L6: Heat transfer and Ultrasound (Physics Module Lecture 2)

Question 1

Which way does the heat flow heat pack?

Answer 1

From hot to cold

Question 2

How much does heat transfer change when 1/2 thickness?

Answer 2

2 times (double)

Question 3

Why does ultrasound at 3 MHz heat tissue three times faster than 1 MHz? A: because tissue has high protein content • B: because all materials will do this • C: because tissue is mostly water and fat

Answer 3

B: because all materials will do this

Question 4

What are the 3 methods of heat transfer?

Answer 4

1. Radiation
2. convection
3. conduction

Question 5

What are 4 characteristics of radiation (heat transfer)?

Answer 5

1. aEvery object radiates heat (infra-red)
2. Exchange of electromagnetic energy between objects (infra-red)
3. Infra-red cannot be observed by eyes (special machines)
4. If at same temp as environment, net flow is 0.

Question 6

What are 2 characteristics of convection (heat transfer)?

Answer 6

1. Exchange of heat with the environment (eg. winding flowing)
2. Material goes pass you Eg. Lose heat with the wind

Question 7

What is a characteristic of conduction (heat transfer)?

Answer 7

Energy transfer between atoms

Question 8

heat transferred Q depends on ______ of object A and the _______ t

Answer 8

surface area; length of time

Question 9

What is power?

Answer 9

rate of energy transfer

Question 10

What is the equation for power?

Answer 10

Question 11

What is the BMR for a human?

Answer 11

≈ 1500 kcal/day = 6279 kJ/day

Question 12

How many watts?

Answer 12

1 day = 24 × 60 × 60 = 86400 secs

6279 × 1000/86400 = 72 W

Question 13

What is the equation for heat?

Answer 13

Question 14

What h- radiation?

Answer 14

Question 15

What is h-convection (for slow air)?

Answer 15

Question 16

What is h-convection (for fast air)

Answer 16

Question 17

Which one feels the biggest?

A: sitting near cold wall

B: sitting in cold wind

C: sitting on a cold concrete seat

Answer 17

B: sitting in cold wind

Question 18

____ waves propagate through solids, liquids and gases

Answer 18

Sound

Question 19

What are sound waves?

Answer 19

Sound waves propagate outwards (away from source)

Question 20

What is the velocity of sound waves in air?

Answer 20

v= 343ms-1

Question 21

How far does sound travel in 3 seconds?

Answer 21

1 km

Question 22

How long does it take for sound to travel 1km?

Answer 22

3 secs

Question 23

What are the 2 varying properties of sound waves?

Answer 23

1. frequency (the sound in your ear)
2. amplitude (the loudness in your ear)

Question 24

What does the sketch of sound absorption?

Answer 24

Question 25

What are characteristics of how a tin phone works?

Answer 25

1. Tin can phone
2. Longitudinal waves
3. Long string
4. Compression and refraction (high and low pressure)
5. Sound is not transverse –> they are longitudinal (pulses in a straight line)

Question 26

Amount of sound energy lost/absorbed depends on amount of local _______

Answer 26

sound energy

Question 27

sound intensity approximately decays _____ (linearly/exponentially)

Answer 27

exponentially

Question 28

Absorption _____ (increase/decrease) with f (eg. half-depth decreases) • eg. 1 MHz ultrasound absorbed less than 3 MHz .

Answer 28

increases

Question 29

Always 1/2 of the energy is absorbed for d < _______.

Answer 29

half-depth

Question 30

If curve drops fast then most energy absorbed near ____ (deep/superficial) tissue. For deep tissue use lower frequency, _____ (deeper/shallower) penetration.

Answer 30

superficial; deeper

Question 31

ADDDD largest attenuation in bone = most rapid absorption

Answer 31