Ohms Law

1. 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).

2. 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).

3. 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.

4. 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".

5. 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,

6. 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.

7. Ohms Law • A further example is an electric hot water system. • All are examples of ohms law at its most basic.

8. 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".

9. 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.

10. 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.

11. 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

12. 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.].

13. 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.

14. 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.

15. 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.

16. 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.

17. 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

18. 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

19. 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.

20. Ohms law is the very foundation stone of electronics! • Knowing two quantities in ohms law will always reveal the third value.

21. 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.

22. 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.

23. Capacitance

24. 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.

25. 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.

26. 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,

27. 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.

28. 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.

29. 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.

30. Time constant of capacitance • The time required for a capacitor to reach its charge is proportional to the capacitance value and the resistance value.

31. 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

32. 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)

33. 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.

34. 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.

35. 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.

36. 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.

37. capacitors passing ac blocking dc