Answer to Question 8

 

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An electron is accelerated by a voltage of 1.5 V.  It travels along a horizontal path that is 15 cm long. 

(a)  Show that the speed of the electron is about 730 km s-1

(b)  Calculate how far the electron deviates from the horizontal path.

 

Mass of an electron = 9.11 10-31 kg; Charge on an electron = 1.6 10-19 C

(a) Formula:

v2 = (2 1.6 10-19 C 1.5 V) (9.11 10-31 kg) = 5.27 1011 m2 s-2

 

v = 7.26 105 m s-1  = 730 km s-1

 

 

(b) Time taken to travel 15 cm:

 

t = s/v = 0.15 m 7.26 105 m s-1  = 2.07 10-7 s

 

Now use s = ut + 1/2at2 to work out the deviation:

 

s = 0 + 1/2 9.81 m s-2 (2.07 10-7 s)2 = 2.1 10-13 m

 

That is about 1000th the diameter of an atom, so the deviation is not that big.