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Worksheet # 4

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A-level Physics 9702

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A-II

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Electrostatics

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Assume o= 8
...
Two point charges X= +5nC and Y = -3nC are placed 40
...
Calculate
(a) the force between the charges
(b) the combined electric field strength and potential at
(i)
their mid point
(ii)
the point that is 100m from X and 60mm from Y
...
(a) State what is meant by the potential at a point in an electric field
...
When in use, charge builds up on a hollow spherical
conducting dome until the dome achieves the required potential difference
between it and the earth
...


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The figure above shows how the potential V varies with distance d from the
center of the charged spherical dome of a Van de Graff generator, for values of
d>0
...
The spherical dome of radius 0
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(i)
use the figure to determine the potential at a distance of 0
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(ii)
Determine the potential at the surface of the dome of the Van de Graff
generator
...

(iii)
Draw lines to represent the electric field near the surface of the dome
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30m from the center
of the dome
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30m from the center
of the dome
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6x10-19C
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The figure below shows a pair of parallel plates separated by a distance of 5
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0
mm

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A potential difference of 100V is applied between the plates
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Sketch graphs to
show the show the variation of electric field strength
(intensity) E at different points along Ox and O`y repectively
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1
...

2
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State the direction of this force
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The point A is unit
distance (1m) from the charge
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Start the graph at a point marked B,
corresponding to the point A at x=1m, and continue it in the
direction of increasing x
...

Also on your axis sketch a graph to show how the electric
potential V varies with x
...

label this graph V
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0C and –2
...
0mm
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Find the position of
D relative to the +1
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Calculate the magnitude of the electric field strength
(intensity) at D
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Worksheet # 4

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Electrostatics

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(i)
(ii)

Copy the diagram and add solid lines to show the electric
field in the space between and just beyond the edges of the
plates
...


(b) Define electric potential at a point
...
The
graph below shows the variation of electric potential V with
distance r from the center of the sphere
...
Mark with an ‘a’ on the distance axis the
point that represents the radius of the sphere
...

5
...
On each graph the vertical axis has a linear
scale
...

(c) At a point where the distance r, from the point charge is 60mm, the
field strength is 2
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Calculate the potential at this point
...
Calculate electrostatic force in each of the following cases
(a) two electrons having a charge of –1
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0x10-10m apart
ina vacuum
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0x10-2mm which have been given a
charge of 1
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0 mm apart in air
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Two small conducting spheres are suspended from the same point by fine,
insulating threads of length 80 cm
...
0cm as shown in the diagram
...
A nucleus of uranium-235 has a radius of 9
...
Assuming that the nucleus can be treated as a point charge,
find
(a) the electric field strength
(b) the potential at the nuclear surface (Proton charge= +1
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Worksheet # 4

Electrostatics

By: Umair Qureshi

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9
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0x10 -9 C through a distance
of 25mm in the opposite direction to a uniform electric field of strength 4
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(b) What is the p
...
0C
gain 4
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(a) Calculate (i) the electric intensity and (ii) the potential at a point midway
between two point charges of +10-9 C and -10-9 C which are 20 cm apart
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0cm apart in air, what PD would be needed between the plates?

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What is the potential difference between P
and Q?
12
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0x10-8 C and surrounded by air, if the sphere’s radius is
2
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How much work would be done in moving a positive charge of 1
...
0cm further from the center?
13
...
What work is done to carry a unit positive
charge completely round it in a circle of radius 10cm
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Two point charges 8C and -12C are separated by a distance 20cm in air
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A pair of equally charged particles A and B are placed a distance 4
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The mass of A is 7
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8x10-7kg
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0m/s 2
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Two charges each of value Q are placed at a distance r from each other
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What is the magnitude sign and position of the charge q?
17
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Given R=6400km
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Worksheet # 4

Electrostatics

By: Umair Qureshi

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Worksheet # 4

Electrostatics

By: Umair Qureshi

11

Question from CIE past Papers

21
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4
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The sphere has diameter 42cm and any charge on its surface may be
considered as if it were concentrated at its center
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(a) By reference to an atom in the air, suggest the mechanism
by which the electric field causes the air to become
conducting
...

