Download homework answers - SPHS Devil Physics

IB PHYSICS
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DEVIL PHYSICS
BADDEST CLASS ON CAMPUS
LSN 5-6 TO 5-8 TEST REVIEW
4.
This question is about forces on charged particles.
(a)
A charged particle is situated in a field of force. Deduce the nature of the force-field (magnetic,
electric or gravitational) when the force on the particle
(i)
is along the direction of the field regardless of its charge and velocity;
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(ii)
is independent of the velocity of the particle but depends on its charge;
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(iii)
depends on the velocity of the particle and its charge.
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(5)
(b)
An electron is accelerated from rest in a vacuum through a potential difference of 2.1 kV. Deduce
that the final speed of the electron is 2.7 × 107 m s–1.
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(3)
The electron in (b) then enters a region of uniform electric field between two conducting horizontal
metal plates as shown below.
+95 V
Path of
electron
P
2.2 cm
7
2.7 × 10 m s
–1
0V
12 cm
The electric field outside the region of the plates may be assumed to be zero. The potential difference
between the plates is 95 V and their separation is 2.2 cm.
As the electron enters the region of the electric field, it is travelling parallel to the plates.
(c)
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(i)
On the diagram above, draw an arrow at P to show the direction of the force due to the
electric field acting on the electron.
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(1)
(ii)
Calculate the force on the electron due to the electric field.
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(3)
(d)
The plates in the diagram above are of length 12 cm. Determine
(i)
the time of flight between the plates.
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(1)
(ii)
the vertical distance moved by the electron during its passage between the plates.
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(3)
(e)
Suggest why gravitational effects were not considered when calculating the deflection of the
electron.
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(2)
(f)
In a mass spectrometer, electric and magnetic fields are used to select charged particles of one
particular speed. A uniform magnetic field is applied in the region between the plates, such that the
electron passes between the plates without being deviated.
For this magnetic field,
(i)
state and explain its direction;
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(3)
(ii)
determine its magnitude.
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(2)
(g)
The electric and magnetic fields in (f) remain unchanged. Giving a brief explanation in each case,
compare qualitatively the deflection of the electron in (f) with that of
(i)
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an electron travelling at a greater initial speed;
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(ii)
a proton having the same speed;
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(iii)
an alpha particle (α-particle) having the same speed.
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(7)
31.
Magnetic and electric fields
A
B
D
C
A proton is accelerated from rest in a vacuum through a
potential difference of 420 V. The proton then enters a
region ABCD of uniform magnetic field as shown.
The magnetic field is directed into the plane of the paper.
The field strength is 15 mT.
(a)
(i)
path of proton
Calculate the speed of the proton as it enters the
region of the magnetic field.
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(2)
(ii)
The path of the proton as drawn on the diagram is in the plane of the paper. The proton enters
the region ABCD of the magnetic field and leaves through the side BC. On the diagram
above, draw the path of the proton within and beyond the region ABCD of the magnetic
field. Label the path P.
(iii)
Determine the magnitude of the force due to the magnetic field that acts on the proton while
the proton is in the region ABCD.
(2)
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(2)
(b)
(i)
Define electric field strength at a point.
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(2)
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(ii)
Determine the magnitude of the electric field strength that would produce a force on a proton
that is equal to the force calculated in (a)(iii).
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(2)
(iii)
The electric field calculated in (b)(ii) is applied in the region ABCD. The electric field is
arranged such that, when a proton enters the region, the force due to the electric field is
opposite in direction to the force due to the magnetic field. Suggest, with a reason, the path
that the proton will follow in the region ABCD.
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(2)
73.
A circular coil of wire of radius r is placed in a uniform magnetic field of flux density B. The angle
between the plane of the coil and the magnetic field is θ.
B
The magnetic flux linking the coil is
A.
B.
C.
D.
r
πr2B.
πr2B sin θ.
πr2B cos θ.
πr2B.
(1)
75.
Two long, vertical wires X and Y carry currents in the same direction and
pass through a horizontal sheet of card.
X
Y
Iron filings are scattered on the card. Which one of the following diagrams
best shows the pattern formed by the iron filings? (The dots show where the
wires X and Y enter the card.)
A.
B.
C.
D.
(1)
83.
An electron is moving in air at right angles to a uniform magnetic field. The
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region of magnetic field
diagram below shows the path of the electron. The electron is slowing down.
Which one of the following correctly gives the direction of motion of the electron and the direction of the
magnetic field?
Direction of motion
clockwise
clockwise
anti-clockwise
anti-clockwise
A.
B.
C.
D.
Direction of magnetic field
into plane of paper
out of plane of paper
into plane of paper
out of plane of paper
(1)
86.
A metal ring is placed in a region of uniform magnetic field such that the plane of the ring is
perpendicular to the direction of the field. The field strength is increasing at a constant rate.
The sketch-graph shows the variation with time t of the magnetic flux  linking the ring.
ring
0
magnetic field
t
0
Which of the following graphs best shows the variation with time t of the induced current I in the ring?
A.
I
B.
0
0
C.
t
D.
I
0
0
I
t
0
0
t
0
0
t
I
(1)
87.
Electromagnetic induction
(a) State Faraday’s law of electromagnetic induction.
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(1)
(b)
A long straight wire carries a constant
current. A rectangular loop of conducting
wire is placed near the wire such that the
wire is on the plane of the loop. The loop is
then moved at constant speed away from
the wire as shown in the diagram below.
wire
current
loop
(i)
Explain why an emf is induced in the
loop.
direction of motion of loop
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(2)
(ii)
On the diagram above, draw an arrow to indicate the direction of the current induced in the
loop. Explain your answer.
