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U08801 2011 Lecture 1: Power, Voltage, Current and

Resistance

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Electronics for Media Applications

U08801

Lecture 1:

Power, Voltage, Current and Resistance


Resistance

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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


Resistance

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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


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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


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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)


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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 . . . .


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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


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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


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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


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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


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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


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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


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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


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Schematic Representations

• Voltage source

• Current Source

• Resistors


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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 ?


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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


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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


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Current Flow Analogy

• Lets go back

to the water

analogy

• Imagine three

pipes meeting

at a junction


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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‟


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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 ?


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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


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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


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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