Introduction to Physical Science Monday, Wednesday, Thursday Tom Burbine [e-mail]

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Introduction to Physical ScienceMonday,
Wednesday, ThursdayTom Burbinetomburbine_at_astr
o.umass.edu
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Quiz on Thursday

• Coulombs Law
• Electric Potential
• Electric current
• Electrical resistance
• Ohms Law
• Voltage current x resistance
• Power current x voltage
• Series Circuit
• Parallel Circuit

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• http//hilaroad.com/camp/projects/lemon/lemon_batt
  ery.html
• 1 lemon can generate approximately 0.83 Volts

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• Electric potential (volt) electric potential
  energy
• 
  charge

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• A conductor is a material which contains movable
  electric charges (electrons)
• Zinc-iron nails and copper are good conductors of
  electricity
• Insulators are non-conducting materials with
  fewer mobile charges (electrons), which resist
  the flow of electric current

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Voltage is like water pressure

• Voltage is analogous to a water pressure
  difference
• A high pressure difference (higher voltage) will
  result in a faster electron flow

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Current

• Electric current, measured in amperes, is the
  rate of flow of electric charge
• One ampere is the rate of flow of one coulomb of
  charge (6.24 x 1018 electrons) per second

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Electrical Resistance

• The electrical resistance is a measure of its
  opposition to the passage of an electric current
• Resistance Voltage
• Current

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• The resistance of a wire depends on three
  factors
• the length of the wire,
• the cross-sectional area of the wire,
• the resistivity of the material
  composing the wire
• The resistance would be greater for a longer
  wire, less for a wire of larger cross sectional
  area, and depends upon the material out of which
  the wire is made

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• Electrical resistance of a conductor is a measure
  of how difficult it is to push the charges along

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Ohms Law

• Current voltage
• resistance
• Units
• 1 Ampere 1 volt
• 1 ohm

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• Usually written as
• V IR
• V is Voltage in Volts (V)
• I is Current in Amperes (A)
• R is Resistance in Ohms (?)

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Example

• How much current flows through a lamp with a
  resistance of 60 ? when the voltage across the
  lamp is 12 V?

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Example

• How much current flows through a lamp with a
  resistance of 60 ? when the voltage across the
  lamp is 12 V?
• V IR
• I V/R 12 V/ 60 ? 0.2 A

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• What is the resistance of a toaster that draws a
  current of 12 A when connected to a 120-V circuit?

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• What is the resistance of a toaster that draws a
  current of 12 A when connected to a 120-V
  circuit?
• V IR
• R V/I 120 V/ 12 A 10 ?

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• Why do you get more of a shock when you touch a
  battery with wet hands than dry hands?

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Example

• Why do you get more of a shock when you touch a
  battery with wet hands than dry hands?
• I V/R
• Resistance of hands soaked in salt water 100 ?
• Resistance of dry hands 100,000 ?
• Lower resistance equals higher current

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Series Circuits

• Electric current passes through a single pathway
• Total resistance is the sum of the resistances
• Current is equal to the Voltage divided by
  the
  sum of the resistances
• A break in the path results in an open
  circuit

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Series Circuits
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Series Circuits

• V I(R1 R2 R3 )

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Series Circuits

• If one lamp burns out, the current through the
  other lamps cease. All lamps go out
• Adding more lamps reduces the current through the
  circuit
• Adding more lamps reduces the brightness of each
  lamp

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Parallel Circuits

• The Voltage is the same across each lamp
• Current divides along the parallel branches

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Parallel Circuits
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Parallel Circuits

• If one of the lamps in a parallel circuit burns
  out, the other lamps are unaffected
• However, the total current in the circuit will
  decrease

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Parallel Circuits

• If you add another lamp, the brightness of the
  other lamps is unchanged
• However, the total current will increase

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Parallel Circuits

• Overloading is when you add too many devices in
  parallel
• The total current increases too much
• The wire can overheat

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• Series
  Parallel

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Power

• Power current x voltage
• Power charge x energy energy
• time charge
  time
• Units
• Watts amperes x volts
• 1 kilowatt 1,000 Watts

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• power energy/time
• energy power x time
• 1 kilowatt-hour amount of energy consumed in
  1 hour at the rate of 1kilowatt

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• If electric energy equals 25 cents/kilowatt-hour,
  how much would it cost to operate a 100-W bulb
  for 10 hours?

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• If electric energy equals 25 cents/kilowatt-hour,
  how much would it cost to operate a 100-W bulb
  for 10 hours?
• cost rate x power x time
• cost (25 cents/kilowatt-hour)(0.1 kW)(10 hours)
• Cost 25 cents

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Example

• Lamp at 120 Watts is operating on a 120 Volt
  line, what current does it draw?

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Example

• Lamp at 120 Watts is operating on a 120 Volt
  line, what current does it draw?
• Power current x voltage
• current power/voltage 120 Watts/120 Volts
• current 1 ampere

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Any Questions?