Lecture 16 Flashcards
Why don’t people get shocked by the 100 V/m ionosphere charge in air?
Air is not very conductive so the charge is static.
What current through the heart can kill you?
A few µA
What is a positive use of electric shocks for healthcare?
Defibrilators
How can an electric field be induced in a conductive body (or closed circuit)?
A changing magnetic field. An EMF can also do this in a closed circuit.
What are ways of changing the magnetic field that a person is in?
- Movements of the body in a uniform field
- Oscillating or switching magnetic fields
Humans operate in a ______________ regime, meaning that there are only induction terms in Maxwell’s equations.
Quasi-static
Describe the shape of a nerve action potential graph
Describe the process of an action potential travelling along a nerve (in the brain or body)?
- 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.
What mechanism is responsible for action potentials?
Sodium-potassium pump
What is the electric field required for nerve stimulation?
6 V/m
What is the shape of the nerve response curve?
What are the 4 mechanisms that could cause vertigo for humans in a magnetic field?
- Magneto hydrodynamic effects (MHD)
- Susceptibility differences
- Induced galvanic vestibular stimulation (iGVS)
- Lorentz forces on hair cell currents
What is the equation for the force on an object in a homogeneous magnetic field?
F = force
χ = magnetic susceptibility
τ = object volume
B = magnetic field
µ = permeability
Why do patients often experience a metallic taste when moving in a magnetic field?
Protons are liberated on the tongue due to currents moving across it.
What is the vestibular system?
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.
How do differences in magnetic susceptibility cause vertigo for humans in a magnetic field?
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.
How do magnetohydrodynamic (MHD) effects cause vertigo for humans in a magnetic field?
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.
How does induced galvanic vestibular stimulation (iGVS) cause vertigo for humans in a magnetic field?
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.
How does the Lorentz force acting on hair cell currents cause vertigo for humans in a magnetic field?
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.
