Calculate the magnitude of the electric field strength between the plates

**Question:**

**(a)**Two flat parallel metal plates, each of length $\displaystyle 12.0 \ cm$, are separated by a distance of $\displaystyle 1.5 \ cm$, as shown in Fig:

http://img236.imageshack.us/img236/4109/08052008vu6.png

The space between the plates in a vacuum.

The potential difference between the plates is $\displaystyle 210V$. The electric field may be assumed to be uniform in the region between the plates and zero outside this region.

Calculate the magnitude of the electric field strength between the plates.

**(b)** An electron initially travels parallel to the plates along a line mid-way between the plates, as a shown in Fig. The speed of the electrons is $\displaystyle 5.0 \times 10^7 ms^{-1}$.

For the electron between the plates,

**(i)** determine the magnitude and direction of its acceleration.

**(ii)** calculate the time for the electron to travel a horizontal distance equal to the length of the plates.

**(c)** Use your answer in **(b)** to determine whether the electron will hit one of the plates or emerge from between the plates.

**Attempt:**

(a) $\displaystyle E = \frac{V}{d} = \frac{210}{1.5 \times 10^{-2}} = 1.4 \times 10^4 \ Vm^{-1}$

(b) $\displaystyle Q.e = m.a$

$\displaystyle Q = 1.6 \times 10^{-19} \ C$

$\displaystyle m = 9.11 \times 10^{-31} \ Kg$

$\displaystyle e = 1.4 \times 10^4 \ Vm^{-1}$

$\displaystyle a = \frac{Q.e}{m} = \frac{1.6 \times 10^{-19} \times 1.4 \times 10^4}{9.11 \times 10^{-31}}$

$\displaystyle a = 2.5 \times 10^{15} \ ms^{-2}$

Direction is RIGHT because the charge is positive.

Are my answers correct? and I need help in solving part (c)

Thnx in advance.

Re: Calculate the magnitude of the electric field strength between the plates

Hey guys,

in b(i),

why did we consider the resultant to be $\displaystyle q.E$,

why didn't we say that $\displaystyle q.E-mg=ma$ and so:

$\displaystyle a=\frac{q.E}{m}-g$?

I understand that $\displaystyle \frac{q.E}{m}$ will be a really big number compared to $\displaystyle g$, but I believe it would matter with smaller numbers; it would make more sense to consider it that way, no? :)

Thanks a lot guys!