Question

An electron falls through a distance d in a uniform electric field of magnitude E. Thereafter, the direction of the field is reversed (keeping its magnitude the same) and now a proton falls through the same distance. Compare, using quantitative reasoning, the time of fall in each case. Contrast this situation with that of objects falling freely under gravity.

Answer 1

Step-by-step explanation:

We know that the electric force can be expressed as: F=qE. According to Newton's Second Law of Motion, force can also be expressed as: F=ma. Therefore: a=F/m. We can substitute the electric force expression for "F" in this equation. We get: a=qE/m. We can see from this equation that acceleration is inversely proportional to mass and directly proportional to the electric field and charge. Since the electric field is being reversed and since the charges on the proton and electron differ only by the + or - sign respectively, the numerator of this fraction will remain constant in this scenario. The only variables that are effectively changing are the mass and the resultant acceleration. From the inverse relationship of acceleration and mass, we can say that the proton - having a significantly larger mass than the electron - should experience a smaller acceleration, and should thus take longer to fall distance "d." The electron, on the other hand, should experience a greater acceleration due to its significantly smaller mass, and should fall through distance "d" in a shorter amount of time.

Under the influence of gravity (on the surface of the Earth, for example), objects released from the same height should fall freely with the same acceleration at any given time, regardless of mass. It makes sense, however, that subatomic particles interacting with the electric field are hardly affected by gravity, given how weak gravitational forces are on the microscopic scale.

Answer 2

Please see below as the answer is self-explanatory

Step-by-step explanation:

Assuming that the electric field is pointing upward this will produce a downward force on the electron. Neglecting the effect of gravity, according to Newton's 2nd Law, the force on the electron due to the field, produces an acceleration, that can be found solving the following equation:

F = me*a = qe*E ⇒ a = qe*E / me

If the electric field is uniform, the acceleration that produces is constant, so, we can use the kinematic equation that relates displacement and acceleration with time:

x = v₀*t + 1/2*a*t² = v₀*t +1/2*(qe*E/me)*t²

Now, for a proton falling, if the direction of the field is reversed (pointing downward) it will accelerate the proton downward.

Using the same reasoning as above, we get the value of the acceleration as follows:

F = mp*a = qp*E ⇒ a = qp*E / mp

The equation for displacement is just the following:

x = v₀*t + 1/2*a*t² = v₀*t +1/2*(qp*E/mp)*t²

We know that qe = qp = 1.6*10⁻¹⁹ coul, but mp = 1,836 me, so, for the same displacement, the time must be much less for the electron, that has an acceleration 1,836 times higher.

When both objects fall freely the same distance under the sole influence of gravity, if the initial velocity is the same, the time must be the same also, as the fall time doesn't depend on the mass of the object.