Endofunit4answers

End of Unit 4
Answers to examination-style questions
Answers

Marks Examiner’s tips

1 (a) (i)
Volume of cylinder = πr2h

= π × 0.302 × 1.50 = 0.424 m3

Mass of cylinder = volume × density

= 0.424 × 7800 = 3310 kg or 3300 kg
1
    v2 =
m

(ii) K gained = EP lost gives _  ____ mg∆h
E
2

∴ velocity v of hammer = 2gh  
_____________
√
  
√2 × 9.81 × 0.80 = 3.96 m s−1
=
or 4.0 m s−1

(b) (i)
Momentum of hammer before impact
= 3310 × 3.96 ( = 1.31 × 104 Ns)

Momentum of hammer and girder after
impact = (3310 + 1600)V

Momentum is conserved

∴ 4910 V = 1.31 × 104

∴ velocity after impact V = 2.67 m s−1

or 2.7 m s−1

(ii)
Loss of EK = work done against
friction

= friction force × distance moved
0.5 × 4910 × 2.672
_______________

Friction force =  
    
  
25 × 10−3

= 7.00 × 105 N

2 (a) (i)
Change of momentum of football
= 0.44 × 32 = 14.1 N s (or kg m s−1)
or 14 N s
Δ(mv)
14.1
______
(ii)
Using F =   Δt   
 
  
gives F = _________ 
 
9.2 × 10−3

∴ average force of impact = 1.53 kN

or 1.5 kN

1
1
1
1

This calculation can be done equally well
using v2 = u2 + 2as, with u = 0,
a = 9.81 m s−2 and s = 0.80 m.

1

Since the hammer does not rebound (or
stay in mid-air), the hammer and girder
move together as one and the same object
after impact. Let their common velocity
be V. Conservation of momentum leads
to this result.

1
1
1
1

1

The ball was at rest before being kicked,
so the initial momentum was zero.

1

The question leads you nicely into this
form of solution.
Alternatively you could use v = u + at
and F = ma

1

1

(ii)
Centripetal acceleration of boot at time
2
v2
__ ____
  0.62 
 

of impact =    =  24  = 929 m s−2
r

or 930 m s−2

1

(iii) efore impact: radial pull on knee joint
B
is caused by centripetal acceleration of
boot.

After impact: radial pull is reduced
because speed of boot is reduced.

1

AQA Physics A A2 Level © Nelson Thornes Ltd 2009

Alternatively the deceleration can be
found from v2 = u2 + 2as, and then the
force from F = ma.
Δ(mv)
______
Or F =   Δt    be used, with ∆t
 
may
1
determined by applying s = _  (u + v) t.
   
2

1

(b) (i)
Use of v = u + at gives

15 = 24 + (9.2 × 10−3)a

and a = −978 m s−2

∴ deceleration of boot = 978 m s−2

or 980 m s-2

If you can’t recall it, the equation for the
volume of a cylinder is given in the Data
1
_

Booklet. Remember that r =     × diameter.
2 

1

During the impact the boot is subject to
two accelerations at right angles to each
other. Part (i) considers the linear
(negative) acceleration along the line of
the impact force. Parts (ii) and (iii) look
at the radial acceleration towards the
centre of the arc in which the boot
travels.
The centripetal force on the boot acts
inwards, towards the knee joint. The knee
joint pulls inwards on the boot but is
itself pulled outwards by the boot. When
the boot slows down this centripetal force
is reduced.

1

End of Unit 4
Answers
3 (a) (i)
Angular speed ω 2πf = 2π × 6.5
=
= 40.8 rad s−1

Force exerted by wheel on mass

F = mω2r = 0.015 × 40.82 × 0.25

= 6.25 N or 6.2 N or 6.3 N

(ii)
Arrow drawn at a tangent to the
circular path, starting on the mass and
directed vertically upwards.

(b) Graph drawn to show:

• xactly one cycle of a negative
E
sinusoidal shape.

• period of 0.15 s clearly marked on
A
time axis.

(c)
Relevant points include:

• orced vibrations occur when a body is
F
subjected to a periodic driving force.

• he frequency of vibrations from the
T
wheel increases as it speeds up.

• he frequency of the forced vibrations is
T
equal to the frequency of the wheel
vibrations.

• esonance occurs at the natural
R
frequency of the mirror.

• t resonance the frequency of the
A
forcing or driving oscillator is the same
as the natural frequency of the mirror.

• he amplitude of vibration reaches a
T
maximum when the wheel rotates at
6.5 rev s−1 (or when the frequency of the
forcing vibration is 6.5 Hz).

• t higher frequencies (or speeds) the
A
amplitude decreases again.

4 (a) (i)
Angular speed ω = 2πf = 2π × 15
= 94.2 rad s−1

Force on an end cap

F = mω2r = 1.5 × 94.22 × 0.55

= 7320 N or 7330 N (about 7 kN)

(ii)
Towards the centre of the rotor


1
1
1

The force exerted by the wheel on the
mass is the centripetal force on the mass,
which acts towards the centre of the
wheel. The mass exerts an equal and
opposite force on the wheel.

1

If the mass is no longer fixed to the
wheel, it will move in the direction it was
travelling at the instant it became
detached, but in a straight line.

