Lect 1 Power,Voltage Current and Resistance
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Transcript of Lect 1 Power,Voltage Current and Resistance
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
1
Electronics for Media Applications
U08801
Lecture 1:
Power, Voltage, Current and Resistance
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
2
Voltage and Current
• Electronics is the business of modifying
the Voltage and Current in a circuit
• What are Voltage and Current ?
– We are going to use an analogy of water flow
to help us understand electricity
– We must be careful to remember that this is an
analogy and cannot explain every aspect of
electricity
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
3
Voltage (a)
• Symbol – V
• Measured in Volts
• Also referred to as Potential Difference(PD) or Electromotive Force (EMF).
• Analogy –
– you can think of the voltage as the pressure difference created by a water pump in a system of pipes
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
4
Voltage (b)
• Voltages exist across two points
– Where voltage is referred to as existing at a
point, the other point is ground or 0 volts
– It is incorrect to refer to the “voltage though” a
component, this wrong.
• Voltages are generated by batteries, solar
panels, electric generators
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
5
Current
• Symbol – I
• Measured in Amperes, commonly referred to as
usually Amps
• Analogy –
– you can think of the current as the amount of water
flowing through a pipe
– For a fixed system of pipes,
the greater the pressure generated by the pump (i.e.
Voltage) the greater the flow of water (current)
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
6
Conventional and Real Current
• Real Current is the flow of electrons from
negative voltage around a circuit
• Conventional current is taken as flowing
from a positive voltage round a circuit
– Direction defined before it was possible to
determine which way electrons actually moved
• In practice it rarely matters – Except . . . .
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
7
Direction of Current
• Many schematic symbols use an arrow indicating
the direction of conventional current
• When analysing a circuit, the direction of the
current is often indicated with arrows
– This allows the polarity of the voltage across a
component to be determined
– Some books uses the direction of real current (i.e. the
direction of electrons) for the direction of the arrows
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Power
• The first important equation
Power = Voltage Current
or
P = V I• Where power is in Watts, the voltage is Volts and
current is measured in Amps
• In electronic components, power often
comes out as heat
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Power is important
• You may think that power is not important
• However, Power is critical
– In specifying individual components
– Making sure that components don‟t get too hot
– Designing for efficiency
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
10
Relationship of Voltage &
Current
• Electronics is the business of modifying the
Voltage and Current to achieve a desired
result or action.
• Essentially, we employ electronic
components to modify the voltage & current
to get a particular outcome.
• The simplest component is a Resistor
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
11
Resistors
• Current through a resistor is proportional to
the voltage across it, and ….
• Ohm‟s Law for resistors relates R, V and I
V = I R (Very Easy If Remembered)
• Very Easy If Remembered (V=I R)
U08801 2011Lecture 1: Power, Voltage, Current and
Resistance
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Current ~ Voltage
Current vs. Voltage for a 200 Ohm Resistor
0
10
20
30
40
50
60
0 2 4 6 8 10 12
Volts
Mil
li-a
mp
s
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Power and Resistors
• We now have two important equations
P = V I
and
V = I R
• We can now work out the power in a resistor
P = (I R) I = I2 R
• and
P= V2
R
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Power ~ Voltage
Power vs. Voltage for a 200 Ohm Resistor
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 2 4 6 8 10 12
Volts
Watt
s
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Schematic Representations
• Voltage source
• Current Source
• Resistors
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Battery and Resistor Circuit
– Wires are single black lines of zero resistance
– Box version of resistor is commonly used, (but some
books use a zigzag version)
– The resistor value usually leaves out the „‟ symbol
What‟s the
current in
this circuit ?
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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A More Complicated Circuit
• Resistors in series have a total resistance of the sum
– What is the current I?
– What are the voltages across each resistor
– Which is the positive end of the each resistor?
– Following the current path, add the voltages
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Kirchhoff’s Voltage Law
• The sum of voltage drops around a circuit is zero
– As long as you are consistent with using conventional
or real current-there is no problem
• The voltage across a resistor is defined by Ohm‟s
law
– This voltage is referred to as a voltage drop
– A voltage source has a negative voltage drop because it
is a source of electrical energy – a resistor is a
dissipator of energy
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Current Flow Analogy
• Lets go back
to the water
analogy
• Imagine three
pipes meeting
at a junction
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
20
Kirchhoff’s Current Law
• The total amount of water entering the
junction equals the total amount leaving
• In electrical terms the sum of the currents
into a junction equals the sum of currents
leaving a junction
– A junction or point is sometimes called a
„Node‟
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
21
Example 1 - Resistors in Parallel
• What are the
currents I1 and I2
• What is I3 ?
• What resistor on
its own would
give rise to I3 ?
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Resistors in Parallel
• Generalising the previous result
• 1/(Parallel Resistance) = 1/R1 +1/R2
• 1 = 1 + 1
(Rparallel) R1 R2
• Rparallel = (R1 R2) /(R1+R2)
– Product over sum
– Rparallel sometimes written R1//R2
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Example 2- Kirchhoff’s Current
Law• In the example
opposite
I1 = I2 + I3
• Note - Black dot
indicates junction.
If two lines cross
without a black dot,
there is no connection
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
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Summary of Lecture 1
• Power = V I (power law)
• V = I R (Ohms law)
• R1 , R2 in series = R1 +R2
• R1 , R2 in parallel = (R1 R2) /(R1+R2)
• Kirchhoff‟s Current law
– Current entering a junction = current leaving
• Kirchhoff‟s Voltage law
– Sum of voltage drops around a circuit = zero