mod 3 rg 3.2 static electricity and coduction

All materials contain both positive and negative charges, which are usually distributed evenly throughout a material so that, at the scale of everyday objects, there is

no overall positive or negative charge detectable.

Static electricity describes a situation where

positive and negative charges are unevenly distributed, leaving materials with an overall positive or negative charge

static electricity can be achieved through either

contact, friction, or induction

opposite charges

attract

like charges

repel

The effect of electrostatic forces of attraction and repulsion can be seen when

charging two pith balls with a rod

Coulomb's Law describes the

force between two electrically charged bodies

Coulomb’s Law states that:

“The magnitude of the electrostatic force of attraction, or repulsion, between two-point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them.”

Coulomb's Law can be used to calculate the force of attraction between two particles, one negatively charged, one positively charged. They are treated as

point charges, so the distance between them is the distance between their centres of mass.

The term k stems from a universal constant, the electric constant. Its value can only vary according to the units it is given, which will depend on

the units used for the charge (Q) and the force (F).

In most circumstances, the units for k shown above are used, meaning it typically takes the value of

9.0 × 10⁹ Nm²/C²

f=

K x Q1Q2/D²

K depends on

the material of the constant?

SI units for Force f

newtons (N)

SI unit for Charge Q

coulombs (C)

the SI units for distance d

meters (m)

Among solids, metals are good conductors of electricity, thanks to the

number of free electrons available for conduction:

Applying an electrostatic force will cause these electrons to flow from the

negative to the positive pole.

All solids will conduct electricity

to some extent

All materials can be shown to conduct a measurable current when

a high enough voltage is applied,

those materials with very low conductivity are classified as

insulators

water is a poor

conductor of electricity

However, in the right circumstances, it can conduct electricity, which is why electrical equipment must not be operated with wet hands. The same applies to mains devices that must not be operated when in the bath.

When an electron enters a conductor, another electron at the opposite end is

pushed out

movement of electrons is called an

electric current

If the current remains constant, the amount of electrons per second that flow through a conductor

remains the same regardless of its cross-sectional area

where a conductor narrows, such as in the filament of an incandescent light bulb, electrons must move

at a higher speed to maintain the same current. This leads to a greater heating effect of the current through the narrower section.

An electric current passing through water decomposes the water, producing

hydrogen( negative cathode) and oxygen (positive anode)

applying Direct Current (DC) will generate hydrogen at the

negative pole

applying Direct Current (DC) will generate oxygen at the

positive pole

in water H2O

The hydrogen accounts for two-thirds of the gas (high development of gas) and the oxygen for one-third (low development of gas).

by adding acids, bases, or salts, water becomes a better conductor, and the compounds in the solution decompose more readily. The decomposition of chemical compounds by passing an electric current through a liquid is called

electrolysis

The conducting substance of a liquid is called an

electrolyte

Electrolysis is used for

electroplating and for charging batteries, as well as producing useful substances such as chlorine, hydrogen, and sodium hydroxide.

When an electric current flows through an electrolyte, there are ions that carry the charge

rather than electrons

Positive ions (cations) flow from

the anode to the cathode

negative anions flow from the

cathode to the anode.

by a chemical reaction at the anode and cathode, where electrons are exchanged, new

substances are formed

A Potential Difference (PD) applied across a low-pressure gas causes a

steady current

The conduction of electricity by gases is called

gas discharge

gas discharge is often accompanied by

the emission of visible light

devices that make use of the visible light from gas discharge is known as

gas discharge tubes, Typically these tubes are filled with gases, such as neon or helium which have characteristic colours.

Under normal conditions, most of the gas atoms enclosed in the bulb are electrically neutral. However,

the electrons of a few atoms are split up by means of external energy supply (heat, light, and radioactivity). This process generates freely moving charge carriers (electrons and positive gas ions).

• Some of these free electrons reach high speeds due to the low pressure and high potential difference.

• Each collision between a high-speed electron and a gas atom ionises the atom and produces several more electrons, which also accelerate. This process repeats through the tube producing large numbers of free electrons and positive ions. This is known as an “electron cascade”.

• Free electrons and positive ions eventually recombine and release energy in the form of electromagnetic radiation, some of which is visible light.

• Some electrons and ions reach the ends of the tube, carrying their charge from one end to the other, meaning that an electric current flows through the gas.

In a fluorescent tube, the inner surface of a gas discharge tube is coated with a

fluorescent material, which absorbs any electromagnetic radiation outside the visible region, and then re-emits a broad spectrum of visible light. This provides a source of white light, rather than the characteristic colours usually emitted by gas discharge tubes.

A vacuum, by definition, contains

no particles

Conduction through a vacuum is actually due to small numbers of

charge carriers moving through what is otherwise a vacuum. For example, charged particles flow through outer space, emitted by objects such as stars.

An oscilloscope provides a means of

visualisation and measurement of rapidly varying processes in electrical engineering, electronics, and other sciences.

the most commonly used oscilloscopes are

electron beam as they are easy to operate, versatile and shock resistant

the electron beam tube, also called the

Braun tube

The picture above shows a cathode ray tube from an oscilloscope, which makes use of an electron gun. The beam of electrons, from the electron gun, travels through a vacuum towards the screen, where a fluorescent paint produces a small spot of light where the beam strikes. The direction of the beam, and, therefore, the position of the spot can be controlled by applying a

PD across the pairs of plates in the vertical (y-axis) direction, or the horizontal (x-axis) direction. This is also the way in which television sets worked up until the advent of flat-screen technologies.