2

At t = 0 the vertical component of F is
zero as F acts towards the centre of the
wheel.. It becomes downwards or
negative at 90°, and returns to zero at
180°. 0 at 270°, and returns to its original
value at 360°. The period of rotation
___
is   1   s.
6.5  

any 5 The vibrating wheel provides a periodic
driving force that sets the mirror in
forced vibration. These vibrations are at
the frequency of the driving oscillator
(the wheel) and they therefore increase in
frequency as speed increases. Eventually
the resonant frequency is reached, when
there is maximum energy transfer from
the driving oscillator to the driven
oscillator, the mirror. At resonance the
amplitude of vibration is greatest, so the
mirror shakes violently. At higher
frequencies the mirror still vibrates, but
with much smaller amplitudes.

1
1
1

The force on each end cap is the
centripetal force required to keep it
moving in a circular path. This acts
inwards, towards the axis about which
the rotor is rotating.

1

2

End of Unit 4
Answers


(iii) ongitudinal stress in blade
L
force
7320
_______

   
  

=  c.s. area  = _________ 
3.5 × 10−4
7

= 2.09 × 10 Pa (about 20 MPa)

tensile stress
 
(iv) oung modulus E = ___________ 
Y
 
tensile strain
2.09 × 107 
_________
 

∴
tensile strain =  
6.0 × 1010

= 3.483 × 10−4
ΔL

tensile strain = ___ 
  L 

∴ change in length of blade ∆L

= 3.483 × 10−4 × 0.55 = 1.92 × 10−4 m

(0.192 mm or 0.19 mm)

(v) Strain energy stored in one blade
1
1
_
F

   

=     ∆L = _  × 7320 × 1.92 × 10−4
2 
2

= 0.703 J or 0.70 J

1

1
1

1

1
1

(b) (i) 1 second volume of air passing
In
over blades = area × length per second
= Av

Mass of air passing over blades = Avρ

Momentum gained by air
= mass × v = Aρv2

1

(ii)
Upward force on helicopter = weight

= 900 N

Area swept out by blades A = πr2

= π × 0.552 = 0.950 m2
= 900 gives

Aρv2 _________ speed of air
900
_________
−1
−1
=  
v
 
=
0.95 ×      27.0 m s or 27 m s
1.3  

1

√


This question now brings in a topic
covered in AS Physics A Unit 2: the
elastic properties of materials. The
centripetal force on the end cap puts the
whole rotor blade in tension, creating a
tensile (longitudinal) stress throughout
the blade. This stress will cause the
length of the blade to increase slightly.

1

1
1

Like a stretched spring, a stretched rotor
blade stores some potential energy.
It is helpful to think about the cylinder of
air that is dragged down over the rotor
blades. In 1 second this air travels a
distance v, so the length of the cylinder
is v. The air was at rest (with momentum
= 0) before being pulled down by the
blades.
The upwards force on the helicopter is
equal to the downwards force on the
column of air. When the helicopter has
no vertical motion, this force is equal to
its weight. Force = rate of change of
momentum = momentum gained in
1 second.

3

End of Unit 4
Answers


5 Relevant points include:
(a)
Why strong vibrations are caused

• he pump creates a periodic driving
T
force which acts on the pipe.

• he pipe has a natural frequency of
T
vibration.

• he frequency of the periodic driving
T
force increases as the pump speeds up.

• t a certain speed, the frequency of the
A
driving force equals the natural
frequency of the pipe.

• nder this condition resonance occurs.
U

any 5 Part (a) illustrates another practical
example of resonance, where the periodic
driving force is caused by the rotation of
the pump in a central heating system.
This will set all of the connected
pipework into forced vibration at the
frequency of the driving force. Every part
of the pipework will have a natural
frequency of vibration. At particular
frequencies, resonance could occur in
different parts of the pipework. This will
cause large amplitude vibrations if the
pipes are free to move.

(b)
Reduction of vibrations

• it extra clamps along pipe.
F

• o eliminate (or change) the resonant
T
frequency.

or

• hange pipe dimensions (or material)
C

• o alter the resonant frequency
T

v2
__
 
6 (a)
Centripetal acceleration a =  r 

1

(7.68 × 10 )
___________
 
 
=  
6750 × 103
= 8.74 m s−2
3 2

Damping, perhaps by fitting padding
around a pipe, would also be effective in
reducing the amplitude of vibrations
produced by this effect. Equivalent credit
would be available for an answer which
referred to damping and explained its
effect.

(b)

• he scientist is in free fall.
T

• he weight of the scientist provides the
T
centripetal force.

• o give the scientist the same orbit
T
radius and acceleration as the ISS.

• he scientist experiences no motion (or
T
force) relative to the ISS.
__
4π2M
m
  2 
    
 
(c) (i) = 2π __  gives k = _____ 
T
k
T

∴
stiffness of spring system
4π2 × 2.0
________
 
 
 

k =  
1.22

= 54.8 N m−1 (about 55 N m−1)

√


1
1

Notice that this question requires the
centripetal acceleration, not the
centripetal force. The radius r of the orbit
is found by adding the height of the orbit
(380 km) to the radius of the Earth
(6370 km).

any 2 ‘Apparent weightlessness’ in a spacecraft
is a similar situation to that of somebody
in a lift whose supporting cables have
snapped. There is no need for contact
with the surrounding capsule, because it
has the same acceleration towards the
Earth as the person inside it.
1
1

The quantity represented by k is usually
called the ‘spring constant’, but that is for
a single spring. The quantity referred to
here as ‘stiffness’ is the effective spring
constant for the two springs.

