Answers to end-of-chapter questions
Chapter 4
[1]
4.2 no resultant force (forces balanced)
[1]
[1]
no resultant moment
4.3 See Activity 4.4 on page 43.
[1]
[1]
[1]
[1]
[1]
[1]
d upward contact force 5 sum of
downward forces
5 200 N + 100 N
5 300 N
[1]
[1]
[1]
pl
Make three small pinholes around the
edge of the lamina.
Suspend the lamina freely from a pin
through one hole.
Mark a vertical line below the pin
using a plumb-line.
Repeat this process for the other
two pinholes.
The centre of mass is where the
three lines intersect.
c moment of weight 5 force × distance
5 200 N × 0.5 m
5 100 N m
moment of force F is F × 1.0 5 100 N m
so F 5 100 N
e
4.1 moment
[1]
[1]
sa
m
[1]
4.4
contact
force
[1]
centre
of mass
A
B
weight
of beam
a centre of mass correctly marked,
as in diagram
[1]
b arrows and labels added correctly
[2]
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Chapter 4: Answers to end-of-chapter questions
Copyright Cambridge University Press 2011
1
Teacher’s notes for activities
Chapter 20
Notes
e
Activity 20.1
Magnetic movements
sa
m
Students will need:
• low-voltage a.c. supply
• 2.0 m thin wire
• C core
• steel hacksaw blade
• blocks
• clamps
• modelling clay
pl
This is a very simple vibrating device that makes use of the
mains frequency.
The electromagnet is magnetised and demagnetised 100 times per
second (for a 50 Hz supply). While magnetised, it attracts the steel
blade. When the field falls to zero, the blade springs back again.
Many variations are possible. For example, you could include
a diode in the circuit to halve the frequency, or use a signal
generator in place of the power supply.
Viewing the device as it operates with light from a stroboscope can
help to show up its movement.
A model relay can be designed based on this system. The hacksaw
blade is the moving part of the secondary circuit. The current
must pass through it. Activating the electromagnet will pull the
blade downwards, where it should touch a contact to complete
the circuit.
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Chapter 20: Teacher’s notes for activities
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Activity 20.2
The catapult field
Notes
pl
Students will need:
• stiff copper wire
• two steel rods
• two magnadur magnets and yoke
• low-voltage d.c. power supply
• two connecting wires
• two crocodile clips
• clamps
e
It can be tricky to arrange for a rod to roll along rails (as illustrated
in Figure 20.7 on page 221). The rod and rails must be straight. A
suitable alternative is the swing version described in this activity.
sa
m
Students who are studying the core syllabus can observe the
phenomenon and note the effect of changing the directions of the
field and current. Students following the extension syllabus can use
Fleming’s left-hand rule to predict directions.
This experiment can be modified to find the polarity of an
unmarked magnet, determined from the direction in which it
pushes a current-carrying wire.
2
Chapter 20: Teacher’s notes for activities
Copyright Cambridge University Press 2011
IGCSE Physics © Cambridge University Press
End-of-block questions
Block 5
What name is given to the central part of the atom?
What particles make up this central part?
What particles orbit around the central part?
Which part of the atom has most of its mass?
Atoms are small. Roughly how many, placed in a line, would
occupy one millimetre – a thousand, a million, a billion or
a trillion?
m
a
b
c
d
e
pl
e
1 The diagram represents an atom.
sa
2 The nucleus of an atom of nitrogen is represented by the
symbol:
15
N
7
a How many nucleons are there in this nucleus?
b What is the proton number of this nucleus and what is its
neutron number?
c A different isotope of nitrogen has one fewer nucleon in its
nucleus. Write the symbol for this nucleus.
d How many electrons are there in a neutral atom of this
second isotope?
Block 5: End-of-block questions
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3 The diagram shows what happens when alpha particles pass
close to the nucleus of a gold atom.
back-scattered
alpha
particles
+
undeflected
+
+
+
+
gold
nucleus
e
a Explain why the path of an alpha particle may bend as it
passes close to a gold nucleus.
b Explain why one of the alpha particles is undeflected.
c One alpha particle has been ‘back-scattered’. Explain why this
has happened.
m
pl
4 We are all exposed to background radiation.
a Name the radioactive gas which accounts for a large fraction
of our exposure.
b What name is given to the radiation which comes from space?
c State another major source of background radiation.
d Explain why the crew of an airliner may be exposed to higher
than average radiation levels in the course of their work.
sa
5 When a radioactive atom emits radiation we say that it ‘decays’.
a Which part of the atom emits radiation?
b Which type of radiation is a form of electromagnetic radiation?
c What particles make up an alpha particle?
d Which type of radiation is the same as an electron?
e Which two types of radiation have an electrical charge?
f Which type of radiation is undeflected if it passes through an
electric or magnetic field?
6 A nucleus of carbon-14 decays to form a nucleus of nitrogen-14.
Here is the partial equation for this:
14
6
C → 147 N +
a Copy and complete the equation.
b Name the particle which the carbon-14 nucleus emits.
c What charge does this particle have?
2
Block 5: End-of-block questions
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Copyright Cambridge University Press 2011
7 Which type of radiation from a radioactive substance …
a is the most penetrating?
b has the strongest ionising effect as it passes through another
material?
c can be absorbed by a few centimetres of air?
d can pass easily through aluminium foil?
sa
m
pl
e
8 A sample of a radioactive substance initially contains 1600
undecayed atoms.
a How many will remain undecayed after one half-life?
b How many atoms will decay during three half-lives?
c If 400 atoms remain undecayed after 30 minutes, what is the
half-life of the substance?
Block 5: End-of-block questions
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Copyright Cambridge University Press 2011
3
Answers to Workbook exercises
Chapter 8
Exercise 8.3 Power
a
60 J
a
. . . gravity (its weight) . . . increases . . .
kinetic . . . 2.0 J
b
3600 J
b
The load is getting higher, so its g.p.e. is
increasing.
c
Most of the energy is transferred as heat, not as
light.
c
The girl provides the energy.
d
120 W
d
The upward pulling force of the rope does work on
the load.
e
It is a bigger force; and it moves further.
e
Exercise 8.1 Forces doing work,
transferring energy
e
pl
contact force of road
air resistance
forward force
of engine
m
Exercise 8.2 Calculating work done
300 J
b
192 J
c
240 J
d
Work must be done against friction on the ramp.
weight
f
48 kJ
g
48 kW
sa
a
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Chapter 8: Answers to Workbook exercises
Copyright Cambridge University Press 2011
1