# 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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