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(2)
(iii)
Energy is dissipated in the wire of the loop. Explain how the movement of the loop gives rise
to energy dissipation.
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(3)
89.
A positively charged particle enters a region of uniform magnetic field. The direction of the particle’s
velocity is parallel to the direction of the magnetic field as shown in the diagram below.
region of uniform magnetic field
charged particle
Which of the following diagrams correctly shows the path of the charged particle while in the region of
magnetic field?
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A.
B.
C.
D.
(1)
90.
A transformer has a primary coil with Np turns and a secondary coil with Ns turns. An alternating voltage
supply of frequency f and r.m.s. value Vp is connected to the primary coil.
Which of the following correctly gives the frequency and r.m.s. voltage in the secondary coil?
A.
B.
C.
D.
Frequency
Ns
f
Np
Np
f
Np
Ns
f
Voltage
Np
Vp
Ns
Ns
f
Vp
Ns
Vp
Np
Ns
Vp
Np
(1)
92.
This question is about induced e.m.f.
A small area A is in a region of uniform magnetic field of strength B. The
field makes an angle  to the normal to the area as shown below.
B
Area A
normal
(a)
With reference to the diagram, define magnetic flux  both in words
and in symbols.
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(2)
(b)
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A thin copper ring encloses an area of 1.8×10–3 m2. The plane of the ring is normal to a uniform
magnetic field. The magnetic field strength increases at a constant rate of
5.0×10–2 T s–1.
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Calculate the e.m.f. induced in the ring.
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(2)
94.
Drops of a liquid are being sprayed
vertically upwards into the air by a
hose in a region where the Earth’s
magnetic field is directed
horizontally as shown in the diagram
below.
As each drop leaves the hose it
becomes negatively charged. Which
of the following describes the
direction of the magnetic force acting
on the drops?
A.
B.
C.
D.
Downwards
Upwards
Out of the paper
Into the paper
motion of drop
direction of magnetic field
hose
(1)
95.
When a coil is rotated in a uniform magnetic field at a certain
frequency, the variation with time t of the induced emf E is as
shown below.
E
The frequency of rotation of the coil is reduced to one half of
its initial value. Which one of the following graphs correctly
shows the new variation with time t of the induced emf E?
0
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0
t
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A.
0
C.
B.
E
0
0
0
t
D.
E
0
t
E
0
t
0
t
E
0
(1)
97.
A bar magnet is suspended above a coil of wire by means of a spring, as
shown below.
Spring
The ends of the coil are connected to a sensitive high resistance
voltmeter. The bar magnet is pulled down so that its north pole is level
with the top of the coil. The magnet is released and the variation with
time t of the velocity v of the magnet is shown below.
Magnet
v
Coil
0
(a)
t
0
On the diagram above,
(i)
mark with the letter M, one point in the motion where the reading of the voltmeter is a
maximum;
(ii)
mark with the letter Z, one point where the reading on the voltmeter is zero.
(2)
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(b)
Explain, in terms of changes in flux linkage, why the reading on the voltmeter is alternating.
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99.
A uniform magnetic field of strength B completely links a coil of area S. The field makes an angle  to the
plane of the coil.
B
The magnetic flux linking the coil is
A.
B.
C.
D.
BS.
BS cos .
BS sin .
BS tan .
(2)
area S
(1)
100. A resistor is connected in series with an alternating current supply of negligible internal resistance. The
peak value of the supply voltage is Vo and the peak value of the current in the resistor is I0. The average
power dissipation in the resistor is
V0 I 0
A.
2
V0 I 0
B.
2
C.
V0 I 0 .
D.
2 V0 I 0 .
(1)
102. The rms voltages across the primary and secondary coils in an ideal transformer are Vp and Vs
respectively. The currents in the primary and secondary coils are Ip and Is respectively.
Which one of the following statements is always true?
A.
B.
C.
D.
Vs = Vp
Is = Ip
VsIs = VpIp
Vs I s
 .
Vp I p
(1)
105. Faraday’s law of electromagnetic induction states that the induced emf is
A.
proportional to the change in magnetic flux linkage.
B.
proportional to the rate of change of magnetic flux linkage.
C.
equal to the change in magnetic flux linkage.
D.
equal to the change of magnetic flux.
(1)
107. The variation with time t of the magnetic flux Φ through a coil is shown below.
0
0
t
Which of the following diagrams best shows the variation with time t of the emf E induced in the coil?
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A.
0
C.
B.
E
0
0
0
t
D.
E
0
t
E
0
t
0
t
E
0
(1)
113. A resistor of resistance R is connected in series with a sinusoidal alternating supply having a maximum
value of emf V0.
The best estimate for the average power dissipated in the resistor during one cycle of the alternating
current is
2
A.
B.
C.
2V0
.
R
2
V
2 0 .
R
2
V0
.
2R
2
D.
V0
.
2R
(1)
115. This question is about an ideal transformer.
(a) State Faraday’s law of electromagnetic induction.
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(2)
(b)
The diagram below shows an ideal transformer.
laminated core
primary coil
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secondary coil
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(i)
Use Faraday’s law to explain why, for normal operation of the transformer, the current in the
primary coil must vary continuously.
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(2)
(ii)
Outline why the core is laminated.
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(2)
(iii)
The primary coil of an ideal transformer is connected to an alternating supply rated at 230V.
The transformer is designed to provide power for a lamp rated as 12V, 42W and has 450
turns of wire on its secondary coil. Determine the number of turns of wire on the primary coil
and the current from the supply for the lamp to operate at normal brightness.
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(3)
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