(c) Under certain conditions, a spark sometimes occurs before
the potential reaches that calculated in (b)(ii)
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(June 2003)

22
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This charge may be assumed to act as a point charge
situated at the centre of the sphere, as shown in Fig
...
1
...
5
...
shows the variation with distance x from the centre of
the sphere of the potential V due to the charge +Q
...

(b) Using the relation in (a), on Fig
...
3 sketch a graph
to show the variation with distance x of the electric
field E due to the charge +Q
...
An isolated conducting sphere of radius r is placed in air
...
This charge may be assumed to act as a
point charge situated at the centre of the sphere
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(ii) State a formula for the electric field strength E at the
surface of the sphere
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(b) The maximum field strength at the surface of
the sphere before electrical breakdown (sparking) occurs is
2
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The sphere has a radius r of 0
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(i) the charge that can be stored on the sphere,
(ii) the potential at the surface of the sphere
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Worksheet # 4

(june 2006)

Electrostatics

By: Umair Qureshi

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24
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0
cm, as shown in Fig
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1
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3
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An electron is emitted with
negligible speed from A and
travels along AB
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(b) The area below the line
of the graph of Fig
...
2
represents the potential
difference between A and B
...
3
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(c) Use your answer to (b) to
calculate the speed of the
electron as it reaches point
B
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3
...

(ii) Without any further calculation, describe the variation
with distance d of the acceleration of the electron
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A small charged metal sphere is situated in an earthed metal
box
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4
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the sphere is 0
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(i) Use the expression

Worksheet # 4

(a) By reference to Fig
...
1, state and
explain
(i) whether the sphere is positively
or negatively charged,
(ii) why it appears as if the charge
on the sphere is concentrated at the
centre of the sphere
...
4
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(c) The radius r of the sphere is 2
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The magnitude of the charge q on

Electrostatics

By: Umair Qureshi

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to calculate a value for the magnitude of the potential V at the
surface of the sphere
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Hence explain whether the actual magnitude of the
potential will be greater or smaller than the value calculated in
(i)
...
4
...
Explain why it is
not possible for the gravitational field to have a similar shape
to that of the electric field
...
(a) Define electric potential at a point
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0 cm, as shown in Fig
...
1
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6 × 10–9 C
...

4
...

(i) State the value
of x at which the
potential is zero
...

(c) A small test
charge is now moved
along the line AB in
(b) from x = 5
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State and
explain the value of x
at which the force on
the test charge will
be maximum
...
(a) Define
electric potential at
a point
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8 MeV
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5
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The α-particle and the gold nucleus may be considered to be point
charges in an isolated system
...
(ii) For the closest approach of the
a-particle to the gold nucleus determine
1
...
the magnitude of the force on the α -particle
...
(a) Explain what is meant by the potential energy of a body
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gravitational potential energy,
2
...

(ii) Use your answers in (i) to show that the initial kinetic energy E K of each nucleus is
0
...

(iii) The two nuclei may rebound from each other
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(c) Calculate the work done by an electron in moving from the midpoint of line AB to
point P
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(a) 84
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00KV/m,0
2
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99x10-4N
3
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20KN 2
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12N (c)(i) 3
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5x109N/C
5
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(a) 2
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2x10-3N
7
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48x10-7N(b) 2
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(a) 4
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0x10-7V
9
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0x10-7J(b) 2400V
10
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8KV/m (ii) zero (b) 36
11
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0V
12
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9KV, 8
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zero
14
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(a) 15ms-2(b) 8
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Q/4, negative, midway
17
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55x104C
18
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15x10-12
19
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7x1010N/C
20
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89m
21
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8 x 10-6 C(ii) 4
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(b) (i) 2
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0x105V (c) electrons are stripped off the atoms electrons
and positive ions move in opposite directions, (giving rise to a current)
24
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2 cm2 ± 0
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(c) (i) 285V
26
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4x10-9C (c) 27cm
27
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7 × 10-14m, 16N

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Worksheet # 4

Electrostatics

By: Umair Qureshi



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