ohms law

Ohms LawOhms law, named after Mr. Ohm,

defines the relationship between power, voltage, current and resistance.

These are the very basic electrical units we work with.

The principles apply to a.c., d.c. or r.f. (radio frequency).

Why is ohms law so very important?Ohms law, sometimes more correctly

called Ohm's Law, named after Mr. Georg Ohm, mathematician and physicist

defines the relationship between power, voltage, current and resistance.

These are the very basic electrical units we work with. The principles apply to a.c., d.c. or r.f. (radio frequency).

Ohms LawOhms Law is the a foundation stone of

electronics and electricity. These formulae are very easy to learn

and are used extensively in this courseWithout a thorough understanding of

"ohms law" you will not get very far either in design or in troubleshooting even the simplest of electronic or electrical circuits.

Ohms LawMr. Ohm established in the late

1820's that if a voltage [later found to be either A.C., D.C. or R.F.]

was applied to a resistance then "current would flow and then power would be consumed".

Ohms LawSome practical every day examples of

this very basic rule are: Radiators (electric fires), Electric

Frypans, Toasters, Irons and electric light bulbs

The radiator consumes power producing heat for warmth,

the frypan consumes power producing heat for general cooking,

Ohms Lawthe toaster consumes power

producing heat for cooking toast, the iron consumes power producing

heat for ironing our clothes and the electric light bulb consumes

power producing heat and more important light for lighting up

an area.

Ohms LawA further example is an electric hot

water system. All are examples of ohms law at its

most basic.

Hot and Cold Resistance encountered in Ohms LawOne VERY important point to

observe with ohms law in dealing with some of those examples is

that quite often there are two types of resistance values.

"Cold Resistance" as would be measured by an ohm-meter or digital multimeter and a "Hot Resistance".

Hot and Cold ResistanceThe latter is a phenomenem of the

material used for forming the resistance itself,

it has a temperature co-efficient which often once heated alters the initial resistance value,

usually dramatically upward.

Hot and Cold ResistanceA very good working example of this

is an electric light bulbIf you measure the first light bulb

with a digital multimeter. It showes zero resistance, in fact

open circuit.

Hot and Cold ResistanceThat's what you get, when for safety

reasons you put a burnt out bulb back into an empty packet and

a "neat and tidy" wife puts it back into the cupboard

Hot and Cold ResistanceO.K. here's a "goodie" and, it's labelled

"240V - 60W", it measured an initial "cold resistance" of 73.2 ohms.

Then measure the actual voltage at a power point as being 243.9V A.C. at the moment

[note: voltages vary widely during a day due to locations and loads - remember that fact - also for pure resistances, the principles apply equally to A.C. or D.C.].

Hot and Cold ResistanceUsing the formula which we will see

below, the resistance for power consumed should be R = E2 / P OR R = 243.92 / 60W = 991 ohms

That is 991 ohms calculated compared to an initial reading of 73.2 ohms with a digital multimeter?

The reason? The "hot" resistance is always at least ten times the "cold" resistance.

Hot and Cold ResistanceAnother example is what is most often

the biggest consumer of power in the average home.

The "electric jug", "electric kettle" or what ever it is called in your part of the world.

Most people are astonished by that news.

Hot and Cold ResistanceMy "electric kettle" is labelled as

"230 - 240V 2200W". Yes 2,200 watts! That is why it boils

water so quickly.

What are the ohms law formulas?Notice the formulas share a common

algebraic relationship with one another.

For the worked examples voltage is E and we have assigned a value of 12V,

Current is I and is 2 amperes while resistance is R of 6 ohms.