4

End of Unit 4
Answers


__
m

(ii) the atom’s vibrations, T = 2π __  
For
    
k
_________
−26
4.7 × 10
  54.8  
 
 

gives period T = 2π _________ 

= 1.84 × 10−13 s

√

√

Frequency of vibration
1
f = __ = __________ 
 1  
T 1.84 ×   −13  
10
= 5.43 × 1012 Hz or 5.4 × 1012 Hz

7 (a) (i)
The period remains unchanged
__
m
    

(ii)
Since T = 2π __  and k is quadrupled,
k
new period T is half the original
the
value __
m
    

(iii) = 2π __  
T
k

and m = total mass on spring, M
4π2M
kT 2

∴ T 2 = _____ giving M = ___ 
     
,

  2  
k
4π
1

(iv) eriod T = __ = ____ (= 1.099) s
P
      1   
f 0.91
kT 2
___
 

using M =   2 
4π
1.9 × 105 × 1.0992

gives M = _______________ 
 
  
 
4π2

∴
total mass on spring M = 5812 kg
and mass of platform = 5820 − 5300

= 512 kg or 510 kg

√

√

(b)

• he system is set into forced vibration
T
by the lorry’s engine.

• he platform vibrates at the same
T
frequency as the lorry.

• esonance occurs at a particular
R
frequency . . .

• hich is around 0.91Hz.
w

• ibrations are of small amplitude at
V
frequencies that are well away from this.

• he amplitude of vibration is a
T
maximum at resonance.


1

1

The whole of part (c) is a good example
of scientific modelling. A system that is
much too small to be observed directly
can be represented by a laboratory model
having similar characteristics. Study of
the model will provide information about
the behaviour of the system represented
by it.

1

For simple harmonic vibrations of small
amplitude, the period is independent of
the amplitude.

1

In greater detail, __
___
____  √k   1
Tnew _____  ____ __

____ √ kold
 
    ___      
 
   
 T  =  
 =
=
knew  4k   2
  √
old
√

1

Part (iii) is a simple exercise in squaring
and rearranging the usual equation for a
mass–spring system.

1
1
1

1

The total load on the spring is the weight
of the lorry together with the weight of
the platform on which it stands. The
equation from part (iii) enables you to
find the total mass supported by the
spring. The mass of the lorry, given in
the question as 5300 kg, must be
subtracted from this in order to find the
mass of the platform.

any 4 The engine vibrates as it runs, and acts as
a source of the driving vibrations. The
platform is driven by the vibrations of the
lorry. The question states that the
frequency of these vibrations is increased
from 0.5 Hz to about 4 Hz. The natural
frequency of the system is 0.91 Hz, i.e.
that of the mass–spring system in part
(a). Resonance occurs as the vibrations
pass through this frequency.

5

End of Unit 4
Answers
8 (a)
T/s: 0.35, 0.44, 0.51, 0.57, 0.62 (all shown
to 2 decimal places)
2/s2: 0.123, 0.194, 0.260, 0.325, 0.384 (all
T
shown to 3 decimal places)

1
1

It is the normal practice in tables of
experimental results to maintain a
consistent number of decimal places
down a column. In this table an example
is set by the first entry, given as 0.25 and
0.063.

1
(b) (i) the mass suspended from the
For
spring,

Mg = k ∆L = k (l − l0)
1

Squaring, and substituting in, equation
2
4π2(l − l0)
4π m
    = ________
 
 

for period gives T 2 = _____    g   
k

The extension ∆l of the spring is found
by subtracting the original length from
the new (measured) length. Hooke’s law
indicates that the tension in the spring is
k ∆L. When the system is in equilibrium,
this tension is equal to the weight of the
suspended masses.

(ii)
Graph drawn to have:

• uitable scales
S

• oth axes labelled: T 2/s2 and l/mm
B

• t least 5 points plotted correctly
A

• n acceptable straight line
A

4

The scales you choose should allow your
line to occupy more than half of the area
of the graph paper. Use a 300 mm ruler
when drawing the straight line, which
should be the ‘best-fit’ line for the
experimental points.

1

When T 2 = 0, l = l0. Hence l0 is the
intercept on the l axis.
Expanding the equation of the line gives
2
4π2l 4π l0
  g   −   g   
T 2 = ____  ____ 

Comparison with the straight line
equation y = mx + c shows that the
4π2
___

gradient (m) is   g   

4π2 0.400
___ _____
 
0.100 
(iii) radient =   g   =  
G

= 4.00 (± 0.05) s2 m−1
4π2
____
 
∴ g =    = 9.87 (± 0.15) m s−2
4.00 
Intercept on l axis = l0 = 300 (± 5) mm

g
___
1   
(c) T = 1.00s, (l − l0) =      = _______ 
If
   
4π2 gradient

= 0.250 m

∴ l = 0.250 + l0 = 250 + 300 = 550 mm

Spring may not obey Hooke’s law when
large masses are added to it.

Beyond limit of proportionality, equal
masses produce larger extensions.


1
1

1
1

1
1

The largest time period in the
experimental results is 0.62 s. The time
period increases when a larger mass is
__
 
placed on the spring. Since T ∝ m  a
√ ,
much larger mass would be needed to
give T = 1.00 s
The extension of the spring would be
almost as great as its original length.
With such a large extension, the spring
would be unlikely to obey Hooke’s law.

