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The electroscope can be unreliable and a bit confusing for

students. We can use it as an indicator of charge or

capacitance or potential difference. But what does it really

measure? It measures the PD between the leaf and the case,

maybe to the nearest 100V, if you are lucky. Granted, one of its

advantages is that its got a very high resistance when dry and

can detect the presence of small charges.

The three experiments we look at here are: The factors affecting the capacitance of a parallel plate

capacitor.

The distribution of charge on different insulated conductors. The photo-electric effect.

In these experiments we use a capacitance meter, a

coulombmeter and a charge sensor (part of a datalogging

system).

What do you do? Set up the apparatus as in the photograph.

Make sure the multimeter is set to capacitance (pF or nF ifpossible).

Measure the capacitance of the parallel plate capacitor.

Change the distance between the plates and measure the

capacitance again.

Change the common area between the plates, keeping the

distance between them constant, and measure the

capacitance.

Thirdly, place the polythene sheet (or other dielectric)

between the plates and measure the capacitance.

So what happened? Your measurements should show that the capacitance of the

parallel plate capacitor increases with common area and

decreases as the distance between the plates increases. The

capacitance should also show an increase with the presence

of the polythene ( whose permittivity is greater than that of

P H Y S I C A L S C I E N C E S M A G A Z I N E

Life without ElectroscopesIn this article I look at three Leaving Certificate Physics experiments where, traditionally, the gold-leafelectroscope is used. The electroscope, in one form or another, dates back to around the year 1600 so

maybe its time to take a look at more modern ways of doing things.

Jimmy Brophy, Our Lady's Secondary School, Castleblayney, Co. Monaghan

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The distribution of charge on different insulated

conductors

What will you will need?Insulated conductors of different shapes, proof plane,

coulombmeter, perspex and polythene rods, dry cloth.

What do you do? Charge one of the rods by rubbing it with the dry cloth

Use this charged rod to charge the pear shaped conductor by

induction. (You could use the Van de Graaff generator to

charge it).

Make sure that the coulombmeter is discharged.

The Photoelectric EffectBackgroundThe photoelectric effect involves the emission of electrons from

the surface of a metal by electromagnetic radiation. Thisradiation must be of a certain minimum frequency (threshold

frequency) for the electrons to be emitted. This frequency

depends on the work function of the metal. In this experiment

we use a zinc plate. The work function of zinc is such that the

threshold frequency is in the UV region of the electromagnetic

spectrum.

What will you will need?

Computer (running DataStudio), Pasco USB Link, Analog Adapter(needed if you have the older Pasco analog sensors), charge

sensor, zinc plate, UV lamp, small sheet of glass, Perspex and

polythene rods.

What do you do? Use sandpaper to thoroughly clean the surface of the zinc

P H Y S I C A L S C I E N C E S M A G A Z I N E

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