Question 3
(a)
Define the coulomb.
[1]
(b)
Two vertical metal plates in a vacuum have a separation of 4.0 cm. A
potential difference of 2.0 × 102 V
is applied between the plates. Fig. 5.1 shows a side view of this arrangement.
Fig. 5.1
A smoke particle is in
the uniform electric field between the plates. The particle has weight 3.9 × 10-15 N and charge –8.0 × 10-19 C.
(i)
Show that the electric force acting on the particle is 4.0 × 10-15 N. [2]
(ii)
On Fig. 5.1, draw labelled arrows to show the directions of the two
forces acting on the smoke particle. [1]
(iii)
The resultant force acting on the particle is F.
Determine
1.
the magnitude of F,
2.
the angle of F to the horizontal. [3]
(c)
The electric field in (b) is switched on at time
t = 0 when the particle is at a horizontal
displacement s
= 2.0 cm from the left-hand plate. At time t
= 0 the horizontal velocity of the particle is zero. The particle is
then moved by the electric field until it hits a plate at time t
= T.
On Fig. 5.2, sketch
the variation with time t of the horizontal
displacement s of the particle from
the left-hand plate.
Fig. 5.2
[2]
[Total: 9]
Reference: Past Exam Paper – November 2017 Paper 22 Q5
Solution:
(a)
1 coulomb is the amount of charges that passes when a
current of 1 ampere flows for 1 second.
(b)
(i)
E = V / d or E = F / Q
F = VQ / d
F = (2.0 × 102 × 8.0 × 10-19)
/ 4.0 × 10-2 = 4.0 × 10-15 N
(ii)
arrow pointing to the left labelled ‘electric
force’ and
arrow pointing downwards labelled ‘weight’
(iii)
1.
F2
= (weight)2 + (electric force)2}
resultant force = √ [(3.9 × 10-15)2 + (4.0 × 10-15)2]
= 5.6 × 10-15 N
2.
angle = tan-1 (3.9 × 10-15 / 4.0 × 10-15) = 44°
(c)
downward sloping line from (0, 2.0)
magnitude of gradient of line increases with
time and line ends at (T, 0)
{The horizontal displacement is affected by the
electric force. The particle accelerates towards the left. So, the velocity
increases. From the s-t graph, the velocity is obtained by the gradient. Thus,
the gradient increases.}
shouldn't the graph start from s=2 on the y-axis
ReplyDeleteyou are right. thanks.
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