Lecture 16

Question 1

Why don’t people get shocked by the 100 V/m ionosphere charge in air?

Answer 1

Air is not very conductive so the charge is static.

Question 2

What current through the heart can kill you?

Answer 2

A few µA

Question 3

What is a positive use of electric shocks for healthcare?

Answer 3

Defibrilators

Question 4

How can an electric field be induced in a conductive body (or closed circuit)?

Answer 4

A changing magnetic field. An EMF can also do this in a closed circuit.

Question 5

What are ways of changing the magnetic field that a person is in?

Answer 5

• Movements of the body in a uniform field
• Oscillating or switching magnetic fields

Question 6

Humans operate in a ______________ regime, meaning that there are only induction terms in Maxwell’s equations.

Answer 6

Quasi-static

Question 7

Describe the shape of a nerve action potential graph

Answer 7

Question 8

Describe the process of an action potential travelling along a nerve (in the brain or body)?

Answer 8

• A section of the nerve feels a signal so becomes depolarised.
• This results in an action potential in this part of the fibre.
• The fibre repolarises and overshoots while returning to its resting potential.
• The adjacent part of the fibre then undergoes the same process, causing the action potential to propagate down the fibre.

Question 9

What mechanism is responsible for action potentials?

Answer 9

Sodium-potassium pump

Question 10

What is the electric field required for nerve stimulation?

Answer 10

6 V/m

Question 11

What is the shape of the nerve response curve?

Answer 11

Question 12

What are the 4 mechanisms that could cause vertigo for humans in a magnetic field?

Answer 12

• Magneto hydrodynamic effects (MHD)
• Susceptibility differences
• Induced galvanic vestibular stimulation (iGVS)
• Lorentz forces on hair cell currents

Question 13

What is the equation for the force on an object in a homogeneous magnetic field?

Answer 13

F = force
χ = magnetic susceptibility
τ = object volume
B = magnetic field
µ = permeability

Question 14

Why do patients often experience a metallic taste when moving in a magnetic field?

Answer 14

Protons are liberated on the tongue due to currents moving across it.

Question 15

What is the vestibular system?

Answer 15

A sensory system that creates a sense of balance and spatial orientation for the purpose of coordinating movement with balance. It is found in the ear for humans.

Question 16

How do differences in magnetic susceptibility cause vertigo for humans in a magnetic field?

Answer 16

A force is generated on the human in an inhomogeneous magnetic field due to the difference in magnetic susceptibility between the vestibular system and the surroundings. This force causes an effective ‘acceleration’ that acts on the hair cells in the system.

Question 17

How do magnetohydrodynamic (MHD) effects cause vertigo for humans in a magnetic field?

Answer 17

It uses the same logic as Lenz’s law where the fluid inside the vestibular system will act against any changes caused by the magnetic field, however, the system is too small for this to actually be the case in reality.

Question 18

How does induced galvanic vestibular stimulation (iGVS) cause vertigo for humans in a magnetic field?

Answer 18

A changing magnetic field with time induces a current density in the vestibular system which can be perceived by vestibular nerves if it persists for long enough.

Question 19

How does the Lorentz force acting on hair cell currents cause vertigo for humans in a magnetic field?

Answer 19

The Lorentz force only depends on the magnitude and direction of the magnetic field. When present, it results in a current being generated within the vestibular system that puts pressure on the cupula.