6

End of Unit 4
Answers


9 (a) (i)
Both obey an inverse square law of
force or both field strengths are
inversely proportional to distance
squared or potential is inversely
proportional to distance.

1

(ii)
Gravitational fields always produce
attractive forces whereas electric fields
can produce attractive or repulsive
forces.
gravitational potential is always
(or
negative but electric potential can be
positive or negative.)

1

(b)
Gravitational potential energy of satellite in
GMm
_____

 

orbit = −   r   

Change in potential energy
 1    1  

= − GMm __  − __   
r1 r2
 1    1  

= −6.67 × 10−11 × 5.98 × 1024 × 165 × __  − __   
r1 r2
1   
1   
 
 
 

= −6.58 × 1016 × _________  − _________ 
8.08 × 106 4.24 × 107

= −6.59 × 109 J

i.e. a decrease of 6.59 × 109 J

1

( 

)

( 

( 

)
)

1

1

The similarities between electric and
gravitational effects are a consequence of
the inverse square law. Many of the
formulae in these topics are similar, the
constant of proportionality being G in
____
one case and   1e  in the other.
4π  0

Gravitational potential energy = mass of
satellite × gravitational potential of orbit.
The change in a quantity is always
calculated as (final value) − (initial
value). Here the initial radius of orbit is
4.24 × 107 m. The change in potential
energy works out to be a negative value,
indicating that it is a decrease. You
should expect the potential energy of the
satellite to decrease as it moves closer to
the Earth! For the final mark, don’t forget
to point out that it is a decrease.

(c)
any 3 At the outset you have to realise the

• igher orbit is geostationary – useful for
H
significance of the original orbit having a
maintaining continuous communication
period of 24 hours. Since it is a
since it can act as a fixed reflector above
geostationary satellite, it would have to
the Earth.
move in an orbit over the Equator. When

• rbit is geosynchronous because the
O
its height was reduced, it would still be
period of the satellite is the same as the
moving above the Equator, and so it
period of the Earth’s rotation on its axis.
would not scan the whole Earth. A

• ower orbit is useful for monitoring
L
satellite in a low polar orbit scans the
– satellite scans the Earth 12 times a day,
whole surface, because the Earth rotates
enabling many parts of the surface to be
beneath its orbit, about an axis in the
observed.
plane of the orbit.

• he low height of the shorter orbit
T
allows closer inspection of the Earth’s
surface.

10 (a)
Factors affecting g are:

• he radius (or diameter) of the planet
T

• he mass (or density) of the planet
T


2

GM
____
 
,
Since g =   2  R and M are the two
R
factors that count. Careless use of words
could cause marks to be lost: ‘size’ of the
planet would not be an acceptable
answer.

7

End of Unit 4
Answers


4πr3
____
(b) (i)
Volume of granite =   3 
 
 
4π × 2003
________
 
 

=   3 = 3.35 × 107 m3

Mass difference between granite
and other material
= 3.35 × 107 × (3700 − 2200)

= 5.03 × 1010 kg

(ii) point A, gA = 9.81 N kg−1
At

At point B, gB = 9.81 N kg−1 +

(field strength due to mass difference
caused by granite sphere)

∴
gB − gA = (field strength due to mass
difference caused by granite sphere)
GM
,

= ____ where R = 400 m
  2 
R
6.67 × 10−11 × 5.03 × 1010
_____________________

=  
     
 
4002
−5
2.10 × 10 N kg−1 or 2.1 × 10−5 N kg−1
=

1
1
1
1

1

1
1
1

The sample of granite rock is spherical.
You can find its radius by reading from
the second diagram, where its diameter is
clearly 0.40 km, or 400 m.
Mass = volume × density, thus M1 = Vρ1
and M2 = Vρ2, leading to mass difference
= V (ρ1 − ρ2). Since you are asked to
show that the answer is 5.0 × 1010 kg, all
working must be written down –
including your answer to 3 significant
figures.
This is a demanding question, but part
(b)(i) sets you thinking in a helpful
direction. The difference in g is caused
by the sphere of granite. But the increase
it produces has to be corrected for the
equivalent volume of matter that it
replaces in the Earth. Hence it is the
difference in mass between the granite
and the surrounding material that
determines the increase in field strength.

(iii) ine drawn on the graph which is the
L
same shape as the original, but with
lower values at all points.

1

Since the granite is now at a greater
distance from the surface, its presence
has a smaller effect.

11 (a) (i)
Wavelength of microwaves
c 3.00 × 108
λ = __ × _________ 
     
 = 0.250 m
 

f 1200 × 106
λ 0.250
 

Angular width of beam = __ =   1.8 
  _____ 
d

= 0.139 rad or 0.14 rad
180
    
 

= 0.139 × ____ = 7.96° or 8.0°
π

1

You should not panic when you read part
(a)(i): the angular width of the beam is
λ
defined as __ at the start of the question!
 
d
You must start by calculating λ, then find
the angular width. It would be easy to
overlook the fact that your final answer is
required to be expressed in degrees.

