Transcript Chapter 9: Magnetism & Inductance

Chapter 3.4-3.8:
Current, Resistance and Ohm’s Law
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 ewhich 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 circuit
2. 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 Values
15KΩ
276Ω
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Resistors: Identification Example
• The value of the resistor shown above is 339Ω ±1%
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Ohm’s Law
• E = E.M.F. = Voltage (Volts)
• I = Current (Amps)
• R = Resistance (Ohms)
E=IxR
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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
R3
When drawing this schematic, I should have (by convention) labeled the
Resistors 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.
R2
R1
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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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Worked Example: Current
• How much current flows in the following
circuit?
E=I/R
Rearrange the equation to:
I=E/R
I = 40V / (5Ω + 25Ω + 10Ω)
I = 40V / 40Ω
I = 1A
• To find the total resistance in a series circuit,
simply add the resistances!
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Worked Example: Voltage Drop
• What is the voltage drop experienced by each
component in the following circuit?
• Recall I = 1A
E1 = I x R1
E1 = 1A x 5Ω
E1 = 5V
E2 = I x R2
E2 = 1A x 25Ω
E2 = 25V
E3 = I x R3
E3 = 1A x 10Ω
E3 = 10V
+
+
= 40V
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Questions?
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