UNIT 10 ELC FLD WS 1
[28 marks]
1. A negatively charged particle falls vertically into a region where there is
an electric field. The equipotentials of this field are shown.
[1 mark]
What is the path followed by the particle?
This question is in two parts. Part 1 is about electric fields and radioactive decay.
Part 2 is about waves.
Part 1 Electric fields and radioactive decay
An ionization chamber is a device which can be used to detect charged particles.
The charged particles enter the chamber through a thin window. They then ionize
the air between the parallel metal plates. A high potential difference across the
plates creates an electric field that causes the ions to move towards the plates.
Charge now flows around the circuit and a current is detected by the sensitive
ammeter.
2a. On the diagram, draw the shape of the electric field between the plates. [2 marks]
The separation of the plates d is 12 mm and the potential difference V between
the plates is 5.2 kV. An ionized air molecule M with charge +2e is produced when
a charged particle collides with an air molecule.
2b. Calculate the electric field strength between the plates.
2c. Determine the change in the electric potential energy of M as it moves
from the positive to the negative plate.
[1 mark]
[3 marks]
226
Radium-226 ( 88 Ra) decays into an isotope of radon (Rn) by the emission of an
alpha particle and a gamma-ray photon. The alpha particle may be detected using
the ionization chamber but the gamma-ray photon is unlikely to be detected.
2d. Construct the nuclear equation for the decay of radium-226.
[2 marks]
2e. Radium-226 has a half-life of 1600 years. Determine the time, in years, it[3 marks]
takes for the activity of radium-226 to fall to 5% of its original activity.
3. A negative charge moves in an electric field. Equipotential lines for the
field and four possible paths of the charge are shown. Which path
corresponds to the largest work done on the charge by the field?
[1 mark]
4. In an experiment, oil droplets of mass m and charge q are dropped into [1 mark]
the region between two horizontal parallel plates. The electric field E
between the plates can be adjusted. Air resistance is negligible. Which is correct
when the droplets fall vertically at constant velocity?
A. E=0
B.
E<
C. E
=
D. E
>
mg
q
mg
q
mg
q
5. What is the unit of Gε0, where G is the gravitational constant and ε0 is the [1 mark]
permittivity of free space?
A. C kg–1
B. C2 kg–2
C. C kg
D. C2 kg2
6. Two parallel metal plates are connected to a dc power supply. An electric [1 mark]
field forms in the space between the plates as shown.
What is the shape of the equipotentials surfaces that result from this
arrangement?
7. An electric field acts in the space between two charged parallel plates.
One plate is at zero potential and the other is at potential +V.
[1 mark]
The distance x is measured from point P in the direction perpendicular to the
plate.
What is the dependence of the electric field strength E on x and what is the
dependence of the electric potential V on x?
8. Two point charges are at rest as shown.
At which position is the electric field strength greatest?
[1 mark]
9. A positive charge Q is deposited on the surface of a small sphere. The
dotted lines represent equipotentials.
[1 mark]
A small positive point charge is moved from point P closer to the sphere along
three different paths X, Y and Z. The work done along each path is WX, WY and
WZ. What is a correct comparison of WX, WY and WZ?
A. WZ > WY > WX
B. WX > WY = WZ
C. WX = WY = WZ
D. WZ = WY > WX
10. A charge of −3 C is moved from A to B and then back to A. The electric
[1 mark]
potential at A is +10 V and the electric potential at B is −20 V. What is
the work done in moving the charge from A to B and the total work done?
11. An isolated hollow metal sphere of radius R carries a positive charge.
Which graph shows the variation of potential V with distance x from the
centre of the sphere?
[1 mark]
12. Four identical, positive, point charges of magnitude Q are placed at the [1 mark]
vertices of a square of side 2d. What is the electric potential produced at
the centre of the square by the four charges?
A. 0
B.
4kQ
d
C.
√2kQ
d
D.
2√2kQ
d
13. The diagram shows 5 gravitational equipotential lines. The gravitational
potential on each line is indicated. A point mass m is placed on the
middle line and is then released. Values given in MJ kg–1.
[1 mark]
Which is correct about the direction of motion and the acceleration of the point
mass?
14. An electron of mass m e orbits an alpha particle of mass m α in a circular
orbit of radius r. Which expression gives the speed of the electron?
A.
√ 2mkeer
B.
√ 2mkear
2
2
4 2
C. √ mker
e
4 2
D. √ mker
a
[1 mark]
15. The diagram shows the electric field and the electric equipotential
surfaces between two charged parallel plates. The potential difference
between the plates is 200 V.
[1 mark]
What is the work done, in nJ, by the electric field in moving a negative charge of
magnitude 1 nC from the position shown to X and to Y?
16. A positive point charge is placed above a metal plate at zero electric
[1 mark]
potential. Which diagram shows the pattern of electric field lines between
the charge and the plate?
17. Two point charges Q 1 and Q 2 are one metre apart. The graph shows the
variation of electric potential V with distance x from Q 1.
What is
A.
B.
[1 mark]
Q1
?
Q2
1
16
1
4
C. 4
D. 16
18. A negative charge Q is to be moved within an electric field E, to
equidistant points from its position, as shown.
[1 mark]
Which path requires the most work done?
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