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Lecture 1: Power, Voltage, Current and Resistance Lecture 1: Power, Voltage, Current and Resistance U08801 2011 1 –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 Lecture 1: Power, Voltage, Current and Resistance U08801 2011 2
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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
8
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
9
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
12
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
13
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
14
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
15
Schematic Representations
• Voltage source
• Current Source
• Resistors
U08801 2011 Lecture 1: Power, Voltage, Current and
Resistance
16
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
17
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
18
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
19
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
22
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
23
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
24
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