Question 23
(a)
Define electric potential at
a point. [2]
(b)
Two positively charged metal spheres A and B are situated in a
vacuum, as shown in Fig. 5.1.
Fig. 5.1
A point P lies on the
line joining the centres of the two spheres and is a distance x
from the surface of sphere A.
The variation with x
of the electric potential V due to the two charged
spheres is shown in Fig. 5.2.
Fig.
5.2
(i)
State how the magnitude of the electric field strength at any point
P may be determined from the graph of Fig. 5.2. [1]
(ii)
Without any calculation, describe the force acting on a positively
charged particle placed at point P for values of x
from x = 0 to x
= 10 cm. [3]
(c)
The positively charged particle in (b)(ii) has
charge q and mass m
given by the expression
q / m = 4.8 × 107 C kg-1.
Initially, the
particle is at rest on the surface of sphere A where x
= 0. It then moves freely along the line joining the centres of the
spheres until it reaches the surface of sphere B.
(i)
On Fig. 5.2, mark with the letter M the point where the charged
particle has its maximum speed. [1]
(ii)
1. Use Fig. 5.2 to determine the potential difference between the
spheres. [1]
2.
Use your answer in (ii) part 1 to
calculate the speed of the particle as it reaches the surface of sphere B.
Explain your working. [3]
Reference: Past Exam Paper – June 2015 Paper 42 Q5
Solution:
(a)
Electric potential at a point is defined as the work done
per unit positive charge in bringing the positive charge from infinity to the
point.
(b)
(i)
The electric field strength is given by the gradient of
the V-x graph.
(ii)
maximum at
surface of sphere A or at x =
0 (cm)
zero at x =
6 (cm)
then increases
but in opposite direction
{Electric
field strength E = (-) potential gradient
Force = Eq =
gradient × q
The force is
maximum at the surface of A (x = 0 cm) (as the gradient has the largest value).
The force
becomes zero at x = 6 cm (as gradient = 0).
The force
then increases but in the opposite direction (as gradient increases but now has
a different sign).}
(c)
(i)
M shown between x = 5.5 cm and x = 6.5 cm
{Gain in KE
= Loss in electric potential energy
Gain in KE =
ΔV × q
The gain in
KE (and thus, speed) is maximum when the change in potential ΔV has the largest value.
Largest ΔV = Vmax (at surface of A) – Vmin (at x = 6.0 cm)}
(ii)
1. ΔV = (570 – 230) = 340 V (allow
330 V to 340 V)
2.
{Considering
the energy changes,}
q(Δ)V = ½mv2
or change/loss in
PE = change/gain in KE or ΔEK
= ΔEP
{q/m ×
ΔV = ½ v2
q/m = 4.8 ×
107 C kg-1 }
4.8 × 107
× 340 = ½ v2
v2
= 3.26 × 1010
v = 1.8 × 105
m s–1
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