Ohm's law

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Ohm’s law

Ohm’s law relates currents to voltage in a circuit
A simple circuit
Why are birds not killed sitting on high-voltage
                power lines?
 A bird stands on an electric transmission line carrying 2100 A.
The line has 2.8x10-5 Ω resistance per meter and the bird's feet
  are 4 cm apart. What voltage difference does the bird feel?
Thousands of hawks, eagles, and other large birds die every
         year from electrocution by power lines.
What happens to people
who touch power lines?
Talk about birds on a power line!
Would it be safe to swing from a power line? What
should you do if a high-voltage power line falls close
                       to you?
***electrocution logo
***Short circuit…
Barefoot in the kitchen: After washing his dishes,
Brian decides to have fun by touching a 120-V line.
            Would this be dangerous?

                                 dry skin: R=104-106 Ω
                                 wet skin: R=103 Ω

                                  I=120/1000=120 mA
 Consider this short circuit. The person touching the case is wet and
presents only 3 kΩ to the ground. What is the voltage on the case if 5
mA flows through the person? If the resistance of the ground wire is
          0.2 Ω, will the 20-A circuit breaker be triggered?

                                                    V = IpR = 15 V
                                                    I = 15/0.20 = 75 A
                                                    Itot = 0.005+75=75 A
Why are two-prong plugs potentially
The third prong provides proper
 How much current will a person who presents a
resistance of 100,000 Ω draw from a 10,000-V,
         improperly grounded device?
The third prong
Fatal attraction? In the summer of 1999, two women were killed
    by lightning in Hyde Park, London. The coroner's report
   implicated underwired bras. What might have been their

          According to the coroner: It's far better to be
          naked in a storm than to wear your bra.
Why are burns due to electrocution often
 characterized by fern-like patterns?
True or false: A 1-amp current is often
 less damaging than a 70-mA current.
Is it correct to say that an electrician is safe from
 shock if he uses the back of his hand when they
                  touch live wires?
 Effects of electrical shock as a function of current*

 Current (mA)                                              Effect

 1                                                         Threshold of sensation

 5                                                         Maximum harmless current

 10-20                                                     Sustained muscular contraction (can’t let go);
                                                           contraction of chest muscles may stop breathing

 50                                                        Onset of pain

 100-300+                                                  Ventricular fibrillation, often fatal

 300-600                                                   Burns

 6000                                                      Sustained ventricular contraction and respiratory
                                                           paralysis; both cease when shock ends; can be useful
                                                           for defibrillation
 *For an average male shocked through trunk of body for 1 s by 60 Hz AC. Values for females are typically 60-
 80% those of males.
Is it correct to say that an electrician is safe from
 shock if he uses the back of his hand when they
                  touch live wires?
     Physiological effects of

      Effects of Current through Human Body

Current (mA)                        Effect

   <10            tingling or imperceptible

   10-20          painful and cannot let go (muscle
    20-30         Muscle paralysis and breathing
    >70           breathing very difficult (could be fatal if
    100           death due to ventricular fibrillation

   >200           Heart and breathing stops; severe burns
 Suppose that the motor of a motorized hospital bed shorts out to
  the bed frame, and the bed frame's connection to a ground has
broken (or was not there in the first place). If a nurse touches the
bed and the patient at the same time, she becomes a conductor and
   a complete circuit can be made through the patient to ground
 through the ECG apparatus, as shown. If V = 220 V, calculate the
                   current through the patient.

I = 220/(3∙104) = 7.3 mA
   An electric current can cause muscular contractions with
    varying effects. Here, the victim is thrown backward by
involuntary muscle contractions that extend the legs and torso.
All the muscles in the hand are being stimulated by the current.
                      Why can’t he let go?
Heart attack or electrocution?
SPEAR, a storage ring 72 m in diameter at the Stanford
   Linear Accelerator, has a 20-A circulating beam of
 electrons that are moving at nearly the speed of light.
          How many electrons are in the beam?
The disk drive in a portable CD player is connected to a battery
  that supplies a current of 0.22 A. How may electrons pass
               through the drive in 4.5 seconds?
The higher the voltage, the greater the
The unit of electrical current is called
A diode does not obey Ohm’s law
Light bulbs turn on at different times. Why
                 the delay?
How does a lightbulb work?
 The electric eel can generate a current as large as 1
ampere at a voltage of up to about 500 volts. What is
            the resistance of the water?
How does the hair drier work?
The falling water
  can do work in
 turning the water
   wheel only as
long as the pump
   maintains the
between the upper
     and lower
High voltage cables such as these distribute
    electricity throughout the country.
  A standard flashlight battery can deliver about 2 W · h of
energy before it runs down. (a) If a battery costs 80 ¢, what
     is the cost of operating a 60 W lamp for 10 h using
batteries? (b) What is the cost if the power is provided by an
            electric utility company, at 6 ¢/kW∙h?

