Also see: Charge

When certain different insulating materials are rubbed against each other they become electrically charged by friction. The charges stay still on the object.. therefore it is called static electricity. This only happens with insulators, with conductors the charge moves through them and doesn't stay put!

Click here for an interactive program that allows you to see buildup and discharge of static charge.

Electrically charged objects attract small light objects placed near to them. If the electric force due to the attraction is bigger that the weight of the small object then this will make them move. They will jump up and stick to the charged object. This is why statically charged objects get so dusty!

When two electrically charged objects are brought close together, they exert a force on each other. Two charged objects may either pull towards each other (attract) or push each other away (repel). These observations can be explained in terms of two types of charge called positive (+) and negative (-). Two objects which have the same type of charge repel. Two objects which have different types of charge attract.

Like repel..opposites attract!

See the animation of lightning

The Van de Graaff Generator makes hair strands all gain the same charge they therefore repel each other. They do so with a force that is greater than the weight of the strands of hair, therefore the strands stand up on end !!

When two different materials are rubbed against each other, electrons, which have a negative charge, are rubbed off one material on to the other. The material which gains electrons becomes negatively charged; the material which loses electrons is left with an equal positive charge.

Charging by Induction

When a charged object is brought near to a neutral one the electric field produced by the charged object disturbs the electron clouds around the atoms in the neutral object, distorting them and causing a disturbance in the balance of charge in the neutral object.

Click here for an interactive Java page

If a negatively charged rod is brought near to a neutral piece of paper the electrons on the surface of the paper retreat giving the surface a partial positive charge. This makes the paper attractive to the rod and it jumps up and sticks to it (providing it is not too heavy - because its weight will pull it down).

If a positively charged rod is brought near to a neutral piece of paper the electrons near the surface of the paper advance toward the rod giving the surface a partial negative charge. This makes the paper attractive to the rod and it jumps up and sticks to it (providing it is not too heavy - because its weight will pull it down).

Remember - the protons don't move! They are locked inside the nucleus... only fliuds could have positive nuclei influenced to move because of electric charge!

You should be able, when provided with appropriate information about a situation in which static electricity is dangerous, to explain why it is dangerous and how precautions can be taken to ensure that the electrostatic charge is discharged safely.

The gold Leaf Electroscope

Electrostatic charges can be useful in everyday life.

For example, in a photocopier:

• a copying plate is electrically charged;
• an image of the page you want to copy is projected on to the plate;
• where light falls on the plate, the electrical charge leaks away;
• the parts of the plate that are still charged attract bits of black powder;
• the black powder is transferred from the plate to a sheet of paper;
• the paper is heated to make the black powder stick;
• there is now a copy of the original page.


There are a few webpages on the photocopier that you might find useful to look at: -

Scientific American (October 1996)
Australian National University Webpage
Photocopier Theory

BBC Bitesize

• tiny droplets of ink are electrically charged as they are forced out of a very fine nozzle;
• the droplets pass between metal plates across which a voltage can be applied so that one plate is negative and the other plate is positive;
• the charged droplets of ink are attracted towards the plate with the opposite charge and away from the plate with the same charge. This means that they are deflected as they pass between the plates;
• the size and direction of the voltage applied across the plates is controlled so that each droplet in turn is deflected to a particular place on the paper;
• each droplet of ink produces a tiny dot on the paper and many such dots, each in exactly the right place, produce the printed characters.

When cleaning the air with an electostatic precipitator

As the waste gases pass the negatively charged wire grid the smoke particles pick up a negative charge.They are repelled by the grid, but attracted to the positive charge on large collecting plates.They stick to the plates, which are banged regularly by a metal striker causing the smoke particles to fall into the dust traps, from which they are removed.

See Bitesize interactive page....

Earth is said to be at zero volts. If a charged object is connected to earth by a conductor, electrons will pass between the charged object and Earth until the object is at zero volts too. If the object lacks electrons (is at positive potential or lacks electrons) then electrons will run from Earth to the object and if it is at negative potential (has a negative charge - too many electrons!) then the opposite happens, electrons run to Earth from the object.

The symbol for for a connection to Earth is:

The greater the charge on an isolated object, the greater the voltage (potential difference) between the object and earth. If the voltage becomes high enough, a spark may jump across the gap between the object and any earthed conductor which is brought near it. A charged conductor can be discharged safely by connecting it to earth with a conductor.

Metal cased appliances are earthed via the plug (see section on the 3-pin plug and earthing of appliances)

Electrolysis

When some chemical compounds are melted or dissolved in water they conduct electricity. These compounds are made up of electrically charged particles called ions. The current is due to negatively charged ions (anions) moving to the positive terminal or electrode (the anode) and the positively charged ions (cations) moving to the negative electrode (the cathode). Simpler substances are released at the terminals (electrodes). This process is called electrolysis.

During electrolysis the mass and/or volume of the substance deposited or released at the electrode increases in proportion to:
 

. the current (I)

. the time for which the current flows (t)

where

m  = the mass of the element deposited at the electrode in kilogrammes.
I    = current (A)
t    = time (s)

z    = a constant called the electrochemical equivalent

Electrolysis in an electrolyzer :

Flow of seawater or brine between sets of electrodes (anode and cathode) in an electrolyser connected to a source of direct current, the chemical reactions which take place in the electrolyzer between chlorine and caustic produced by electrolysis.

Electrochlorination Reaction :

The direct current, passing through the seawater or brine flowing in the electrochlorinator (electrolyzers), where the sodium chloride is completely dissociated into the ions Na⁺ and Cl^-, causes the following electrochemical and chemical reactions:

Reaction at the Electrochlorinators Anode :
2Cl^- ----> Cl₂ + 2e^- (chlorine generation in the electrolyzer)

Reaction at the Electrochlorinator Cathode :
2H₂O + 2e^- ----> H₂ + 2OH^- (hydrogen evolution from electrochlorinator)

Chemical Reaction in Electrolyzer :
Cl₂ + 2OH^- ----> Cl^- + ClO^- + H₂O
NaCl + H₂O = NaOCl + H₂ (hypochlorite or hypo generation)

Electrolysis of water - a solution to our fuel needs?

Current Electricity

In solid conductors, an electric current is a flow of electrons.

Metals are good conductors of electricity because some of the electrons from their atoms can move freely throughout the metal structure. If the atom of a substance holds on tightly to its electrons then the substance is an insulator.

(See current electricity section).