AS Practicals

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Estimating the power of an electric motor

  • If an electric motor is used to ift a known weight through a measured height we can calculate the useful energy output of the motor from the equation
    Work done = force x distance
  • We can also calculate the energy input from the motor using
    Electrical energy = VIt
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Photoelectric effect

  • A charged zinc plate is illuminated with a UV lamp through glass and the leaf remains charged (violet light only hitting it) but when the piece of glass is removed the leaf drops because UV light hits the zinc.
  • UV photons have energy greater than the work function and cause photoelectric emission.
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Measurement of resistance and production of I-V gr

  • A circuit with the component to be investigated and a variable power supply is needed.
  • An ammeter is connected in series with the component and a voltmeter in parallel.
  • Measurements of I and V can be taken as required and resistance calculated using the formula R = V/I.
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EMF and internal resistance of a cell

  • A circuit is set up with a cell, a resistor, an ammeter and a voltmeter in parallel with the cell.
  • The resistance of the circuit is changed to obtain a set of values of I the current through the cell and V the potential difference  across the cell.
  • A graph is plotted of V on the y-axis and I on the x-axis and the y-intercept gives the EMF and the gradient gives the negative value of the internal resistance (V = -rI + E).
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Demonstation of drift velocity of ions

  • Use of copper sulphate and potassium pomanganate on a piece of soaked filter paper to measure how quickly the different coloured ions travel from plus to minus or vice versa.
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Resistivity of a metal wire

  • Set up a circuit with a 1m length of wire and an ammeter in series and a voltmeter in parallel.
  • Measure the resistance of the wire and repeat for lengths 90cm, 80cm etc down to 10cm.
  • Measure the diameter of the wire and calculate the cross sectional area.
  • Plot a graph of resistance on the y-axis against length on the x-axis.
  • Find the gradient and multiply it by the c.s.a to find the resistivity.
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Resistivity of polythene

  • Polythene is a very poor conductor of electricity so to lower the resistance a very large area and very small length is used (R = pl/A).
  • This enables us to measure the very small current produced and calculate the resistivity.
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Measurement of refractive index

  • Draw around a rectangular glass block
  • Use a protractor to mark in the normal
  • Measure the angle of incidence i and the angle of refraction r for a number of incident rays.
  • Plot a graph of sin i against sin r.
  • The gradient = the refractive index
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Rotation of the plane of polarisation using a suga

  • Set up two polarising filters whose planes of polarisation are at 90 degress.
  • Measure the angle using a protractor.
  • Insert the sugar solution in a boiling tube between the two filters and rotate until minimum light level is reached.
  • Measure the angle of rotation using a protractor.
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