MCAT Physics 5

Question 1

A proton (charge +e) moves perpendicular to a magnetic field B with speed v. What is the radius of its circular orbit? (mass = m)

Accepted Answer

mv/(eB)

Answer

The magnetic force provides centripetal force: evB = mv²/r → r = mv/(eB).

Question 2

Which phenomenon explains why the sky appears blue?

Accepted Answer

Rayleigh scattering of shorter wavelengths

Answer

Rayleigh scattering is proportional to 1/λ⁴, so shorter (blue) wavelengths scatter far more than longer (red) wavelengths.

Question 3

A 1000 W electric motor lifts a 50 kg load. What is the maximum speed at which the load can be raised? (g = 10 m/s²)

Accepted Answer

2 m/s

Answer

Power = Fv = mgv; v = P/(mg) = 1000/(50 × 10) = 1000/500 = 2 m/s.

Question 4

In the photoelectric effect, increasing the intensity of incident light (without changing frequency) will:

Accepted Answer

Increase the number of emitted electrons per second

Answer

Higher intensity means more photons per second, which ejects more electrons per second, but does not change each electron's kinetic energy.

Question 5

Two parallel plates are separated by 2 mm and connected to a 100 V battery. What is the electric field between the plates?

Accepted Answer

50,000 V/m

Answer

E = V/d = 100 / (2 × 10⁻³) = 50,000 V/m.

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