Transcript
Basic Electricity
• Current & Ohm’s Law
E-mail ivan.tandberg@shaw.ca
Meaning of Current!!
• ‘Current’ is the raterate of electron Flow.• The symbol for current is I.I.• The raterate of electron flow is expressed
in ‘amperesamperes’.• The symbol for an ampere is A.A.
Interesting Electrons !!• 1 ‘FREE ELECTRON’ is the
electron that orbits in the valence shell of a conductive atom.– Silver has a greater number of free
electrons per unit volume than any other conducting element or matter.
– More ‘COULOMBS’ of free electrons would be present in a given volume of an atom of Silver than other conducting matter..
Electron Drift !!• Electron ‘DRIFT’ more closely defines
the ‘FLOW’ of electrons.– Electron movement is a chain reaction along
a conducting medium.– This movement or drift resembles the Pool
Ball that moves at the end of a row of pool balls when the first one is hit.
Meaning of Ampere !!• 1 ‘AMPERE’ is the rate of electron
flow that is equal to 1 coulomb per second.
• Current is expressed in amperes.E.g. The current through the resistor is 5 amperes. OrOr The The currentcurrent through the through the resistorresistor is is 5 5 coulombs per second.coulombs per second.
Or IIRR = 5 A - - - > (Simple)5 A - - - > (Simple)
Meaning of Coulomb !!
• 1 ‘COULOMB’ is a specific number of electrons.
• A coulomb is the equivalent of 6.24 x 1018 electrons.
• For a given volume of any conducting element, the number of electrons would not be the same.
Current shown Graphically!!
6.24 x 1018 electrons
ConductorGiven Point
0
1 second
Second Timer
1 Coulomb1 Coulomb
Rate of Electron flow(CURRENTCURRENT) is 1 coulomb per second(1 AMPERE1 AMPERE)
II = 1C/s1C/s = 1 ampere1 ampere
1 Coulomb moved . . 1 Coulomb moved . .
Speed up Electron Flow Graphically!!
6.24 x 1018 electrons
ConductorGiven Point
Second Timer
1 Coulomb1 Coulomb
Rate of Electron flow(CURRENTCURRENT) is 1 coulomb per 1/2 second(2 AMPERES2 AMPERES)
II = 1C/.5s1C/.5s = 2 amperes2 amperes
1 Coulomb moved . . 1 Coulomb moved . .
0.5 second
24VDC
24VDC
For current to be present in a circuit . . .
+ -
AC Ammeter
Voltage Source Voltage Source and aand a
Complete Path Complete Path
Current Measurement !!
• The device used to measure the value of current in a circuit is referred to as an “AMMETER”.
• It is connected in series with the load.• AC AMMETERS need not have the correct
polarity as in DC circuits.
AC Ammeter
No Polarity Shown
Ohm’s Law then & now!• Ohm’s original Law was determined when capacitance and
inductance was still not studied in detail.• So that Ohm’s Law can be used in any circuit, a
modification is required.• Modification:
– “The current is directly proportional to the voltage and indirectly proportional to the impedance.”
– Using the original law in some AC circuits does not return the value of current, and the modified version does.
Clamp-On Ammeter• This ammeter is simply clamped
around any current carrying wire.• There are 2 types of clamp-on
ammeter.– AC (depends on induction)– DC (depends on strength of field)
• When current to be read is very small, just wrap the wire around one tong and divide the readout by the number of wraps of wire around the tong.
10A
Current Facts!• A current can not be applied.• Current is the rate of electron flow and therefore
the term ‘Current Flow’ is redundant but seemingly acceptable.
• Current through a resistive component will result in heat produced by the resistive component.
• Current requires an electrical path.• The symbol for current is ‘I I ’ (Intensity)• The symbol for an ampere is ‘AA’.
1A
10V
+ -
Ohm’s Law states that . .
The current is directly proportional to the voltage and indirectly proportional to the resistance.
102010
2A
20V
1A.25A.5A1A
5V10V
Voltage UP Current UP
Resistance UP
Current DOWNVoltage DOWN
Current DOWN
Resistance DOWN
Current UP Voltage UP
Current UP
Ohm’s LawOhm’s Law
• This is a law that helps in the understanding of how voltage and opposition in a circuit affect currentcurrent.
• It does not help in the determination of how a voltage is produced.
• It does not help in the determination of the factors affecting resistance.
Original Ohm’s LawOriginal Ohm’s Law• When this law was ascertained by George Simon Ohm,
the only circuit property known at the time was ResistanceResistance.
• I = E / R . . . . . Good I = E / R . . . . . Good onlyonly in resistive circuits!! in resistive circuits!!• Michael Faraday, John Henry and others later
discovered InductanceInductance and CapacitanceCapacitance• Inductance and/or capacitance in an alternating current
circuit also exhibit ‘Opposition” to circuit current. Z = (R2 +(XL - XC)2)
• To be able to use the Ohm’s Law concept in all circuits one must use I = E / Z . . . Good in all Circuits! I = E / Z . . . Good in all Circuits!
Using Ohm’s Law• Since the equation for Ohm’s Law is mathematical,
any of the variables in the formula of the equation may be found by manipulating the equation to make a variable the subject.
• E = IR (current & resistance do not affect voltage)• R = E/I (voltage & current do not affect resistance)• The 2 equations above do not state the LAW but are
simply relationships in a circuit!• The LawThe Law - -> I = E / RI = E / R or I = E / ZI = E / Z
Ohm’s Law in Action (I)!
+ - 2A
10
20V
A. I = E / R = 20 / 10 = 2A
Q. What is the current through the resistor?
Ohm’s Law in Action (V) !
+ - 2.8A
5
14V
Ans. E = IR = 2.8 x 5 = 14V
Q. What is the voltage across the resistor?
Ohm’s Law in Action () !
+ - .1A
240
24V
Ans. R = E/I = 24 ÷ .1 = 240
Q. What is the resistance of the resistor?
Hydraulic Analogy• Electricity might be compared to water:• A battery could be likened onto a water pump.
– Voltage (V) or water pressure (pounds per square inch).
• CurrentCurrent could be likened onto the flow rateflow rate of water.– Coulombs per second or gallons per minute.
• A wirewire could be likened onto a water hosehose.– Small diameter wire is like a small diameter water hose.
• An open switchopen switch is like a closed valveclosed valve . . Either way you have infinite opposition (resistance)(impedance).
• Electrons move in a circuit at a given rate if a voltage source is present and there is a closed circuit.
• The rate is determined by the voltage and the opposition by the circuit components.
• 1 ampere is a rate of 1 coulomb per second.• Current is the rate of electron flow.• I = E / R (Resistive Circuits Only)• I = E / ZI = E / Z (All circuits)• Ohm’s Law:Ohm’s Law: “The current is directly proportional to the
voltage and indirectly proportional to the impedance.” (any single phase or DC circuit)
Summary . . .
I = ER
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