(  )

1
1

(ii)
Beam width at 15 000 km

= 1.50 × 107 × 0.139 = 2.09 × 106 m

(which is 2100 km to 2 significant
figures)

1

(b) (i)
Gravitational force on satellite
GMm
 
 

F = _______ 
 
(R + h)2
GMm
mv2
   
 
the circular orbit, _______ = ______ 
For
 
2   (R + h)  
(R + h)

Cancelling m and taking square root
_______
GM
 
  
 

gives speed of satellite v = ______  
(R + h)

1

√


1
1

( 

)

arc length
 
 
  
Angle θ in rad = _________ 
radius
∴ arc length = radius × θ
You can also draw a triangle and work
with tangent or sine.
This is the standard theory for the orbit of
a satellite, but expressed in terms of the
linear speed v rather than the (more
usual) angular speed ω. If it was not
already evident, the final result should
make it clear that the higher the orbit, the
lower the speed of the satellite.

8

End of Unit 4
Answers


(ii)
Speed of satellite
_____________________
−11
24
GM
______
5.98

v =  
    
  _____________________
  
  
= 6.67 × 10 6 ×   × 10   
7
(R + h)
6.37 × 10 + 1.5 × 10 )

= 4320 m s−1 or 4300 m s−1

1

1

√

1

(iii) peed of beam over surface
S
circumference of Earth
 
  

= ___________________ 
 
T 
6
2π × 6.37 × 10
_____________
    
 

=  
3.11 × 104
−1

= 1290 m s (1.3 km s−1)

( 

Period of satellite
2πr 2π × 2.14 × 107
  
= ___ = _____________ 
  v     

4320 
= 3.11 × 104 s or 3.1 × 104 s

)

( 

)

beam width
2.09 × 106
   
 
  
  
 
Time = ____________   = _________ 
speed of beam
1290
= 1620 s = 27 min

12 (a) (i)
Force of attraction
Q1Q2 _____
e

F = _____ =   2  2 
 
2 
4πe0r 4πe0r
(1.6 × 10−19)2
= __________________________ 
 
   
  
4π × 8.85 × 10−12 × (5.3 × 1011)2

= 8.19 × 10−8 N or 8.2 × 10−8 N

mv2
 
(ii) = ____ 
F   r  gives

Fr 8.19 × 10−8 × 5.3 × 10−11
 m   ____________________
    
  
v2 = ___ =  
9.11 × 10−31
v = 2.18 × 106 m s−1 or 2.2 × 106 m s−1

( 

)

de Broglie wavelength
h/mv
 
  
(ii) ___________________   =  ____ 
2πr 
circumference of orbit
−34
6.63 × 10
   
    
= ___________________________________ 
 
9.11 × 10−31 × 2.18 × 106 × 2π × 5.3 × 10−11
= 1.00

( 

)


This calculation simply makes use of the
equation from part (b)(i). At the end of
the working, do not forget to take the
square root.
The satellite travels at speed v round an
orbit whose length is the circumference
of a circle. In this equation, r is the
previous (R + h).

1

The time taken by the beam to move
once round the surface of the Earth is the
same as the time taken for one orbit by
the satellite.

1

The beam width at the Earth’s surface for
a satellite at height h (15 000 km) was
calculated in part (a)(ii).

1

The charge of the electron is −e, whilst
the charge of the proton is + e. According
to this model of the hydrogen atom, this
force provides the centripetal force to
keep the electron in orbit around the
proton.

1
1

(b) (i)
Energy released by transition
 12    12  

∆E = − 2.2 × 10−18 × __  − __   
2 1

= 1.65 × 10−18 J
c hc
       

Wavelength emitted λ = __ = ___  
f ΔE
−34
8
6.63 × 10 × 3.00 × 10
____________________
    
  

=  
1.65 × 10−18

= 1.21 × 10−7m or 1.2 × 10-7 m

1

1
1

1
1

1

It is not necessary to go back to first
principles, because F has already been
determined in part (a)(i). Do not forget to
take the square root when working out
the final answer.
It may be surprising, but the de Broglie
wavelength of the electron turns out to be
exactly equal to the circumference of the
orbit. Because both quantities are lengths,
the ratio has no unit.
The electron returns to the ground state
(n = 1) from the first excited state (n = 2).
Substitution of these values for n into the
given energy equation allows the energy
difference to be calculated. This part then
asks you to recall work on emission
spectra covered in Unit 1 of AS Physics A.

1

9

End of Unit 4
Answers


(ii)
The wavelength calculated in (i) is in
the ultraviolet region of the
electromagnetic spectrum.
other transitions to the ground state
All
will produce wavelengths that are
shorter than this.

13 (a) (i)
Mass of α particle

= 4 × 1.66 × 10−27 = 6.64 × 10−27 kg

or 2 × (1.673 + 1.675) × 10−27 kg

= 6.70 × 10−27 kg

Kinetic energy of α particle EK

= 2.8 × 106 × 1.6 × 10−19 = 4.48 × 10−13 J
1
_
m

Use of EK =     v2 gives
2 
____

√

_____________

Q1Q2
 4πe r
(b) (i)
Loss of EK = gain of EP = _____ 

1

1
1

√

1

2E
2 × 4.48 × 10−13
  K   _____________
     
  
  
v = ____    
  =
m
6.64 × 10−27
= 1.16 × 107 m s−1 or 1.2 × 107 m s−1

(ii) Broglie wavelength of α particle
De
6.63 × 10−34
h
___

λ =       = ____________________ 
   
     

mv 6.64 × 10−27 × 1.16 × 107

= 8.61 × 10−15 m or 8.6 × 10−15 m

1

0

Q1 = 79e and Q2 = 2e
79 × 2 × (1.6 × 10−19)2
∴ __________________ 4.48 × 10−13
 
  
  
 =
4π × 8.85 × 10−12 × r
gives least distance of approach
r = 8.12 × 10−14 m or 8.1 × 10−14 m

(ii)
Diffraction is negligible because the de
Broglie wavelength of the α particle is
much smaller than 2r (which is the
approximate diameter of the nucleus).

