What is current?A flow of electrons forced into

motion by voltage is known as current.

The atoms in good conductors such as copper wire have one or more free electrons of the outer ring constantly flying off.

CurrentElectrons from other nearby atoms

fill in the holes. There are billions of electrons

moving aimlessly in all directions, all the time in conductors.

Electrons from other nearby atoms fill in the holes.

CurrentWhen an emf (voltage) is impressed

across a conductor it drives these free electrons away from the negative force toward the positive.

This action takes place at near the speed of light, 300,000,000 metres per second

although individual electrons do not move far they have a shunting effect.

CurrentThis is similar to a number of cars

pulled up at traffic lights when the last vehicle fails to stop

and hits the second last vehicle which in turn hits the third last

vehicle...............

Current

CurrentThe amount of current in a circuit is

measured in amperes (amps). Smaller units used in electronics are

milli-amps mA (1 / 1,000th of an ampere) and micro-amps uA (1 / 1,000,000th of an ampere).

An ampere is the number of electrons going past a certain point in one second.

CurrentThe quantity of electrons used in

determining an ampere is called "coulomb" which one ampere is one coulomb per

second. A coulomb is 6,280,000,000,000,000,000

or 6.28 X 10 18 electrons. This (a coulomb) is the unit of measuring

electrical quantity or charge.

Current vs. Electron Flowcurrent flows from the positive to the

negative terminal, but electrons actually flow from the

negative to the positive terminal. In other words, current and electrons

flow in opposite directions.

AC and DC CurrentAC stands for alternating current, and DC for direct current.

In AC circuits the current regularly switches direction.

In DC circuits the current always flows in the same direction

DC Current

AC Current

AC and DC CurrentYour home uses AC circuits at approximately 110 volts (larger appliances like dryers and electric stoves require 220 V AC).

The electrical outlets for higher voltage devices look different from the normal household outlet.

AC and DC CurrentThe Electricity and Power in Space activities involve DC circuits only,

using voltages ranging from 1.5 to 6 volts.

The International Space Station is powered by DC circuits at approximately 110 volts.

VoltageVoltage should be more correctly

called "potential difference". Voltage is actually the electron

moving force in electricity (emf) and the potential difference is

responsible for the pushing and pulling of electrons or electric current through a circuit.

Sources of electromotive force (EMF) or voltageTo produce a drift of electrons, or

electric current, along a wire it is necessary that there be a

difference in "pressure" or potential between the two ends of the wire.

This potential difference can be produced by connecting a source of electrical potential to the ends of the wire.

EMFThere is an excess of electrons at the

negative terminal of a battery and a deficiency of electrons at the positive terminal, due to chemical action.

Then it can be seen that a potential difference is the result of the difference in the number of electrons between the terminals.

EMFThe force or pressure due to a

potential difference is termed e.m.f. or voltage.

An emf also exists between two objects whenever there is a difference in the number of free electrons per unit volume of the object.

EMFIf the two objects are both negative,

current will flow from the more negatively charged to the less negatively charged when they are connected together.

There will also be an electron flow from a less positively charged object to a more positively charged object.

EMFThe electrostatic field, i.e. the strain of

the electrons trying to reach a positive charge or from a more highly negative charge is emf or voltage.

It is expressed in units called volts, short for voltage.

A volt can be defined as the pressure required to force a current of one ampere through a resistance of one ohm.

EMFTo make this easier to visualise, consider the water pressure

(voltage) required to pass a litre of water

(current) through a copper pipe of a certain

small diameter (resistance).

EMFAlso try and visualise water going

through other pipes of varying diameters (smaller to larger in size).

Either the water pressure required would vary or the volume delivered would vary, or both.

Electrical potential'Electrical potential' is a condition, which determines the direction of the flow of charge.

Let us consider the experiment outlined below to understand electrical potential

Electrical potential

Electrical potentialPipe A is connected to the container B through a stopcock.

The quantity of water in A is less than the quantity in B, but the level of water is higher than the level in B.

When the stopcock is opened, this water begins to flow from A to B, till the levels of water in both A and B are equal.

Electrical potentialThe above observation, determines that it is not the quantity of water, but the level of water, which decides the direction of flow of water.

Here the water in 'A' is at a higher 'gravitational potential' and the water in 'B' is at a lower gravitational potential.

Electrical potentialIt is the 'potential difference' that is responsible for the flow of water.

Similarly 'electrical potential' is the direction of the flow of charge.

voltage can be generated in many different waysChemical (batteries) e.g. dry cell

1.5V, wet cell storage about 2.1V Electromagnetic (generators) Thermal (heating junctions of dis-

similar metals) Piezoelectric (mechanical vibration

of certain crystals) Photoelectric (light sensitive cells)

ResistanceIn the topic current we learnt that

certain materials such as copper have many free electrons.

