Circuits

1
Circuits

• Electric Circuit a closed path along which
  charged particles move
• Electric Current the rate at which a charge
  passes a given point in a circuit.
• For current to flow the circuit must be complete
  and there must be a potential difference between
  two points in the circuit.
• The unit of current is the ampere (also known as
  an amp)

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• Conductor a material, usually a metal, in which
  electric charges move easily. There are plenty of
  free electrons.
• Insulators have few or no free electrons.
• Current can be described in two ways
  conventional current and electron flow
• Conventional current current flows from the
  positive to negative, against the flow of
  electrons
• Electron flow current flows from negative to
  positive, a flow of electrons

3

• I q/t where I Current (amps), q charge
  (Coulomb), and t time (seconds)
• Example During a thunderstorm, a lightning
  strike transfers 12 coulombs of charge in 2.0 x
  10-3 second. What is the average current produced
  in this strike?
• Example Charge flowing at the rate of 2.50 x
  1016 elementary charges per second is equivalent
  to a current of ?

4
Some Circuit Components

• Switch a device for making, breaking, or
  changing the connections in an electric circuit.
  The switch must be closed and there must be a
  complete circuit for current to flow.
• Light Bulb
• Cell a device that converts chemical energy to
  electrical energy.

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• Battery a combination of two or more cells.
• Batteries and cells can provide the potential
  difference needed for current to flow.
• Ammeter a device used to measure current (Placed
  in the circuit)
• Voltmeter a device used to measure the potential
  difference between two points (Placed over the
  circuit)

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• Resistance the opposition that a device or a
  conductor offers to the flow of electric current.
• Unit is the ohm (O)
• resistor a device designed to impede the flow
  of current
• Used to represent an appliance like a toaster or
  hairdryer in a circuit
• Ohms law for a constant temperature, the
  resistance of a conductor is the ratio of the
  potential difference applied to its ends and the
  current that flows through it.
• R V/I where R resistance (Ohm), V potential
  difference (Volt), I current (Amp)

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• Example A student measures a current of 0.10
  ampere flowing through a lamp connected by short
  wires to a 12.0V source. What is the resistance
  to the lamp?
• Example An automobile headlight with a
  resistance of 30 ohms is placed across a 12-V
  battery. What is the current through the circuit?
• Example A motor with an operating resistance of
  32 ohms is connected to a voltage source. The
  current in the circuit is 3.8 A. What is the
  voltage of the source?
• Example A 20 ohm resistor has 40 C of charge
  passing through it in 5.0 seconds. What is the
  potential difference across the resistor?

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• Electrical power the rate at which electrical
  energy is transferred by an electrical circuit
• P VI where P power (watt), V potential
  difference (volt),
• I current (amps)
• Given P VI and V IR we can also say that P
  I2R
• Given P VI and I V/R we can also that that P
  V2/R
• kilowatt hour equal to 1000 watts delivered
  continuously for 3600 seconds (one hour) or
  3.6x106 J

9

• Electrical Energy/Work The power consumed during
  a specific amount of time
• W Pt VIt I2Rt V2t/R where W
  energy/work (joules), P power (watts),
• t time (seconds)
• Given W Pt and P VI we can also say that W
  VIt
• Given W Pt and P I2R we can also say that W
  I2Rt
• Given W Pt and P V2/R we can also say that W
  V2t/R

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• Example A television set draws 2.0 A when
  operated on 120V.
• a.) How much power does the set use?
• b.) If the set is operated for an average of
  7.0 hours per day, what energy is kWh does it
  consume per month?
• c.) At 0.08 per kWh, what is the cost of
  operating the set per month?
• Example A 60W light bulb is connected to an 115V
  power source.
• a.) What is the current through the light
  bulb?
• b.) What is the resistance of the light bulb?
• Example A 240V water heater has a resistance of
  15 ohms.
• a.) What is the power of the heater?
• b.) What thermal energy is supplied by t he
  heater in 15 minutes?
• c.) Compare the power of the same 240V water
  heater to a 120V water heater, also with a
  resistance of 15ohms.
• d.) Compare the thermal energy of the 240V
  water heater to a 120V heater in a 15 minute
  period.

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• Resistivity a characteristic of a material that
  depends on its electronic structure and
  temperature (?)
• unit is the ohmmeter
• Resistance varies directly with length
• Resistance varies inversely with its
  cross-sectional area (Area of a wire is p r2 )
• Resistance varies directly with resistivity

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• Good conductors have low resistivities.
• As temperature increases, resistivity increases
• R ?L/A, where ? resistivity (ohmmeter), L
  length (m), A cross sectional area (m2), R
  resistance (ohm)
• Cross sectional area, A pr2

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• Example A 10.0 meter length of copper wire is at
  20C. The radius of the wire is 1.0x10-3 meter.
• a.) Determine the cross-sectional area of the
  wire.
• b.) Calculate the resistance of the wire.

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• Example A 10.0 meter long wire has a cross
  sectional area of 3.0x10-6m2 and a resistance of
  9.4x10-2 ohm at 20C. What material could this
  wire be made of?
• Example A 12.0-meter length of copper wire has a
  resistance of 1.50 ohms. How long must an
  aluminum wire with the same cross-sectional area
  be to have the same resistance?