The Physics
Hypertextbook
Opus in profectus

# Electric Current

## Problems

### practice

1. A stun gun delivers 3 amperes of peak current in pulses lasting 80 microseconds. One pull of the trigger delivers 95 pulses in a 5 second cycle. A sustained 3 amps is more than enough to kill a person, but because it is delivered in such short bursts a stun gun is only capable of delivering intolerable pain.
1. Determine the total charge delivered by the stun gun in one cycle.
2. Determine the average current of the stun gun in one cycle.
2. Write something.
3. Write something.
4. Write something completely different.

### conceptual

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 is the sign of the net electric charge on the current carrying wires in your home? Explain your reasoning.
3. 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?
4. What is the source of the electrons when an electric current flows through a circuit?
5. 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.)

### numerical

1. 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?
2. 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 1000 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?