Graduate Record Examinations

The Graduate Record Examination score is an important factor in graduate school admissions decisions. This document offers advice on preparing for and taking the exams, gleaned from the literature (Physics Today, October 1991, page 9) and from recent Oberlin College physics seniors.

The GRE has two parts: the general test and the subject tests. Graduate departments in physics usually require both the general test and the physics subject test, whereas departments in materials science and electrical engineering usually require only the general test. The exams are given three times a year: in April, October, and December. Exact dates are avaliable from the ETS web page.

Registration

Many students take the general test in the April of their Junior year and the physics test in the October of their Senior year. (It is not a good idea to take both exams on the same day.) You can get a GRE registration packet from Oberlin's Office of Career Development and Placement or by mail from: GRE/ETS, P.O. Box 6000, Princeton, NJ, 08541-6000. Many scholarship programs--and some graduate departments--require that applicants take both parts of the GRE on or before the October testing date. Be aware that the registration deadline falls more than a month before the test date; in particular the deadline for the October test offering often falls before the first day of fall semester classes at Oberlin.

Preparation (subject matter)

The general test is an aptitude test similar to the SAT, and usually requires no preparation (other than a good night's sleep) on your part. The physics subject test is another matter altogether. Most of the questions concerning classical physics (mechanics, electricity and magnetism, thermodynamics, waves, etc.) are at the level of Halliday and Resnick's text. It makes sense to skim your copy of this book, paying special attention to the chapter summaries. Most of the questions concerning quantum physics are at the level of Physics 212. In particular, there are many questions concerning the facts of atomic physics. We recommend a thorough review of your Physics 212 text, or some other text on modern physics. (Some students recommend Serway, Moses, and Moyer's Modern Physics for such review; others recommend Eisberg and Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. It's probably best to use whatever you're familiar with.) Consider doing this review during the summer before you take the physics test.

Preparation (practice)

The GRE is a timed, multiple choice exam, similar to the SAT. You have probably not taken such an exam for several years, so it is necessary to get some practice. Some sample questions are available in the free GRE General Test Descriptive Booklet and some will be mailed to registrants for the physics test, but it is very useful to also study the ETS booklet Practicing to Take the GRE Physics Test. This booklet contains two GRE physics exams that were actually administered in previous years. Many students have found it useful to take these trial exams individually, respecting the time limit, and then to discuss their answers collectively. You can also purchase mock GREs from publishers other than ETS, but these are generally not accurate reflections of the real GRE. In particular, the sample tests given in J.J. Molitoris's GRE Physics ("the purple book") are uncharacteristically advanced and difficult.

Test taking strategy

For some questions on the physics test you are not expected to know the answer or even be able to derive it, but instead you are supposed to be able to eliminate some or most of the candidate answers through physical reasoning. Such reasoning includes:

If the answer is a numerical value, you can eliminate candidates with the wrong order of magnitude. For example, any energy in an atomic physics problem will be about
1 eV = 1.6 x 10^{-19} Joule. A candidate answer of 10^{-13} Joule is a million times larger than reasonable.
Some formulas will be incorrect dimensionally. For example the formula
```
     distance = speed x time
```
is dimensionally correct, although it doesn't hold for accelerated motion. But the formula
```
     distance = speed/time
```
could never be correct under any circumstances, because it has the units of [meters] on the left and [meters]/[second]^2 on the right.
Some formulas can be ruled incorrect on the basis of qualitative behavior. For example, does the electric field in the candidate answer increase or decrease with charge? Do the candidate answers reduce to the correct initial conditions at t = 0? Do they exhibit the correct symmetry? The correct classical limit? Can the quantity in the denominator ever equal zero? If so, the candidate is probably wrong!

Units in electricity and magnetism

GRE questions on these subjects may be presented in either the MKS system (the one used in Halliday and Resnick) or the cgs system. You need to be familiar with both forms of the equations. For example, Coulomb's law takes the form

             1    q1 q2                  q1 q2
     F = -------- -----  in MKS and  F = -----  in cgs.
         4  π e₀   r^2                    r^2

Consequently, Gauss's law is

     div E = ρ/e₀     in MKS and  div E = 4 π ρ  in cgs.

The Lorentz force law is

     F = q v x B         in MKS and  F = q (v/c) x B  in cgs.

where c is the speed of light.