Electrostatics

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Electrostatics

K Warne

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Rubbing materials

Electron clouds from the atoms interact with each other.

Atoms from different substances usually attract electrons with different strengths.

Electrons from the atoms in one substance can get rubbed off onto the atoms of the other substance.

The substance receiving electrons becomes NEGATIVE the substance loosing electrons becomes POSITIVE.

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ELECTRON TRANSFERRED

CHARGED ATOMS (IONS) FORMED

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Separating Charge

If a polythene rod is rubbed with cloth electrons are transferred to the rod.

The total number of negatives and positives are still equal.

Charges have simply been separated – charge has not been created.

The Law of Conservation of charge states that:

CHARGE CANNOT BE CREATED OR DESTROYED ONLY TRANSFERRED FROM ONE PLACE TO ANOTHER.

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(+5) + (-5) = 0

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A charged object experiences a force in the vicinity of a second charged object.

We say there is an electric field present in the space around the charged object.

An electric field is defined as a space in which a charged object will experience an electrostatic force.

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Force of repulsion

Electric field

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Factors affecting force

Size of the charge: +1, +2 etc.

As the charge INCREASES the force INCREASES.

Distance between charges.

As the distance INCREASES the force DECREASES.

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Force of repulsion

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More charge = bigger force!

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Bigger distance = smaller force!

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Polarization An uncharged object is ATTRACTED by a charged object.

This is due to POLARISATION.

The charged object attracts opposite charges in the object.

This causes a CHARGE SEPARATION or POLARISATION in the object.

An electrostatic force is then established between the two objects.

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The distribution of charge on a conductor depends on its shape.

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Electric charge is concentrated around a point.

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1 Coulomb of charge is an _______ charge.

1 electron has a charge of ___________ - this is called the _______________ charge.

The number of electrons in 1 C is:

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When the collecting sphere of a large Van der Graaffgenerator is charged sufficiently and the spark is 10 cm long, there is only 0,01 C on the sphere.

Typical everyday charges are of the order of

picocoulombs (pC)(x10-12C) ornanocoulombs (nC) (x10-9C).microcoulombs (C) (x10-6C)

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Charged

Neutral (polarised)

Each!Final charge on each sphere = ½(-3x10-6 )

= -1.5 x 10-6 C

How many electrons were transferred onto the neutral sphere???

No. Of e- = charge/(-1.6x10-19 ) = -1.5x10-6 /(-1.6x10-19 ) = 9.4x1012 electrons!

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Charge Problems

A sphere A with a charge of 2 nC was brought into contact with an identical sphere B having a charge of -3 pC. The spheres were then separated.

Calculate the final charge on each sphere and the number of electrons that were transferred.

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+2 nC

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-3 pC

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B

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Electroscopes

Gold leaf electroscopes are used to detect the presence of electrostatic charge.

When the electroscope is charged the gold leaf is repelled by the shaft (LIKE CHARGES) and it stands out to the side.

Metal disk

Metal shaftGold

foil (leaf)

Container with glass windows

Simplified diagram

UNCHARGED

CHARGED!

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INSULATION

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Charging Electroscopes

If a negatively charged object is brought close to the electroscope plate -electrons would be repelled by the platemaking the bottom of the shaft and gold leaf, negative.

The gold leaf would now repel the shaft.

THE POSITIVE CHARGES DO NOT MOVE!!!!

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Some of the electrons are now transferred to the ELECTROSCOPE.

The electroscope has now acquired a NEGATIVE charge.

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V d Graaf Generator

The brushes transfer electrons to and from the belt.

These electrons then accumulate on the metal dome.

The insulating stand keeps the charge from being transferred to the ground.

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Insulating stand

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Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides (with all the gaps filled in) as well as

other resources, are freely available on my resource sharing website:

www.warnescience.net

(paste into your browser if link above does not work)

Have a look and enjoy!

Keith Warne