Note that "*" means multiply by, while "/" means divide by.

ohms law formulasFor voltage [E = I * R]   E (volts) = I

(current) * R (resistance)   OR   12 volts = 2 amperes * 6 ohms

For current [I = E / R]       I (current) = E (volts) / R (resistance)   OR   2 amperes = 12 volts / 6 ohms

For resistance [R = E / I]   R (resistance) = E (volts) / I (current)   OR   6 ohms = 12 volts / 2 amperes

ohms law formulasNow let's calculate power using the

same examples. For power        P = E2 /

R   OR   Power = 24 watts = 122 volts / 6 ohms

Also        P = I2 * R   OR   Power = 24 watts = 22 amperes * 6 ohms

Also        P = E * I   OR   Power = 24 watts = 12 volts * 2 amperes

ohms law formulasThat's all you need for ohms law -

remember just two formulas: for voltage E = I * R and; for power P = E2 / R You can always determine the other

formulas with elementary algebra.

Ohms law is the very foundation stone of electronics!Knowing two quantities in ohms law

will always reveal the third value.

What is capacitance?In the topic current we learnt of the unit

of measuring electrical quantity or charge was a coulomb.

Now a capacitor (formerly condenser) has the ability to hold a charge of electrons.

The number of electrons it can hold under a given electrical pressure (voltage) is called its capacitance or capacity.

CapacitanceTwo metallic plates separated by a

non-conducting substance between them make a simple capacitor.

Here is the symbol of a capacitor in a pretty basic circuit charged by a battery.

Capacitance

CapacitanceIn this circuit when the switch is

open the capacitor has no charge upon it,

when the switch is closed current flows because of the voltage pressure,

this current is determined by the amount of resistance in the circuit.

CapacitanceAt the instance the switch closes the

emf forces electrons into the top plate of the capacitor from the negative end of the battery and

pulls others out of the bottom plate toward the positive end of the battery.

CapacitanceTwo points need to be considered

here. Firstly as the current flow

progresses, more electrons flow into the capacitor and

a greater opposing emf is developed there to oppose further current flow,

Capacitancethe difference between battery

voltage and the voltage on the capacitor becomes less and less

and current continues to decrease. When the capacitor voltage equals

the battery voltage no further current will flow.

CapacitanceThe second point is if the capacitor is able to

store one coulomb of charge at one volt it is said to have a capacitance of one Farad.

This is a very large unit of measure. Power supply capacitors are often in the

region of 4,700 uF or 4,700 / millionths of a Farad.

Radio circuits often have capacitances down to 10 pF which is 10 / million, millionths of a Farad.

CapacitanceThe unit uF stands for micro-farad

(one millionth) and pF stands for pico-farad (one million, millionths).

These are the two common values of capacitance you will encounter in electronics.

Time constant of capacitanceThe time required for a capacitor to

reach its charge is proportional to the capacitance value and the resistance value.

Time constant of capacitanceThe time constant of a resistance -

capacitance circuit is: T = R X C where T = time in seconds

where R = resistance in ohms where C = capacitance in farads

Time constant of capacitanceThe time in this formula is the time to

acquire 63% of the voltage value of the source.

It is also the discharge time if we were discharging the capacitance.

Should the capacitance in the figure above be 4U7 (4.7 uF) and the resistance was 1M ohms (one meg-ohm or 1,000,000 ohms)

Time constant of capacitancethen the time constant would be T =

R X C = [1,000,000 X 0.000,0047] = 4.7 seconds.

These properties are taken advantage of in crude non critical timing circuits.

Capacitors in series and parallelCapacitors in parallel ADD together

as C1 + C2 + C3 + ..... While capacitors in series REDUCE by:

1 / (1 / C1 + 1 / C2 + 1 / C3 + .....) Consider three capacitors of 10, 22,

and 47 uF respectively.

Capacitors in series and parallelAdded in parallel we get 10 + 22 +

47 = 79 uF. While in series we would get:

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

Note that the result is always LESS than the original lowest value.

A very important property of CapacitorsCapacitors will pass AC currents but not DC.

Throughout electronic circuits this very important property is taken advantage of to pass ac or rf signals from one stage to another

while blocking any DC component from the previous stage.

capacitors passing ac blocking dc