# Electrostatic Physics Notes by akmalsfather

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```									Contents
1.   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

Concept Map

Physics                              Electrostatics - 1
1. Electrostatic Phenomena
Electrostatics 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?
_______________________________________________________________
_

_______________________________________________________________
_

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?
_______________________________________________________________
_

_______________________________________________________________
_

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.

Physics                                Electrostatics - 2
Further Experiments
Experiment 1

Experiment 2

Experiment 3

Experiment 4

Physics               Electrostatics - 3
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:

Physics                               Electrostatics - 4
2. The Atom
To 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?

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

Physics                                Electrostatics - 5
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
Insulators
Materials that do not allow electrons to move about inside them are called electrical
insulators.

Electrical insulator - electrons all in fixed positions.
Examples of insulators:

Physics                                 Electrostatics - 6





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.

Conductors
Some 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.

Semiconductors
Not all materials are so easy to classify. Some of materials allow only a very few
electrons to move around. These are called semiconducting materials.

Superconductors
Some 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.

Physics                             Electrostatics - 7
5. Charging and Discharging
Method 1 — Charging by Friction
The 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.

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

Physics                                 Electrostatics - 8
Method 2 — Charging by Induction
Charge Two Conductors with Equal and Opposite Charges
Step 1

Step 2

Step 3

Step 4

Physics                       Electrostatics - 9
Charge One Conductor
Step 1

Step 2

Step 3

Step 4

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.

Physics                           Electrostatics - 10
Earthing
In 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

In both cases we end up with a neutral sphere.

Physics                             Electrostatics - 11
Discharging a Charged Insulator
Q1.       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?
P                  Q
-       -
A Uncharged           Positive
-       -
B Positive            Positive
C Positive            Negative                    P -       -   Q
D Negative           Positive                          -       -
2 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?
+ +                     + +                     + +               + +
+                       +                       +                 +
+ +                     + +                     + +               + +
no charge           +                       -                 +
on foil         -                       +                 +
A                      B                       C         D

Physics                                 Electrostatics - 12
6. The Electroscope
An electroscope is used to test for charge and to test the sign of a charge (positive or
negative).

Detecting a Charge
When 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?

Physics                                 Electrostatics - 13
7. Electric Fields
Just 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.

+                                              +

+                                               -

+         +           +              +   +

-        -            -            -   -

Physics                                 Electrostatics - 14
8. Uses and Dangers of Electrostatics
Uses for Electrostatics

Flue-ash removal
Flue 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 Painting
Many 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 Generators
A Van de Graff generator can be used to produce very large voltages up to 14 million
volts.

Dangers of Electrostatics

Lightning
Lightning 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 Explosions
Charge 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.

Physics                             Electrostatics - 15

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