14 (a)
Decreases from 40 V to 20 V in 4.7 s

Decreases from 20 V to 10 V in

(9.0− 4.7) = 4.3 s

∴ average time for pd to halve = 4.5 s


The shortest visible wavelength is violet
light (≈ 400 nm). All the other transitions
to the ground state will release more
energy than that in (i), and higher energy
photons have a shorter wavelength.

An α particle consists of 4 nucleons
(2p + 2n). Its mass is therefore
approximately 4u. The kinetic energy is
given in MeV in the question; you must
change this energy into J. Application
1
m
of _  v2 then leads to the result.
   
2
α particles from radioactive sources
usually have energies ranging from 4 to
9 MeV.

1

Since an α particle has a much larger
mass than an electron, it has a much
smaller de Broglie wavelength when
travelling at a similar speed to an
electron.

1

As the α particle approaches the nucleus
it gains electric potential energy and
continues to lose kinetic energy until it
stops. The potential energy gained is
(charge × electric potential). The gold
nucleus has 79 protons; the α particle has
2 protons.
This is a slightly simplified model
because the nucleus recoils and the α
particle is not stationary at the distance of
closest approach.

1
1
1

1

For significant diffraction, this
wavelength has to be comparable with
the diameter of the diffracting obstacle.
Here it is only 5% of the diameter of the
nucleus.

1
1

It is a characteristic of exponential decay
that the quantity always decreases to half
its original value in the same period of
time.
Other voltage pairs can be chosen but it
is important to measure at least two
values and show the values are roughly
equal and have an average of about 4.5 s.

10

End of Unit 4
Answers


(b)
Time constant RC

V0
__

= time for V to fall to    
 
e

time for pd to fall from 40 V to
14.7 V = 6.6 s
6.6
6.6
___ _________
   
 

capacitance of A =   R   =  
10.0 × 103
−4

= 6.6 × 10 F (660 µF)

1

1

(c)
Capacitance of B = 290 µF (found by
method in (ii) or a similar method)

Energy lost by capacitor B when pd falls
1
_
C

=     (V12−V22)
2 
1
_

=     × 2.9 × 10−4 × (402 − 102)
2 

1

1

∴
energy dissipated in resistor = 0.218 J
or 0.22 J

1
1

15 (a) (i)
Energy lost by capacitor when pd falls
1
_

=     (V12−V22)
C

2 
1
_

=     × 2.0 × 10−2 × (4.52 − 2.52)
2 

= 0.14 J

1

(ii)
Potential energy gained by mass

= mg ∆h = 0.015 × 9.81 × 0.35

= 5.15 × 10−2 J

efficiency of transfer
5.15 × 10−2

= __________  36.8% or 37%
  0.14  =
 
 

1

( 

)

This could be calculated by substituting
values from the graph into V = V0e−t/RC,
but it would take longer. Note that
capacitor B discharges more rapidly than
A, showing that B has the smaller
capacitance.

The energy dissipated in the resistor must
be equal to the energy lost by the
capacitor. Any energy calculation
involving a capacitor requires the value
of its capacitance to be known, so the
first step is to find it.

The energy delivered to the motor is
equal to the energy lost by the capacitor.

1
1

1

The ‘efficiency of transfer’ of energy is
useful work output
________________ × 100%. The useful
 
     
  
energy input
work output is the increase in
gravitational potential energy of the
mass.

( 

)

(iii) easons for inefficiency:
R
any 2 All of this wasted energy eventually

• nergy lost due to resistance of
E
passes to the surroundings as thermal
wires in the circuit and motor
energy. There will be frictional forces at

• ork done against friction
W
the bearings of the motor. The amount of

• ork done against air resistance as
W
energy converted into sound energy is
the mass rises through the air
incredibly small.

• ound energy losses due to vibration
S
of the motor
(b) (i)
Average useful power output
energy transferred
5.15 × 10−2
_______________
__________
      =   1.3 
  
 
 

=  
time taken
−2

= 3.96 × 10 W (39.6 mW or 40 mW)

( 

)


1

Work, energy and power are covered in
Unit 2 of AS Physics A.

1

11

End of Unit 4
Answers

(ii)
Use of V = V0 e−t/RC gives

2.5 = 4.5 e−1.3/RC
4.5
   

∴ e1.3/RC = ___ = 1.80,
2.5
1.3
___

   = ln 1.80 = 0.588, and RC = 2.21 s
RC 

Resistance of motor
2.21
  
R = _________ = 111 Ω or 110 Ω
 
2.0 × 10−2

1
1

1

16 (a) (i)
Magnetic field exerts a force on the
wire when current passes through it.

The current repeatedly reverses, so the
force repeatedly reverses.

1

(ii) 290 Hz the driving frequency is
At
equal to the fundamental frequency of
the wire.

The wire is set into resonance.

