## Tuesday, January 21, 2014

### Sample GRE Physics Questions (1)

It's that time of year when many new college grads get ready to take the Graduate Record Examination (GRE) to get into grad school. Most will take the verbal, quantitative and analytical tests but many others will also take one or more subject tests, including physics. For those who might be taking the physics subject GRE I will be sharing some sample questions for you to practice on. The questions-problems will be given in each of several blog posts, alternating with the solutions.

Be assured that the problems you encounter will be nowhere near the sort given on typical departmental qualifying exams (which are given before one undertakes graduate study) such as the Modern Physics  M.Phil. qualifying exam I took from the University of the West Indies .

Here goes, for the first set  (You should allow for no more than 30 minutes, max):

1. The photo-electric threshold of tungsten is 2300A (where A denotes Angstrom). Estimate the energy E of the electrons ejected from the surface by ultraviolet light of wavelength 1800A.

2. Find the minimum energy in electron volts (eV) for an electron in a spark discharge to strip a sodium (Na) atom (Z = 11) of its last electron, assuming the other ten are already removed.

3. Let r^ be a position vector. Then find the divergence of r^.

4. Let r^ be a position vector and a be a constant vector. Then find the gradient of the scalar product: a*r^.

5. Consider the matrix:

(0........0........1)
(0........1.........0)
(1.........0 .......0)

a) Find Tr, the trace of the matrix

b) Find the eigenvalues of the matrix.

6. Let U1 and U2 be orthonormal functions. Find the value of N which normalizes:

F = N(U1 + 2i(U2)

7. Two men together support a uniform plank of wood. At the instant one of the men lets go of his end, what is the force the other man feels?

Questions 8-9:
Consider two coordinate systems whose origins are non-accelerating. Assume that one of these systems (denoted by primes) is rotating with constant angular velocity w with respect to the other which is non-rotating. (Let i' be one of three orthogonal unit vectors in the rotating system)

8. Find the time derivative in the non-rotating system.

9. Find the second order time derivative in the non-rotating system.

10. If two strings, whose densities are 25 g/cm and 9 g/cm are joined together then find the reflection coefficient for the vibration waves.

11. A particle of mass m moves in a plane  under the influence of a force F = - kr directed toward the origin. Show a polar coordinate system (r, theta) to describe the motion of the particle and thence or otherwise give the Lagrangian for the system.

12. According to relativistic mechanics the actual velocity of the electrons whose kinetic energy is 0.25Mev is what?

13. Compute the effective mass of a photon of wavelength 6000 A using the Einstein mass-energy equation.

Questions 14 and 15:

When the Sun is directly overhead, a given square meter surface of the Earth receives about 1300 watts of radiant energy. Assume this energy is in the form of a plane-polarized monochromatic wave.

14. Find the rms (root mean square) magnitude of the electric field of the wave.

15. Find the rms magnitude of the magnetic field for the wave.