6A- Physics Mr. Gooden

Electrostatics:

1. Differentiate between charging by friction and charging by induction2. What is an electroscope? Explain how it works?3. Describe two hazards and applications of static electricity.4. Draw and describe I-V graph characteristics for:

a. Ohmic deviceb. Diodec. Filament lampd. thermistor

Types of Charging with Examples

Types of Charging

Charging means gaining or losing electron. Matters can be charged with three ways, charging

by friction, charging by contact and charging by induction.

Charging by Friction

When you rub one material to another, they are charged by friction. Material losing electron is

positively charged and material gaining electron is negatively charged. Amount of gained and

lost electron is equal to each other. In other words, we can say that charges of the system are

conserved. When you rub glass rod to a silk, glass lose electron and positively charged and

silk gain electron and negatively charged. 

 

Charging by Induction

We can also charge conductors without contact. Examine the given picture, it shows this type

of charging.

 

 

 

A and B

conductors are

neutral at the

beginning. When

we put a

positively charged plate near them, it attracts the electrons in the conductors. Electrons move

to the left part and protons stays. Thus, when we separate plates A and B they are charged by

induction, A is negatively charged and B is negatively charged. Be careful, there is no contact,

they are charged only by induction.

Basic Theory of Electrostatics - Today's Accepted Model

Matter is composed of protons, neutrons and electrons. In solids, protons and neutrons can't move and electrons might be able to move. In materials called conductors (metals), there are electrons that are free to move around. In materials called insulators (glass, wood, plastic), electrons can't move freely, rather they usually can only redistribute themselves within the molecules they are in (charge polarization - remember from chemistry?).

There are two electric charges. They are called positive and negative. The protons are positive and the electrons are negative.

The rules that govern the behaviors are:

- Like charges repel.

- Opposite charges attract.

- Only the negatives (elelctrons) move in solid conductors.

- In a neutral object there are equal numbers of positives and negatives. (Note: charge is a conserved quantity, so we can't create new charge in an interaction, just move some around.)

III. Charging by Friction

We charge the lucite by rubbing it with fur (or taffeta). When the two are rubbed together, electrons are ripped from the lucite and depositied on the fur. This makes the lucite positively charged and the fur becomes equally negatively charged. PVC when rubbed with flannel takes electrons from the flannel to become negatively charged.

IV. Charging by Induction

When an object is charged by induction, it acquires an opposite charge to that of the charging object.

Example: Charge sphere A by induction.

Frame 1:   two conducting neutral spheres.

 

Frame 2:

The negative PVC forces the electrons (also neg.) into sphere B (since the spheres are conductors, some of the electrons are free to move).

Frame 3:

In the presence of the PVC, the spheres are separated. This leaves the sphere A positively charged. The rod can then be removed. Sphere A is charged opposite to the charging object (PVC). Note: Sphere B can be any conductor like your finger or a wire that has been grounded.

Here is an animation showing the process of charging by induction:

What is Static Electricity?

Static electricity is generated when any material gains or loses electrons and becomes

positively (when it loses electrons) or negatively charged (when it gains electrons). The

acumulated charges are whats called static electricity.

It is called static because the charges do not move from where they where generated, unlike the

electricity used to power electronic devices that flows from one pole of a battery or power

source to the other. Most of the time it is produced when two objects come in contact or are

rubbed together.

For a static charge to remain in an object, it must have a high resistance to the flow of

electricity. That is why plastic balloons and hair remain charged, they are poor electric

conductors. Other materials like aluminum can get a static charge, but it will dissipate very

quickly because of its low resistance to the flow of electrons.

Images: Uses of Static Charges

Click thumbnail to view full-size

Static charges are used to remove dust and other pollutants in smokestacks

Practical Uses of Static Charges

The ability of opposite charges in static electricity is what's used when designing applications for

it.

Dust removal: There are some appliances that can eliminate dust from the air, like air

purifiers. They use static electricity to alter the charges in the dust particles so that they

stick to a plate or filter of the purifier that has an opposite charge as that of the dust

(opposite charges attract each other).

This effect is also used in industrial smokestacks to reduce the pollution that they generate,

altough they work in a very large scale, the effect is basically the same as the home air

purifier.

Photocopy: Copy machines use static to make ink get attracted to the areas where we

need the information copied. It uses the charges to apply the ink only in the areas where

the paper to be copied is darker (usually this means text or other information) and not

where the paper is white, this process is called xerography.

Car painting: To make sure a car's paint is uniform and that it will resist the high speeds

and weather to protect the car's metal interior, it is applied with a static charge. The metal

body of the car is submerged in a substance that charges it positively, and the paint is

charged negatively with the paint sprayer.

This process ensures a uniform layer of paint, since when there is enough negative paint in

the car the extra will be repelled by the paint already in the car.

It also ensures that the paint won't fall off, since the electrical attraction between the paint

and the car is stronger than if it was just sprayed.

All of these processes use electronic circuits to generate and control the static charges

generated. If you are interested in electricity and electronics, check out my electronic circuits for

beginners site, where you will find simple circuits to get you started, When you have learned

enough you can build your own practical static electricity circuits for your own use!

Even though lightning has a lot of energy, it can't be efficiently used or stored

Power from Static Electricity?

Current Voltage Characteristics

A resistor at constant temperature (ohmic conductor) 

Current is directly proportional to potential difference. Doubling the potential

difference doubles the current in the circuit. The resistance remains the same.

Plotting a graph of potential difference against current gives a straight line passing

through the origin (0,0). 

 

Ohm’s Law 

“The electrical current in  a conductor is proportional to the potential

difference applied to it provided the temperature remains the same.” 

 

V = IR

Potential difference =        current         x            resistance 

       (V, volts V)                  (I, amps A)                     (R, ohms ) 

Measuring current and potential difference  

Current is measure with an ammeter, ammeters are always connected in series

with the component of interest. 

Potential differences are measured using a voltmeter, voltmeters are connected in

parallel with the component of interest. 

By measuring the current and potential difference you can calculate the

resistance. 

   

A Filament Lamp    

Here the graph curves because as the filament heats it’s resistance goes up (the

resistance of the filament is changing).  

 

A diode  

A diode only allows current to flow in one direction through it (forward biased),

when the current tries to flow the other way (reverse biased) no current is allowed

to flow through the diode.  

    

 When the diode is reversed biased if we keep increasing the potential difference

the diode will eventually begin to conduct in the reverse direction, this is called the

break down voltage.

Thermistor  

The resistance of a thermistor decreases as it’s temperature increases.  

Thermistors can be used as thermostats, the thermistor is used in circuits which

monitor and control the temperature of rooms, freezers & fridges etc. 

    

Thermistors can have a positive or a negative temperature coefficient. A

negative temperature coefficient means that its resistance decreases with an

increase in temperature, this is caused by the release of extra charge carriers in

the thermistor.

LDR – Light Dependant Resistor  

The resistance of an LDR decreases as the light intensity falling on it  increases.  

LDR’s are used in circuits which automatically switch on lights when it gets dark,

for example street lighting.