chapter 3.4-3.8: current, resistance and ohm’s law
TRANSCRIPT
Chapter 3.4-3.8:Current, Resistance and Ohm’s Law
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Current: Going with the flow
• What is current?
– At its simplest, Electric current is the rate of charge flow past a given point in an electric circuit, measured in Coulombs/second – more commonly known as Amperes
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The Ampere (A)
• Current is measured as the number of e- which flow past a particular point per unit time (generally 1 second)
• Saying that a device “draws” 6.24 x 1018 e-/s is unwieldy
• 1A = 1 Coulomb / second– Note: 1 Coulomb = 6.24 x 1018 e-
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50:50 Chance … but they got it wrong!
• Early electronics pioneers assumed that current flowed from (+)ve to (-)ve– This is known as “conventional current”– Comes up multiple times in E.E.
• Turned out to be exactly opposite• We will only consider the correct assertion that
electromotive force is generated by the flow of electrons:– (-)ve battery terminal to (+)ve– Electrons flow anode → cathode
• ACID: anode current into device
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Anodes ..• ACID: Anode current into device– This applies to batteries which are discharging!
• In electronics, the anode is generally the (+)ve terminal of a component such as a diode– Consider how the electrons flow for a moment ..– See how this is maddening?
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Conductors & Insulators
• Conductor:– Any medium which allows the flow of electrical
charge (ie. Electrons)
• Insulator:– Any medium which (ideally) does not allow the
flow of electrical charge– Air breaks down at ~3.3 x 106 V/m or 3.3kV/mm
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Controlling Current
• Two methods to control the current in a circuit:
1. Change the voltage applied to the circuit2. Provide resistance to the flow of electrons
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Controlling Current: Voltage
• By stacking cells of a battery in series, you increase the voltage potential!
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Controlling Current: Resistance
• To influence the flow of electrons (current), you can increase or decrease the ease at which they flow
• Hallway analogy– Long, narrow hallway limits the number of people
which can walk by a point in any given unit of time– Resistors work much the same way
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Resistance: Ohms
• Resistance is defined as the ratio between Voltage (E) and Current (I):
R = E I
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Conductance: mohs ( )℧
• The ability of a material to conduct electricity is measured in Siemens (G)– Conductance is seldom used
• Conductance is effectively the inverse of resistance:– where G = I / E
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Resistors: Common Formats
• There are many resistor packages, depending on design needs
• Resistance value often identified by resistor colour code
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Resistors: Identifying Values15KΩ
276Ω
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Resistors: Identification Example
• The value of the resistor shown above is 339Ω ±1%
93 3 x 10^0 ±1%
Note:10^0 = 1
x 1
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Ohm’s Law
• E = E.M.F. = Voltage (Volts)• I = Current (Amps)• R = Resistance (Ohms)
E = I x R
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Example: Calculate Current
• If a circuit has a 12V battery and a “load” which has a resistance of 10Ω Ohms, what is the current observed in the circuit?
• Recall: E = I * R• I = E / R
• I = 12V / 10Ω• I = 1.2A
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Energy And Work
• Mechanical forms of energy:– Potential– Kinetic
• Electrical energy parallels mechanical– Voltage is often also referred to as potential– Current can be thought of some quantity of
electrons in motion (kinetic)
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Series Resistor Circuit
R1
R2
R3When drawing this schematic, I should have (by convention) labeled theResistors R1 through R3 as the electrons (EMF) flow. I inadvertently labeled them in the direction of conventional current. This is more stylistic than anything else, though it is worth mentioning.
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Series Resistor Circuit
• What do we need to know in order to calculate how much current flows in this circuit?
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Kirchhoff’s Laws
• Loop Rule:– The sum of voltages across all resistors in a series circuit is
equal to the applied EMF– Put another way, the total voltage drop equals the supply
voltage
• Point Rule:– At any node (junction) in a circuit, the sum of currents flowing
into that node is equal to the sum of currents flowing out of that node
– Restated, the current in a loop is the same at every component
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• How much current flows in the following circuit?
• To find the total resistance in a series circuit, simply add the resistances!
Worked Example: Current
E = I / RRearrange the equation to:
I = E / RI = 40V / (5Ω + 25Ω + 10Ω)I = 40V / 40ΩI = 1A
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Worked Example: Voltage Drop
• What is the voltage drop experienced by each component in the following circuit?
• Recall I = 1AE1 = I x R1
E1 = 1A x 5ΩE1 = 5V
E2 = I x R2
E2 = 1A x 25ΩE2 = 25V
E3 = I x R3
E3 = 1A x 10ΩE3 = 10V
+ + = 40V
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Questions?