Electrostatics

Contents1. Electrostatic Phenomena

2. The Atom

3. Charges

4. Insulators and Conductors

5. Charging and Discharging

6. The Electroscope

7. Electric Fields

8. Uses and Dangers of Electrostatics

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Concept Map

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1. Electrostatic PhenomenaElectrostatics is the study of non-moving electric charges, sometimes called static electricity.

A simple experiment will demonstrate the phenomena.

1. Take a polythene rod and place one end of it near some pieces of paper.

Does anything happen?

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________________________________________________________________

2. Rub the rod with a cloth and again place it near some pieces of paper as shown in the diagram below.

Does the rod affect the paper after being rubbed?

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________________________________________________________________

This experiment tells us that the friction produced by rubbing the rod must have affected the rod in some way. We can do further experiments to discover the properties of such rods.

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Further ExperimentsExperiment 1

Experiment 2

Experiment 3

Experiment 4

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Experiment 5

Experiment 6

Questions:Q1. What do you think would happen if we brought two charged

ebonite rods next to each other?

Q2. There appears to be more than one type of charge. How can we tell this from the above experiments?

Conclusion:From the experiments we can conclude that:

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2. The AtomTo understand electrostatics it is first important to understand the basic structure of an atom.

An atom is made up of three different particles. This is demonstrated in the following diagram showing an atom of beryllium.

Protons - having a _______________ charge and present in the nucleus.

Neutrons - having no charge and present in the nucleus.

Electrons - having a _______________ charge and orbit the nucleus.

Q1. How many electrons are there present in the beryllium atom?

Q2. How many protons are there present in the beryllium atom?

Q3. Is the beryllium atom positively charged, negatively charged or neutral?

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This is true for all elements they are electrically neutral, having an equal number of electrons and protons.

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3. Charges

Measuring Electric Charge

The SI unit for measuring electric charge is the _______________.

The symbol for this is ____.

This is used for both positive and negative charges.

The charge can be found from the equation:

Q =I =t =

Q. How many coulombs of electricity pass through a lightbulb in 2 minutes if there is a current of 2 A passing through the circuit?

The charge on one electron is a negative charge of 1.6 × 10-19 C.

Q. What is the electric charge on a proton?

4. Insulators and Conductors

InsulatorsMaterials that do not allow electrons to move about inside them are called electrical insulators.

Electrical insulator - electrons all in fixed positions.

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Examples of insulators:

The method of charging by friction will only work when two insulators are rubbed against each other. When an insulator is charged by the friction method the charge remains on the surface of the material. This is because the charge cannot move through the insulator.

A charged insulator can be discharged by passing it quickly through a Bunsen flame.

ConductorsSome materials allow electrons to move about easily inside them. These are called electrical conductors.

Electrical conductor - ‘Free electrons’ can move.

Examples of insulators:

A way to charge a conducting object is by induction. Once a conductor is charged the charge will move throughout the object.

SemiconductorsNot all materials are so easy to classify. Some of materials allow only a very few electrons to move around. These are called semiconducting materials.

SuperconductorsSome materials, at very low temperatures (about -270 ºC), have zero resistance. There is a lot of interest in these materials as they could be used to transport energy with zero loss, and save a lot of money.

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5. Charging and Discharging

Method 1 — Charging by FrictionThe polythene rod starts off being neutral - having an equal number of positive charges (____________) and negative charges (_____________).

When a soft cloth rubs polythene the friction causes some of the electrons in the atoms to break free of their bonds and move from one object to the other.

The polythene rod ends up with a negative charge. This means that the electrons have moved from the _______________ to the ________________.

The soft cloth will also be left with a charge. It will be _______________ charged.

NEGATIVELY CHARGED MATERIALS POSITIVELY CHARGED MATERIALS

MATERIAL RUBBED WITH MATERIAL RUBBED WITH

Polythene Soft cloth

(duster)

Glass Silk

Ebonite Fur Perspex Soft Cloth

Plastic Comb Hair Acetate Soft Cloth

Q. List two materials that will gain electrons when rubbed.

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Q. Why can conductors not be charged using the friction method?

Method 2 — Charging by InductionCharge Two Conductors with Equal and Opposite ChargesStep 1

Step 2

Step 3

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Step 4

Charge One ConductorStep 1

Step 2

Step 3

Step 4

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The metal sphere is left with a _______________ charge.

The charge left on an object such as this will always be _______________ to that of the charged rod used.

EarthingIn the above example (step 3) the positive charge was removed from the sphere.This was achieved through a process called ______________ or ______________ .This means giving a path for the charges to flow between the charged object and the Earth.Earthing a Negatively Charged Conductor

Earthing a Positively Charged Conductor

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In both cases we end up with a neutral sphere.

Discharging a Charged InsulatorQ1. Why will touching the end of a charged rod not discharge it?

Q2. Explain how such a charged insulator rod can be discharged quickly.

Quick Revision Questions:1 Two metal balls, P and Q, each hangs from a nylon thread as

shown below. A negative charged rod is then placed between them. While P is repelled by the rod, Q is attracted to the rod. What are the charges of P and Q?

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P Q

A Uncharged Positive

B Positive Positive

C Positive Negative

D Negative Positive2 What will happen if a positive charge of static electricity is placed

in an electric field?A The charge will remain positive but become smaller.

B The charge will remain positive but become larger.

C The charge will become negative.

D A force is produced on the charge.3 A piece of metal foil, which is initially uncharged, is picked up by

a charged rod. Which one of the following diagrams is correct?

A B C D

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P Q

-

-

-

-

-

-

-

-

6. The ElectroscopeAn electroscope is used to test for charge and to test the sign of a charge (positive or negative).

Detecting a ChargeWhen the electroscope is uncharged it may be used to see if a material is charged or uncharged.

Q. Why does the gold leaf move?

Q. Can we tell if the charge is negative or positive?

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7. Electric FieldsJust like magnets have fields around them charges also have fields surrounding them. Electric fields are similar to magnetic fields in many ways.

Electric fields are represented by lines. Electric fields travel from a positive charge towards a

negative charge. Electric fields cannot cross each other or touch. Electric field lines represent the path a small positive charge

would follow is set free in the electric field.

Complete the following diagrams to show the electric field lines around the following charges.

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+

-

+

+

+ + + + +

- - - - -

8. Uses and Dangers of Electrostatics

Uses for Electrostatics

Flue-ash removalFlue ash is a mixture of dust and smoke produced by many factories and power stations.Charged metal plates in the chimney attract these particles and remove them from the exhaust gases.

Spray PaintingMany mass produced objects such as cars are spray painted.To increase efficiency and reduce paint usage the paint particles and the car body are given opposite charges. Thus, the paint will be attracted to parts of the car not yet covered by paint.

High Voltage GeneratorsA Van de Graff generator can be used to produce very large voltages up to 14 million volts.

Dangers of Electrostatics

LightningLightning is caused by charging produced by the friction between water droplets and air molecules.When the charge built up is large enough, the air will ionise allowing the charge to discharge to the ground.

Fires and ExplosionsCharge can build up on many objects such as planes and petrol tankers.If not discharged carefully a spark, similar to that produced by lightning, can start a fire or cause an explosion.

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