physicsa.com
class code: SPS21 teacher: Mr. Elert
classroom: n/a office: n/a
test day: unknown phone: n/a
office hours: n/a email: n/a

Physics A: Problem Set 7: Electric resistance

recommended reading

High Marks: 3:39–3:40, 3:47–3:48
Barron's Let's Review: 9.4–9.5 Resistance
physics.info: Electric resistance
Wikipedia: Electrical resistance and conductance, Ohm's law
HyperPhysics: Resistance, Electric shock
Khan Academy: Circuits (part 1)
Physics Girl: Circuits, Voltage, Resistance, Current
Mr. Machado: 06 Ohm's Law, 07 Ohm's Law - Sample Problems, 10 Resistivity, 11 Resistivity - Sample Problems

classwork

  1. An electric current of 1 mA through the human body is just barely detectable. At 5 mA, the sensation becomes painful. A current above 15 mA is sufficient to cause paralysis in major voluntary muscles. At 75 mA, the current will begin to scramble the normally coordinated contractions of the heart — a condition that is fatal if it persists for more than a few seconds. This information is summarized in the table below along with some information on the arm-to-arm resistance of the human body.
    Physiological effects of current
    current (mA) resistance (Ω)
    sensation 01 dry skin 100,000
    pain 05 wet skin 005,000
    paralysis 15    
    fibrillation 75    
    1. What voltage across the arms of a human with dry skin would result in a barely detectable current?
    2. The voltage you calculated in part a. is now applied to a human with wet skin. What is the effect?
    3. Why is it dangerous to handle electrical equipment with wet skin?
    4. What minimum voltage is needed to produce sensation in wet skin?
    5. How could the situation described in part d. be demonstrated? (No exotic or dangerous equipment, please.)

homework

  1. Which device in your home has the highest resistance: a flood light, a reading lamp, or a night light? All three devices are plugged into standard North American 120 V outlets.
  2. You are a cruel and unusual electrical engineer and have been given the cruel and unusual task of designing an electric chair for the purposes of executing criminals in a cruel and unusual fashion.
    1. How is your victim to be attached to the chair and what special preparations would you do to ensure the chair worked most effectively?
    2. What minimum voltage is sufficient to achieve your task?
    3. What do the terms DC and AC refer to and which of these was used in electric chairs? Explain the reasoning behind this choice.
  3. Given a wire with a resistance R, what will be the new resistance if…
    1. the wire is cut in half and only one half is used to conduct electricity?
    2. the wire is folded in half and both halves are used to conduct electricity?
  4. A tungsten rod and an aluminum rod have the same length and resistance.
    1. What is the ratio of the cross sectional area of the tungsten rod to the aluminum rod?
    2. What is the ratio of the diameter of the tungsten rod to the aluminum rod?