering the fraction of the total p.d. across this part of the circuit, lowering Vout.
  107. 1073.6: E.M.F. & INTERNAL RESISTANCE.
  108. 108What is the potential difference between the external terminals of the source?The p.d. is the potential energy difference per coulomb as the charges move 'downhill', releasing energy as they do so.
  109. 109What pushes the charges 'uphill' to start with?Within battery cells, chemical processes liberate electrons at the negative terminal (cathode) and consume them at the positive terminal (anode);
    Effectively moving electrons from the anode to the cathode and providing the energy to 'lift them' up the potential hill.
  110. 110What is the E.M.F.?The e.m.f. is the energy that the source gives for every coulomb of charge.
  111. 111What does E.M.F. stand for? Why is it better to use the abbreviation?E.M.F. stands for 'electro-motive force', but it is not strictly a force, so it is better to use the abbreviation.
  112. 112By applying the principle of conservation of energy to a circuit, what can you see?You can see that:
    Energy supplied by the battery = Energy dissipated by the circuit resistance.
  113. 113This is true for every coulomb passing, so what can we say for any complete circuit?Since p.d. = Energy transfer per coulomb,
    E.M.F. = The sum of all the p.d.s across the resistances of the circuit.
  114. 114What is this known as?It is known as Kirchoff's second law.
  115. 115What does the e.m.f. produced by a battery result in?A current through the entire circuit, including the body of the battery itself.
  116. 116How do the chemicals in the battery affect the flow of charges?The chemicals in the battery provide resistance to the flow of charges.
  117. 117So what does the battery have? What does this mean?The battery has internal resistance, r. This means that a p.d. develops across r, so there is a p.d. drop inside the battery itself ('lost volts' because not available to the circuit)
  118. 118In the circuit, what does the e.m.f provide? Form an equation from this.The e.m.f. provides the total p.d. across both the external and internal resistances, so:
    ε = V + Vr
  119. 119What is Vr and what is V?Vr is the p.d. drop / 'lost volts' within the battery itself,
    and V is the p.d. across the resistor R, and is also the p.d. across the battery terminals (the terminal p.d.)- provided the resistances of the connecting cables is negligible.
  120. 120What do ε and r both have in common?They are both constants of the battery.
  121. 121What 2 things happen in any real circuit?1) The terminal p.d. < the e.m.f (but this difference is not constant).
    2) As the load resistance R (the resistance of the circuit attached to battery terminals) decreases, the current I will increase.
  122. 122Why does the current increase as the load resistance decrease?Because the total circuit resistance, R+r, has been reduced.
  123. 123What other affects will this increase in current have?It will increase the 'lost volts' Ir and so reduce the terminal p.d.
  124. 124Why is it difficult to measure internal resistance in some battery cells?Because some battery cells have such a low internal resistance.

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AS - Level Physics (AS) FlashCards sobre Chapter 3: Sensing, criado por Matt Clancy em 03-04-2016.
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Matt Clancy
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