Medical Physics Answers
1. (a) A dot 3 squares up from centre line; 1
B any two from
4 squares vertically
negative direction line; 1
(b) any two from
6V
time base off
alternating/a.c.;; 2
allow 12V battery/d.c. for 1 mark
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2. (a) minimum amount of energy to maintain life (per day)(OWTTE); 1
Allow:
• is the amount of energy expended while at rest
• is the number of calories you’d burn if you stayed
in bed all day
• number of calories needed to do nothing all day and
remain alive
• minimum amount of energy needed by the body to maintain
life
Reject heart rate
(b) take more exercise;
increase muscle to fat ratio;
eat a balanced diet;
if 4 ticks, deduct 1 mark from the total
if 5 ticks, then 0 marks 3
(c) (i) heart and other organs shown correctly; 1
(ii) the heart; 1
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3. (a) (i) breathing, respiration, kidney action; 1
(ii) 200 × 2;
400 J; 2
(iii) energy transferred is equal to work done;
400 × 10 / 4200 = 1; 2
(iv) Any 2 of:
1. to reduce the effect of interference due to movement
2. to reduce the effect of electrical noise
3. back-up strategy
4. to help eliminate the reading due to non-pulsatile
(veinous or capillary) blood; 2
(b) (i) dangerous to the volunteer / unethical / OWTTE; 1
(ii) unreliable;
has been extrapolated / is a guess 2
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4. all correct = 3 marks
2 correct = 2 marks
1 correct = 1 mark
action Number?
two gamma rays are produced 4
the radioactive isotope emits positrons 2 given
gamma rays are detected 5
a computer puts the images together 6
positrons annihilate electrons 3 given
a 3-D image is produced 7 given
the patient is injected with a radioactive isotope 1
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5. (a) One from
• action potential (caused by heart muscle)
• p.d. / voltage (of the heart) with respect to time
• electrical activity (of the heart); 1
Accept:
• p.d./ voltage across the skin caused by heart activity
Ignore: heart beat/pulse
• heart rate/ pulse rate
ignore heart action
• heart activity
(b)
P = atria contract;
QRS = ventricles contract;
T = recovery wave; 2
all three correct = 2 marks
any one correct = 1 mark
(c) in order
allow descriptions in terms of
• the frequency or period of the heart beats
• specific section of the wave being too long/short
heart beats too slow/eq;
heart beats too fast/eq;
heart beat is irregular/eq; 3
for 3rd mark accept abnormal or (fast but) missing a beat
Ignore random
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6. (a) KE = 1.6 x 10-19 x 25 x 103;;
[Evidence of 103 needed – deduct 1 mark if not used 103] 2
(b) A calculation to include:
1. K.E. = ½ x m x v2;
2. 4 x 10-15 = 0.5 x 9.1 x 10-31 x v2;
3. v = 9.4 x 107 ; 3
(c) Any four valid points, for example,
1. the electron will fall;
2. gravitational field is acting on it;
3. moving so fast;
4. (X/Y) plates used to compensate / position electron;
5. time to reach screen is very small / distance to screen short;
6. distance fallen will be negligible; 4
plus one communication mark for presenting relevant information in a 1
form that suits its purpose
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7. (a) waves closer in glass;
emerging waves parallel to incident waves;
are the same wavelength as the incident beam;
3
(b) Any two of:
some reflected back into the water;
because angle greater than critical angle / 41° – 45° ;
these rays are totally internally reflected ; 2
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8. (a) inserted down throat into/for internal examination of stomach; 1
(b) total internal reflection; 1
(c) angle 75 < x < 80; 1
(d) flexible/can bend easier etc. 1
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9. (a) (i) change of speed (of light); 1
Allow change of direction /bending when qualified, e.g.
bends towards/away the normal, bends when it changes medium, bends when it enters a different medium
Ignore light bends / change of direction without qualification
(ii) correct angle at first reflection within set tolerance;
reasonable attempt to show continuous TIR along the fibre; 2
Allow:
• tolerance range is vertically between the first pt of impact,
and the end of the leader line
• straight by eye
(b) (i) mechanism is (T)IR;
Ignore endoscope
any one from
• idea of passage of light along a fibre
Allow tube for fibre
• idea of reflection inside the body; 2
Allow look for ans that describe mechanism not purpose or description
(ii) any valid suggestion; 1
Allow:
eg
more operations (possible)
idea of cosmetically better (scarring, hole size)
reducing pain
less anaesthetic/stay awake
less bleeding
reduced infection risk
fewer complications
faster recovery time
Ignore:
any implication of money
unqualified ‘safety’
unqualified ‘quicker’
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10. calc. of distance (in m or in cm); 3.3(m)
330(cm)
Accept no alternatives
check the diagram for the calc of distance method
Reject incorrect units for one mark
e.g. N/m
substitution;
3.3 × 805
Acceptable Substitutions:
Distance
(m) Ans Distance
(cm) Ans
3.3
(or ecf) 2656 330
(or ecf) 265650
0.9 725 90 72450
1.4 1127 140 112700
1.8 1449 180 144900
1.9 1530 190 152950
2.4 1932 240 193200
2.8 2254 280 225400
Unit: J or
Nm Ncm
(Allow ecf into answers for 3.3m or 330cm)
answer in correct units; 2 660 J
2656.5(J)
2.656 kJ
ecf from substitution, as shown in above table. Answer plus correct unit.
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11. (a) (i) A - heater filament
B - X deflecting plates
C - fluorescent screen
D - Y deflecting plates
E - accelerating anode 4
All five correct - 4 marks
four/three correct - 3 marks
two correct - 2 marks
one correct - 1 mark
(ii) An explanation to include:
1. electrons would not reach the screen;
2. undergo many collisions with air particles; 2
(b) A suggestion to include any two valid reasons, for example:
1. vacuum not good enough;
2. not a high enough voltage across the plates;
3. plates too far apart;
4. plates not long enough;
5. charged particles did not leave the region of the plates; 2
(c) Any two examples of where cathode rays are used, for example:
1. TV;
2. computers;
3. electron microscope; 2
[Allow CRO]
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