                               60 10 / 2  300 batteries  $240
                               60 10 / 1000   $0.06  3.6 ¢
Charge tends to flow
downhill much like a
By convention, current is a flow of
         positive charge.
   The falling
  water can do
 work in turning
the water wheel
 only as long as
    the pump
  maintains the
  between the
upper and lower
An EMF, often a battery, is the charge
 A simple electric circuit has a voltage source (such as a generator or
battery) that maintains the electrical potential, some device (such as a
 lamp or motor ) where work is done by the potential, and continuous
                  pathways for the current to follow.
  A simple electric circuit carrying a current of
1.00 coulomb per second through a cross section
    of a conductor has a current of 1.00 amp.
 A conventional current describes positive charges moving from the
positive terminal (+) to the negative terminal (-). An electron current
describes negative charges (-) moving from the negative terminal (-)
                       to the positive terminal (+)
  ***What is the nature of the electric current
    carried by these conducting lines? It is an
  electric field that moves at near the speed of
light. The field causes a net motion of electrons
   that constitutes a flow of charge, a current.
   Determine the number of electrons that pass
between the terminals of a 12-V car battery when a
       60-W headlight burns for one hour.

                              P = I•V or I = 60/12 = 5 A
                              Q = I•t = 5 •3600 = 1.8x104 C
***Ohm’s law: bra animation
How does a digital voltmeter work?
A 12-V storage battery consists of
 six 2-V cells connected in series
Current vs resistance: Water flow
Example of Ohm’s Law
Ohm’s law applet
Applet: bulb amperage
Any voltage source (in this case, a
   carbon-zinc dry cell) has an
       internal resistance
Electrical power…
If you connect a 40-W light bulb in series with a 60-
       W light bulb, which would be brighter?
Why is the 60-W
 lightbulb the
If my bill for Nov of 2002 was about $72, what was
            the average cost of one kWh?
Electric pickles
Charging electric cars
Why do batteries warm up when a flashlight is in use?
Typical neuron
Membrane charge

Positive and negative charge layers form on the outside and
inside surfaces of a membrane during its resting state.
  In the technique of impedance plethysmography, the
electrical resistance of the calf is measured to diagnose
  deep venous thrombosis (blood clotting in the veins).
What is the resistance the light bulb if the
             current is 0.4 A?
 An electrical heating coil is immersed in 4.2 kg of
 water at 22°C. The coil, which has a resistance of
210 W, warms the water to 35°C in 3.5 min. What is
the potential difference at which the coil operates?
A heating coil has a resistance of 3.7 and operates
 on 117 V. What does is the cost of operating the
     heating coil 30 min per day for 30 days?
A 50 W electrical heating element is placed in a well-insulated
 container into which has just been placed 500 g of water at
  10°C and 280 g of ice at 0°C. How long will it take for the
contents to evaporate completely? (The latent heat of fusion
 for water is 333 J/g, the heat of vaporization is 2260 J/g,
              and the specific heat is 4.18 J/g.)
A standard toaster has a power cord with a resistance of 0.02 W
 connected in series with a 10-W nichrome heating element. The
potential difference between the terminals of the toaster is 120
  V. How much power is dissipated in the power cord and in the
                        heating element?
Extension Cords and a Potential Fire Hazard

  When an extension cord is used with a space heater, the cord must have a
  resistance that is sufficiently small to prevent overheating of the cord.
Electrical Power Sent to a Loudspeaker
***Ohm’s law: carbon button…Darren’s presentation
***ohm…How do turn signals work?
Battery: Chemical energy of a battery can be
converted to mechanical energy by an engine.
Plasma ball

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