Voltage, Current and Resistance

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• Voltage, Current and Resistance
• A basic introduction
• George South
• 2 Feb 2006

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Voltage, Current and Resistance

• What is voltage ?
• it can be described as a sort of electrical
  pressure
• it is usually denoted by V and is measured in
  Volts
• What is current ?
• electrical current is simply a measure of how
  much electricity passes a given point in a fixed
  amount of time
• It is usually denoted by I and is measured in Amps

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Voltage, Current and Resistance

• What is resistance?
• The ability of all conductors of electricity to
  resist the flow of current, turning some of it
  into heat
• it is usually denoted by R and is measured in
  Ohms or ?
• What is power ?
• the rate of doing work
• It is usually denoted by P and is measured in
  Watts

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• Ohms Law
• Ohms Law simply states that the more voltage that
  is applied to a conductor then the more current
  will flow
• I V divided by R
• Power
• Power can be calculated by using
• P I times V

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Impedance

• Electrical components such as loudspeakers are
  quite complex and contain other components in
  addition to resistance
• The more correct term to use for specifying a
  speaker is impedance
• Impedance a unit of measure, expressed in Ohms,
  of the total opposition (resistance, capacitance
  and inductance) offered to the flow of an
  alternating current.

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Loudspeaker characteristics

• Loudspeakers do not have the same impedance at
  all audio frequencies
• Different speakers can have totally different
  characteristics
• Note
• For the ElecNet Principles unit we will consider
  loudspeakers to be ideal and to only contain
  resistance
• Symbol for a loudspeaker

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DC and AC

• Batteries and power supplies provide DC voltages
• Ideally the value is constant with time

Voltage

5 V
5 Volts
-
Time
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DC and AC

• Amplifiers etc provide AC Voltages
• An AC signal often used in testing is a sine wave
  mains electricity is sinusoidal

Voltage
!
G
5 Volts
Time
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AC and DC

• Working out the power with DC supplies is easy
  P I times V
• Working out the power with AC signals is more
  challenging what voltage etc do we use ? Peak,
  Average etc ??
• The answer is called the RMS value (look it up if
  you are curious) and when we talk about AC and
  sine waves we say, for example, 240 Volts mains
  this is 240 Volts RMS

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Power dissipation in a speaker

• Suppose we apply an 8 Volt RMS signal at 1 KHz to
  an 8 Ohm speaker
• The current I V/R 8/8 1 Amp
• Power V times I 8 times 1 8 Watts
• Most of this generates heat in the speaker
  components but a small proportion generates sound
  energy