Lecture 15

Question 1

What is the equation for the differential form of Gauss’ law?

Answer 1

E = electric field
ρ = charge density
ε = permittivity of free space

Question 2

What is the equation for the differential form of Gauss’ law for magnetism?

Answer 2

B = magnetic field

Question 3

What is the Maxwell-Faraday equation for the differential form of Faraday’s law of induction?

Answer 3

E = electric field
B = magnetic field
t = time

Question 4

What is the equation for the differential form of Ampere’s circuital law?

Answer 4

B = magnetic field
µ = permeability
ε = permittivity of free space
E = electric field
t = time

Question 5

What is the equation for current density?

Answer 5

J = current density
σ = conductivity
E = electric field

Question 6

What is the wave equation in a conductive medium?

Answer 6

Question 7

What is the equation for specific absorption rate (SAR)?

Answer 7

SAR = specific absorption rate
σ = conductivity
E = electric field
ρ = charge density

Question 8

What is the equation for radiation penetration depth (sometimes called skin depth)?

Answer 8

δ = penetration depth
σ = conductivity
µ = permeability
f = frequency

Question 9

What are the biological uses of radio frequencies?

Answer 9

• Radio-frequency heating
• Bacteria (in vitro) experiments
• Psychology and cognitive experiments

Question 10

What are the safe and maximum allowed degrees of heating of tissue?

Answer 10

Safe: 0.1ºC
Maximum: 1ºC

Question 11

What is the equation for the specific absorption rate (SAR) using experimental measurements?

Answer 11

SAR = specific absorption rate
c = specific heat capacity
T = temperature rise
∆t = time for increase

Question 12

Give 2 ways of measuring specific absorption rate (SAR)

Answer 12

• Numerically using finite element calculations
• Experimentally in anthropomorphic phantoms

Question 13

What is the SAR limit in the EU?

Answer 13

2 W/kg averaged over 10g of tissue

Question 14

What is the SAR limit in the US?

Answer 14

1.6 W/kg averaged over 1g of tissue

Question 15

How are SAR limits determined?

Answer 15

Using the metabolic rate of tissue (1.6 W/kg) and completing experiments using phantoms that don’t allow for a temperature rise exceeding 0.1ºC.

Question 16

Why aren’t mobile basestations dangerous?

Answer 16

Even though they are high power transmitters, they are at least 10m off the ground and direct radiation radially so it isn’t directed at the ground.