A sculptor has asked you to help electroplate gold onto a brass statue. You know that the charge carriers in the ionic solution are singly charged gold ions, and you've calculated that you must deposit 0.50 g of gold to reach the necessary thickness. How much current do you need, in mA, to plate the statue in 3.0 hours?

The biochemistry that takes place inside cells depends on various elements, such as sodium, potassium, and calcium, that are dissolved in water as ions. These ions enter cells through narrow pores in the cell membrane known as ion channels. Each ion channel, which is formed from a specialized protein molecule, is selective for one type of ion. Measurements with microelectrodes have shown that a 0.30-nm-diameter potassium ion (K⁺) channel carries a current of 1.8 pA. How many potassium ions pass through if the ion channel opens for 1.0 ms?

You've decided to protect your house by placing a 5.0-m-tall iron lightning rod next to the house. The top is sharpened to a point and the bottom is in good contact with the ground. From your research, you've learned that lightning bolts can carry up to 50 kA of current and last up to 50 μs. How much charge is delivered by a lightning bolt with these parameters?

Lightning occurs when there is a flow of electric charge (principally electrons) between the ground and a thundercloud. The maximum rate of charge flow in a lightning bolt is about $20,000$ C/s; this lasts for $100$ ms or less. How much charge flows between the ground and the cloud in this time? How many electrons flow during this time?

(I) A current of 1.75 A flows in a wire. How many electrons are flowing past any point in the wire per second?

(II) What is the current in amperes if 1200 Na⁺ ions flow across a cell membrane in 2.8 μs? The charge on the sodium is the same as on an electron, but positive.

The LHC accelerator at CERN (Switzerland) is designed to carry a 0.58-A beam of protons ( q = 1.6 x 10^-19 C ) traveling at very nearly the speed of light ( 3.0 x 10⁸ m/s) around a ring 27 km in circumference. How many protons are in the beam?

For the wire in Fig. 25–38, whose diameter varies uniformly from a to b as shown, suppose a current I = 2.5 A enters at a. If a = 2.5 mm and b = 4.0 mm, what is the current density (assume uniform) at each end?