In Chapters 9 and 11 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss shot noise.

9.8.1 Shot Noise

The first (and smallest) limitation [on our ability to measure current] is called shot noise. It is due to the fact that the charge is transported by ions that move randomly and independently through the channels….

11.16.2 Shot Noise

Chapter 9 also mentioned shot noise, which occurs because the charge carriers have a finite charge, so the number of them passing a given point in a circuit in a given time fluctuates about an average value.

The metric system is based on three fundamental units: the kilogram (kg, mass), the meter (m, distance), and the second (s, time). Often a combination of these three is given a name (called a derived unit), usually honoring a famous scientist. For example, a newton, the unit of force named after the English physicist and mathematician Isaac Newton (1642–1727), is a kg m s^−2; a joule, the unit of energy named for English physicist James Joule (1818–1889), is a kg m^2 s^−2; a pascal, the unit of pressure named for French mathematician and physicist Blaise Pascal (1623–1662), is a kg…

Hart and Gandhi (1998)
Phys. Med. Biol.,

Sometimes proponents of pseudoscience embrace nonsense, but other times they propose plausible-sounding ideas that are wrong because the numbers don’t add up. For example, suppose you are discussing with your friend about the biological effects of power-line magnetic fields. Your friend might say something like this:

“You keep claiming that magnetic fields don’t have any biological effects. But suppose it’s not the magnetic field itself, but the electric field induced by the changing magnetic field that causes the effect. We know an electric field can stimulate nerves. Perhaps power-line effects operate like transcranial magnetic stimulation, by inducing electric fields.”


The central slice theorem is key to understanding tomography. In Intermediate Physics for Medicine and Biology, Russ Hobbie and I ask the reader to prove the central slice theorem in a homework problem. Proofs are useful for their generality, but I often understand a theorem better by working an example. In this post, I present a new homework problem that guides you through every step needed to verify the central slice theorem. This example contains a lot of math, but once you get past the calculation details you will find it provides much insight.

The central slice theorem states that…

Schenck, J. F. (2005)
Prog. Biophys. Mol. Biol.


In Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss biomagnetism, magnetic resonance imaging, and the biological effects of electromagnetic fields. We don’t, however, talk about the safety of static magnetic fields. If you want to learn more about that topic, I suggest an article by John Schenck:

Schenck, J. F. (2005) “Physical interactions of static magnetic fields with living tissues,” Prog. Biophys. Mol. Biol. Volume 87, Pages 185–204.

This paper appeared in a special issue of the journal Progress in Biophysics and Molecular Biology analyzing the health effects of magnetic fields. The abstract states:

Clinical magnetic resonance…

This week I spoke at the American Association of Physics Teachers 2021 Summer Meeting. Getting to the meeting was easy; I just logged onto a website. Because of the Covid-19 pandemic, the entire conference was virtual and all the talks were prerecorded. A video of my talk — “Two-Semester Intermediate Course Sequence in Physics for the Life Sciences” — is posted below. If you want a powerpoint of the slides, you can find it here. As readers of this blog might suspect, the courses I describe are based on the textbook Intermediate Physics for Medicine and Biology.

“Two-Semester Intermediate Course…

In the November 13, 2020 episode of ( Season 12, Episode 5), two earnest entrepreneurs, Ken and Allyson, try to persuade five investors, the “sharks,” to buy into their company. The entrepreneurs sell LIFTiD, a device that applies a small steady current to the forehead. Ken said it’s supposed to improve “productivity, focus, and performance.” Allyson claimed it’s a “smarter way to get a… boost of energy.”

The device is based on transcranial direct current stimulation (tDCS). In 2009 I published an editorial in the journal Clinical Neurophysiology to accompany a paper appearing in the same issue by Pedro Miranda

Voodoo Science,
by Robert Park

These days — when so many people believe crazy conspiracy theories, refuse life-saving vaccines, promote alternative medicine, fret about perceived 5G cell phone hazards, and postulate implausible microwave weapons to explain the Havana Syndrome — we need to understand better how science interacts with society. In particular, we should examine similar controversies in the past to see what we can learn. In this post, I review the power line/cancer debate of the 1980s and 90s. I remember it well, because it raged during my graduate school days. …

In last week’s post, I discussed the Bragg peak: protons passing through tissue lose most of their energy near the end of their path. In Chapter 16 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I present a homework problem in which the student calculates the stopping power (energy lost per distance traveled), S, as a function of depth, x, given a relationship between stopping power and energy, T. This problem is a toy model illustrating the physical origin of the Bragg peak. Often its helpful to have two such exercises; one to assign as homework and one…

Energy loss versus depth for a 150 MeV proton beam in water, with and without straggling (fluctuations in the range). The Bragg peak enhances the energy deposition at the end of the proton range. Adapted from Fig. 16.47 in Intermediate Physics for Medicine and Biology.

In Chapter 16 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss the Bragg peak.

Protons are also used to treat tumors (Khan 2010, Ch. 26; Goitein 2008). Their advantage is the increase of stopping power at low energies. It is possible to make them come to rest in the tissue to be destroyed, with an enhanced dose relative to intervening tissue and almost no dose distally (“downstream”) as shown by the Bragg peak in Fig.16.47.

Brad Roth

Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.

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