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class code: SPS22-10 teacher: Mr. Elert
classroom: 319 office: A214
test day: Friday phone: (718) 724-8500 ext. 2141
office hours: 8:05–2:40 email: [email protected]

Physics A: Problem Set 6: Electric current

recommended reading

High Marks: 3:35–3:39
Barron's Let's Review: 9.2–9.3 Electric current
physics.info: Electric current
Wikipedia: Electric current, Ampere
Mr. Machado: 04 Introduction to Circuits - Current and Resistance, 05 Current - Sample Problems

classwork

  1. The magnitude of the electric field needed to produce a spark in air (its dielectric strength) is about 3 × 106 V/m. As Benjamin Franklin showed in his famous experiment of 15 June 1752, lightning is basically a very, very large spark. A good sized bolt could travel 1 km and transfer 1,000 C of charge in half a second.
    1. What voltage is needed to make a typical lightning bolt?
    2. How much current flows along its jagged path?
    3. How much energy does it deliver?
    4. What is the power of a lightning bolt?
    5. Movie trivia question: Could you power a "flux capacitor" with a lightning bolt and go Back to the Future?
  2. An average human brain has a power consumption of about 20 W.
    1. How much current flows within the brain as its neurons switch from resting potential (−70 mV) to action potential (+40 mV)? Hint: a watt is a joule per second, a volt is a joule per coulomb, and an ampere is a coulomb per second.
    2. Would you blow a fuse if you wired your brain into a 20 A circuit in a typical North American home?
    3. Movie trivia question: Could you power The Matrix using humans as batteries?

homework

  1. Does more electric charge flow out of a battery or into a battery when it is in use? What about when it is being recharged? Explain your reasoning.
  2. What happens to the electrons in a wire as they pass through a light bulb (or any other electrical device)? That is, what changes as electric current flows through a circuit?
  3. What is the source of the electrons when an electric current flows through a circuit?
  4. A typical Van de Graaff generator or Wimshurst machine used for classroom demonstrations produce electric potentials of 100,000 V or more. They make impressively large sparks that hurt like hell, but will not kill you. Household electrical outlets provide a potential difference of 120 V in the US (240 V in the UK). It almost doesn't need to be said, but one should never touch bare wires in a house or any other building. The risks are just too great. This seems like a contradiction. Why doesn't the higher voltage of a classroom demonstration come with a higher risk of death? (There are two factors at work here.)