# CIE AS & A Level Physics : 24.1 Production and use of ultrasound – Exam style question – Paper 4

### Question

Outline the use of ultrasound to obtain diagnostic information about internal body structures.
……………………………………………………………………………………………………………………………………….
……………………………………………………………………………………………………………………………………….

Ans:

(ultrasound) pulse
reflected at boundaries
gel is used to minimise reflection at skin
or
generated and detected by quartz crystal

time delay between generation and detection gives information about depth

### Question

(a) Two long straight wires P and Q are parallel to each other, as shown in Fig. 8.1. There is a current in each wire in the direction shown.
The pattern of the magnetic field lines in a plane normal to wire P due to the current in the wire is also shown.

(i) Draw arrows on the magnetic field lines in Fig. 8.1 around wire P to show the direction of    the field.                                                                                                                                                      [1]
(ii) Determine the direction of the force on wire Q due to the magnetic field from wire P.           [1]
(iii) The current in wire Q is less than the current in wire P.
State and explain whether the magnitude of the force on wire P is less than, equal to, or greater than the magnitude of the force on wire Q.                                                                        [2]

(b) Nuclear magnetic resonance imaging (NMRI) is used to obtain diagnostic information about internal structures in the human body.
Radio waves are produced and directed towards the body. The radio waves affect the protons  within the body.
(i) Explain why radio waves are used.                                                                                                       [2]
(ii) Explain why the radio waves are applied in pulses.                                                                         [2]
[Total: 8]

Ans

(a) (i) at least one anticlockwise arrow and no clockwise arrows
(a) (ii) (force is to the) left
(a) (iii) force is the same
Newton’s third law (of motion)
or force depends on the product of the two currents

(b) (i) frequency of radio waves is equal to natural frequency of protons
resonance of protons occurs / protons absorb energy
(b) (ii) in between pulses / when pulse stops
Any 1 from:
• protons de-excite
• protons emit r.f. pulses

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