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Ohms Law
 Ohms 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 Law
 Ohms Law is the a foundation stone of
electronics and electricity.
 These formulae are very easy to learn and
are used extensively in this course
 Without 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 Law
 Mr. 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 Law
 Some 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 Law
 the 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 Law
 A 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 Law
 One 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 Resistance
 The 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 Resistance
 A very good working example of this is an
electric light bulb
 If you measure the first light bulb with a
digital multimeter.
 It showes zero resistance, in fact open
circuit.
Hot and Cold Resistance
 That'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 Resistance
 O.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 Resistance
 Using 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 Resistance
 Another 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 Resistance
 My "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 formulas
 For 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 formulas
 Now 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 formulas
 That'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.
Capacitance
 Two 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
Capacitance
 In 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.
Capacitance
 At 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.
Capacitance
 Two 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,
Capacitance
 the 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.
Capacitance
 The 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.
Capacitance
 The 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 capacitance
 The time required for a capacitor to reach its
charge is proportional to the capacitance
value and the resistance value.
Time constant of capacitance
 The 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 capacitance
 The 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 capacitance
 then 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 parallel
 Capacitors 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 parallel
 Added 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
Capacitors
 Capacitors 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