Other materials have fewer free electrons and substances such as glass have practically no free electron movement

therefore making good insulators.

ResistanceBetween the extremes of good

conductors such as silver, copper and

good insulators such as glass and rubber lay other conductors of reduced conducting ability,

they "resist" the flow of electrons hence the term resistance..

ResistanceThe specific resistance of a

conductor is the number of ohms in a 1' (305mm) long, 0.001" diameter round wire of that material.

Some examples on that basis are Silver = 9.75 ohms, Copper = 10.55 ohms, Nickel = 53.0 ohms and Nichrome = 660 ohms

ResistanceFrom this information we can

deduce that for a voltage applied to a piece of Nichrome wire ,

only around 10.55 / 660 = 0.016 of the amount of current will flow as opposed to the the current flowing in the same size copper wire.

ResistanceThe unit of resistance is the ohm

and 1 ohm is considered the resistance of round copper wire, 0.001" diameter, 0.88" (22.35 mm) long at 32 deg F (0 deg C).

Resistance in series and parallelIt follows if two such pieces of wire

were connected end to end (in series)

then the resistance would be doubled,

on the other hand if they were placed side by side (in parallel)

then the resistance would be halved!

Resistance in series and parallelThis is a most important lesson

about resistance. Resistors in series add together as

R1 + R2 + R3 + ..... While resistors in parallel reduce by

1 / (1 / R1 + 1 / R2 + 1 / R3 + .....)

Resistance in series and parallelConsider three resistors of 10, 22,

and 47 ohms respectively. Added in series we get 10 + 22 + 47

= 79 ohms. While in parallel we would get 1 /

(1 / 10 + 1 / 22 + 1 / 47) = 5.997 ohms.

Resistance and PowerNext we need to consider the power handling

capability of our resistors. Resistors which are deliberately designed to

handle and radiate large amounts of power are electric cooktops, ovens, radiators, electric

jugs and toasters. These are all made to take advantage of power

handling capabilities of certain materials.

Resistance in series and parallelFrom our topic on ohms law we

learnt that P = I * I * R that is, power equals the current squared

times the resistance. Consider our example above of the

three resistors in series providing a total resistance of 79 ohms.

Resistance in series and parallelIf these resistors were placed across a 24

volt power supply then the amount of current flowing, from ohms law, is I = E / R = 24 / 79 = 0.304 amperes.

Resistance in series and parallelUsing any of our power formulas we

determine that 0.304 amperes flowing through our 79 ohm resistance dissipates a combined 7.3 watts of power!

Worse, because our resistors are of unequal value the power distribution will be unequal with the greater dissipation in the largest resistor.

Resistance in series and parallelIt follows as a fundamental rule in

using resistors in electronic circuits that the resistor must be able to comfortably handle the power it will dissipate.

A rule of thumb is to use a wattage rating of at least twice the expected dissipation.

Resistance in series and parallelCommon resistors in use in electronics

today come in power ratings of 0.25W, 0.5W, 1W and 5W.

Other special types are available to order. Because of precision manufacturing

processes it is possible to obtain resistors in the lower wattage ratings which are quite close in tolerance of their designated values.

Resistance in series and parallelTypical of this type are the .25W

range which exhibit a tolerance of plus / minus 2% of the value.

Resistors come in a range of values but the two most common are the

E12 and E24 series.

Resistance in series and parallelThe E12 series comes in twelve

values for every decade. The E24 series comes in twenty four

values per decade.E12 series - 10, 12, 15, 18, 22, 27,

33, 39, 47, 56, 68, 82 E24 series - 10, 11, 12, 13, 15, 16,

18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91

Resistance in series and parallelYou will notice with the E12 values

that each succeeding value falls within

the plus / minus 10% of the previous values.

This stems from the real old days when resistances were stated as within 20% tolerance (accuracy).

Resistance in series and parallelLater values of plus / minus 5%

tolerance led to the E24 range of resistance.

Quite common today are 2% tolerance metal films types but for general purpose use we tend to stick to

E12 values of resistance in either 1%, 2% or 5% tolerance.

Resistance in series and parallelCost is the determining factor and

many retailers now stock the 2% range of resistance as a standard

to minimise stocking levels and also at reasonably low cost.

Resistance in series and parallelAs examples of say the "22" types

(red - red) from the E12 series we get 0.22, 2.2, 22, 220, 2,200, 22,000, 220,000 and 2,200,000 or eight decades of resistors.

symbol of Resistors