1

(iii) avelength of waves on wire
W
λ = 2 L = 2 × 0.71 = 1.42 m

Speed of waves on wire c = f λ

= 290 × 1.42 = 412 m s−1 or 410 m s−1
60
(b) (i)
Mass per unit length µ = __  = ____ 
 T       
c2 4122
3.54 × 10−4 kg m−1
=
or 3.5 × 10−4 kg m−1 or 3.6 × 10−4 kg m−1

1

1
1
1

1
1

(ii)
Cross-sectional area of wire A = πr2

= π × (0.12 × 10−3)2 = 4.52 × 10−8 m2

Volume of a 1 metre length of wire

= A × 1.0 = 4.52 × 10−8 m3

Density of steel in wire
3.53 × 10−4
 
 = 7810 kg m−3

=   mass   
_______ = __________ 
volume 4.52 × 10−8

or 7800 kg m−3

1

17 (a)
Path drawn on Figure 1:

Curving downwards towards (or meeting)
the negative plate.

Path drawn on Figure 2:

Curving upwards towards the top of the
page.

1


1
1

1

The time of discharge from 4.5 V to
2.5 V is stated to be 1.3 s. This
calculation involves solution of the
equation for exponential voltage decay,
using logs. It is assumed that the motor
resistance is the total circuit resistance. In
practice there will be a small resistance in
the wires and terminals of the circuit, so
the motor resistance will be slightly less
than the calculated value.
The alternating current produces an
alternating force on the wire, making it
vibrate up and down between the poles of
the magnet.
In this experiment the wire will resonate
at any of its natural frequencies of
vibration. The fundamental (lowest)
frequency produces vibrations of the
largest amplitude.
In the fundamental mode, the length of
λ
the wire between the fixed ends is __  This
 2 
.
is the condition for the establishment of a
fundamental stationary wave.
Rearrangement of the equation given in
the question gives this result. Note that
the quantity you calculate is the mass per
unit length, with unit kg m−1.
This type of calculation is often done
most easily when you consider a unit
length (1.0 m) of the wire.
Alternatively you could calculate the
density by finding the mass and volume
of the 0.71 m length of the wire in the
question.

The path is parabolic in an electric field,
but circular in a magnetic field. The
direction of the force in the magnetic
field, and hence the deflection, is given
by Fleming’s left-hand rule.

12

End of Unit 4
Answers


(b) (i)
The electric field strength is the force
that acts per unit charge on a small
positive test charge when placed in the
field.

The field strength is uniform if the
field strength has the same magnitude
and direction at all points in the field.

1

V
__
(ii) a uniform field E =    ∴V = E d
In
d

= 2.0 × 104 × 0.045

= 900 V

1

(c) (i)
Force on particle in electric field

F = E Q = 2.0 × 104 × 6.4 × 10−19

= 1.28 × 10−14 N or 1.3 × 10−14 N

1

1

1

1

There are two things to explain here:
what is meant by field strength, and what
is meant by uniform. Since electric field
strength is a vector, ‘uniform’ means
having the same magnitude and
direction at all points.
The electric field is uniform over most of
the region between parallel plates,
although it does weaken towards the
outer edges.
This force acts vertically upwards, in the
same direction as the field, because the
charge is positive.

(ii) the electric force is equal to the
If
1
This principle is embodied in the velocity
magnetic force, E Q = B Q v
selector component of a mass
spectrometer: see the earlier examples in
2.0
E  =   × 104 
__ ________
 

∴ initial velocity v =  
B
0.17 
Chapter 7.
1

= 1.18 × 105 m s−1 or 1.2 × 105 m s−1
(d)
any 6 The speed remains constant in a magnetic

In an electric field
field despite the fact that the charged

• orce always acts in the same direction.
F
particle is subjected to a force. The force

• here is an acceleration in the direction
T
always acts at right angles to the velocity
of the force.
of the particle, causing a centripetal

• his acceleration increases the speed in
T
acceleration. The magnetic force has no
the direction of the force . . .
component in the direction of the

• ut the horizontal speed at right angles to
b
particle’s movement. There is no
the field remains constant.
acceleration along the path travelled and

In a magnetic field
no change in speed. In an electric field, a

• he force is always perpendicular to the
T
charged particle moves like a projectile
direction of movement.
subject to gravity. It continues at constant

• his force has no component in the
T
speed at right angles to the force, whilst
direction of movement.
being accelerated in the direction of the

• here is no acceleration in the direction
T
force.
of movement, so speed is constant.

• otion is in a circular path.
M

18 (a)

• ravitational force acts towards the
G
centre of the orbit.

• his force is at right angles to the
T
velocity of the satellite.

• here is no displacement in the direction
T
of the gravitational force.

• o work is done; there is therefore no
N
change in EK and no change in speed.


any 3 The force acting on the satellite is always
directed at right angles to the
instantaneous velocity. This force
therefore has no component in the
direction of the velocity. In these
circumstances the acceleration produced
by the force has the effect of changing
the direction of the velocity but not its
magnitude.

13

End of Unit 4
Answers


(b) (i)
Resultant magnetic flux density
________
 

Bres = 562 + 172 = 58.5 µT or 59 µT
√

1

(ii) the angle is between the rod and Bres
If
17

is θ, then tan θ = ___ 
   
56

giving θ = 16.9° or 17°

1

(iii) n emf is induced in the rod because it
A
cuts through the magnetic flux/field of
the Earth.

