The two 5.0-cm-long parallel electrodes in the figure ( Figure 1 ) are spaced 1.

Question

The two 5.0-cm-long parallel electrodes in the figure(Figure 1) are spaced 1.0 cm apart. A proton enters the plates from one end, an equal distance from both electrodes. A potential difference ?V=500V across the electrodes deflects the proton so that it strikes the outer end of the lower electrode.

http://session.masteringphysics.com/problemAsset/1385291/3/38.P32.jpg

What magnetic field strength will allow the proton to pass through undeflected while the 500 V potential difference is applied? Assume that both the electric and magnetic fields are confined to the space between the electrodes.

What magnetic field direction will allow the proton to pass through undeflected while the 500 V potential difference is applied?

Explanation / Answer

accelaration a = Eq/m

E = dV/d = 500/0.01 = 50000 N/C

y = 0.5*at^2

x = vt===========> t = x/v

y = 0.5*E*q*x^2/v^2*m

v^2 = (E*q*x^2)/(2*y*m)

v^2 = (50000*1.6*10^-19*0.01^2)/(2*0.005*9.1*10^-31) = 87.91e+12

v= ?87.91e+12 = 9.38e+6 m/s

Fb = Fe

qvB = qE

B = E/v

B = 5.33*10^-3 T

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