Current,%20Voltage%20and%20Resistance - PowerPoint PPT Presentation

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Current,%20Voltage%20and%20Resistance

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Current, Voltage and Resistance Electricity Current Electricity What do turning on a light, turning on a radio, and turning on your television have in common? – PowerPoint PPT presentation

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Title: Current,%20Voltage%20and%20Resistance


1
Current, Voltage and Resistance
  • Electricity

2
Current Electricity
  • What do turning on a light, turning on a radio,
    and turning on your television have in common?
  • In each case, you are completing an electric
    circuit. In the last section, we studied static
    (stationary) charges.
  • we are going to learn about charges that are on
    the move and the paths they follow!

3
  • Alessandro Voltas work led to the invention of
    the electric battery (voltaic cell), which
    produced the first steady flow of charged
    particles.

4
Conversion of Energy
  • Generator coverts KE to electrical energy
  • Battery converts chemical energy to electrical
    energy
  • Photovoltaic cell converts solar energy to
    electrical energy

5
Electric Circuit
  • a closed loop of a device (battery, generator,
    photovoltaic cell), that will increase the PE of
    electric charges, connected to a device that
    reduces the PE of the charges (radio, lamp, hair
    dryer) while converting electrical energy into a
    form of "useful" energy (sound, light, heat).
  • In order to keep charges flowing, a potential
    difference must be maintained.

6
Electric Current
  • The rate of flow of electric charge is called
    electric current.
  • Electric current in a wire can be defined as the
    net amount of charge that passes through it per
    unit time at any point.
  • Symbol I unit amperes,amps ,A

7
Calculating Electric Current
  • I ?Q
  • ?t
  • I - electric current ( C/s or A)
  • Q amount if charge passing through conductor
    (C )
  • t time interval (s)

8
  • Conversion tips
  • 1 A 1 C/s
  • Often measured in mA and µA

9
Sample Problem
  • A steady current of 3.5 A flowed in a wire for
    2.0 minutes. How much charge passed through the
    circuit?

10
Give it a go
  • A battery was charged using a current of
  • 5.7 A. How long did it take to charge the
    battery if 1.2 x 105 C of charge passed through
    it? ( answer 5.8 h)
  • What is the current in amperes if 2000 Na ions
    were to flow across a cell membrane in 9.8 µs?
    The charge on a sodium ion is the same as on an
    electron, but positive. (answer 3.3 x 10-11 A)

11
Resistance and Ohms Law
  • Players so far current (I) and potential
    difference (V)
  • Georg Ohm experimentally determined
  • that I was proportional to V. (I ? V)
  • For example
  • If you connect a wire to a 9 V battery, the
    current will be three times what it would be if
    the wire were connected to a 3 V battery.

12
  • The amount of current that flows in a circuit is
    also dependent on the resistance offered by the
    circuit.
  • hallway example
  • Current (I) is inversely proportional to
    resistance (R) I ? 1/R
  • as resistance increases, current decreases

13
Ohms Law
  • I V/R
  • Usually written and V IR
  • I current (A)
  • V potential difference (V)
  • R resistance (O) (Greek letter omagha)

14
Resistors
  • Connecting wires generally have very low
    resistance compared to the coils or wire
    filaments in some electrical devices like heaters
    and light bulbs.
  • Resistors are devices designed to have a specific
    resistance and are often used in electronic
    devices to control the amount of current that
    flows..

15
Rresistor
  • The first two colors represent the first two
    digits in the value of resistance,
  • the third represents the power of ten that it
    must be multiplied by
  • the fourth is the manufactured tolerance.
  • Red is 2,
  • green is 5,
  • orange is 103
  • silver indicates a tolerance of 10.
  • This resistor, then, has a resistance of 25 x 103
    , or 2.5 k , give or take 10.

16
Sample Problems
  • What is the resistance of a toaster if 110 V
    produces a current of 3.1 A?
  • A 4.5 V battery is connected to a bulb whose
    resistance is 2.5 O. What is the current?
  • A hair dryer draws 11.0 A when plugged into a 120
    V line. How much charge passes through it in 10
    min?

17
Resistance in a Metal Wire
  • resistance of a metal wire is directly
    proportional to its length, and inversely
    proportional to its cross-sectional area, A
  • R resistance (O)
  • ? - resistivity(O m)
  • L length (m)
  • A cross sectional area (m2)

18
How to Find Cross-Sectional Area of a Wire
  • The cross-sectional area of a circular wire can
    be calculated using
  • where d is the diameter of the wire or
    where r is the radius of the wire.

19
Sample Problem
  • What is the resistance of a 3.5 m length of
    aluminum wire 1.5mm in diameter?

20
Give these a go..
  • What is the length of a copper wire that has a
    cross-sectional area of 3.4 x 10-6 m2 and a
    resistance of 7.1 x 10-2 O ? (answer 14 m)
  • What is the radius of a 1.00 m length of tungsten
    wire whose resistance is 0.25 O ? (answer 2.7
    x 10-4 m)
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