1
19 (a) (i) k gained = Ep lost ∴ _  v2 = e V
E
   
m
2

Speed of electron
____
2eV 2 × 1.60 × 10−19 × 12 × 103
  m    ______________________
 
   
  
 

v = ____ =  
9.11 × 10−31
7
−1
7

= 6.49 × 10 m s or 6.5 × 10 m s−1

1
1

The emf induced would vary as the
satellite moved round the Earth, because
Bres (and its components) would be
different in different locations.

1
1

A charged particle ‘travels’ across an
electric field in much the same way that a
mass falls through a gravitational field. In
each case, the force from the field causes
kinetic energy to increase.

1

Remember that Q = It (or that
1 A = 1 C s−1), from Unit 1 of AS
Physics A. Each electron has a charge
of e ( = −1.60 × 10−19 C), so number of
electrons per second = __ 
 I .
e

√

(ii)
Charge arriving at screen in 1s

= 25 × 10−3 C

∴ in 1s, number of electrons striking
25 × 10−3
 
 

screen = __________ = 1.56 × 1017
 
1.60 × 10−19

or 1.6 × 1017

(b) (i)
The changing magnetic field causes
changes in the magnetic flux linked
with the circuits

These changes in flux linkage induce
emfs in the circuits

(ii)
Possible causes of faults:

• hen there are induced currents,
W
these could lead to heating effects
and/or additional magnetic fields.

• arge induced emfs could cause
L
circuit components to fail by
breaking down their insulation.
(iii) aximum change in magnetic flux
M
∆B = 7.0 × 10−4 T

Maximum change in flux linkage ∆(Nf)

= 250 × 7.0 × 10−4 × 4.0 × 10−3

= 7.0 × 10−4 Wb turns

Maximum induced emf =
∆Nf
____ 7.0 × 10−4
  ∆t    _________ = 14 V
   
 
=
50 × 10−6

(iv) he coil might not be perpendicular to
T
the changing magnetic field.


Part (b) starts with some of the work
involving vectors covered in Unit 2 of
AS Physics A: vector addition (from the
resolved components of a vector) and
angles in a vector triangle (or vector
parallelogram).

1

1
1

Faraday’s law: the induced emf is equal
to the rate of change of flux linkage. Note
that strictly it is emfs that are induced.
There will be induced currents if the
circuits are complete.

any 1 Changing magnetic fields will always
induce emfs. Sometimes metal shielding
is fitted around the internal components
of electronic equipment in order to
protect the components from induced
emfs.
1
1
1

1

The maximum induced emf is given by
the maximum rate of change of flux
linkage. The flux density changes from
0.35 mT in one direction to 0.35 mT in
the other direction (a change of
0.70 mT) in 50 µs.
Magnetic flux f = B × area A, and flux
linkage = Nf.
If it was not perpendicular, the induced
emf would be smaller.

14

End of Unit 4
Answers


Induced currents in other circuits in the
television might reduce the magnetic
flux through the coil.
20 (a) (i)

• current that circulates in a loop
A
within a conductor.

• here are alternating currents in the
T
coils of a transformer…

• hich produce changing magnetic
w
fields in the core.

• his effect induces emfs in the core,
T
which cause the eddy currents.

• he eddy currents cause energy to
T
be wasted because the currents
produce heating of the core.
(ii)
The core of the transformer is
laminated i.e. made from alternate
layers of metal and insulator.

Eddy currents are reduced because the
metal is no longer continuous (or the
resistance of the core is increased).

__ ___
P 36
(b) (i)
Current in each lamp I =     =    = 3.0 A
V 12 

1

any 4 The core is a magnetically soft material,
which is also a conductor. It can be
thought of as ‘chains’ of atoms. These
chains are continuously cut by the
changing magnetic fields caused by the
alternating currents in the primary and
secondary coils. Any conductor subjected
to a changing magnetic flux will have an
emf induced in it. The emfs in the core
cause the eddy currents, which dissipate
some of the energy that is supplied to the
transformer by heating its core.
1
1

1

Ammeter reading = 3 × 3.0 = 9.0 A

1

(ii)
Total output power I V = 9.0 × 12
=
= 108 W

Input power = 108 W
P 108 
__ ____

∴
primary current Ip =     =   48 
V

= 2.25 A

1

Ns Vs
(iii) urns ratio equation ___ = ___  gives
T
       
N Vp

1

240 × 12
________

Ns =  

p

These induced currents would produce
magnetic fields that would oppose the
change producing them (Lenz’s law).

1

A lamina is a plane sheet, such as a sheet
of cardboard. The core is made from a
stack of metal sheets, separated by
insulating material. This means that eddy
currents can only flow within each single
sheet, instead of in the whole core. Much
reduced eddy currents means greatly
reduced energy losses.
The secondary of the transformer
supplies three identical lamps that are
connected in parallel. The total current is
the sum of the currents in each lamp.
Since this transformer is assumed to be
perfectly efficient, the input power is
equal to the output power. This implies
that the current is stepped up by a factor
of 4 when the voltage is stepped down by
a factor of 4.
When the voltage is stepped down in the
ratio 4:1, the turns ratio must also be 4:1.

 
 
48 = 60

Nelson Thornes is responsible for the solution(s) given and they may not constitute the only possible solution(s).

15

Endofunit4answers

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