L7: Physics of Electricity (Physics Module Lecture 3) Flashcards
Which of the 3 are ‘physical’? (ie. follows the laws of physics). Why?
EXAM QUESTION
Answer: C
- Tissue absorbs very little
- Muscle absorbs more
- Bone absorbs everything
What does absorption = heating look like in a graph?
- Very little heat in the first area (tissue)
- A lot of heat getting put into the muscle
- Bone will also get heating
- 1 –> 3 MHz
- Frequency
- Where the heat is targeted
What is electrcity?
All electrical phenomena are due to electric charge.
The basic element of charge is that carried by a _____ (+) or an ______ (-).
proton; electron
Atoms are _____(neutral/positive/negative). equal numbers of ____ and _____.
neutral; protons and electrons
What are insulators?
In some materials, charges are not free to move
What are 2 examples of insulators?
- Rubber
- plastic
What are conductors?
charge carriers are free to move electrons in metals
What are examples of conductors?
ions in fluids in human body
What are semiconductors?
(in computer chips, etc) have behaviour in between conductors and insulators
Which would be a ‘good’ conductor of electricity?
- Copper
- Air
- Pure water
- Salty water
- Dry skin
- Moist skin
EXAM QUESTION
- Copper (good)
- Air (not good)
- Pure water (not good conductor)
- Salty water (able to conduct)
- Dry skin (not very well)
- Moist skin (good if its due to sweat)
How does voltage make charge move?
- Need something to “push” charge to make it move.
- Voltage (eg. battery) generates an electric field
- Need path for charge to move along:
How do we need something to “push” charge to make it move?
- Voltage (eg. battery) generates an electric field
- Between 2 electrodes
- Electric field
- Positive ions go toward negative electrode (vice versa)
How can we make a path so that charge can more along it?
Wire
What is the electric current?
Rate of movement of charge over time
What is the net flow of charge?
What is the units for impedance/resistance?
Ohms
What is resistivity?
p (“rho”) is a property of the material.
What is the individual resistance of the electrode?
Small l ==> small R
What is the individual resistance of the skin in contact with electrode?
Larger p, smaller l ==> small R
What is the individual resistance of the subcutaneous tissue between electrodes?
Smaller p, larger l ==> larger R
Where would be the electrodes be place if following this circuit?
Apply both side of arm
Where would be the electrodes be place if following this circuit? What is the benefit of this placement?
Path of least resistance E
g. apply only one side of arm
- Diffuse area
- Stronger electric field close to surface
Where would be the electrodes be place if following this circuit?
Small electrode and large electrode
What is the purpose of TENS nerve signalling?
- Stimulating the nerves
- Regulations of movements of ions through membrane (flow from top to bottom - vice versa)
- Sends TENS to override the body’s signals = reduce pain
What are the 4 steps in an action potential?
- Usually, net –ve charge inside → -ve voltage across membrane.
- At stimulus, potential rises.
- At threshold voltage, Na+ channels open → influx of Na+ → potential rises.
- At peak voltage, Na+ channels close; K+ channels open → outflow of K+ → potential drops.
What is DC current? What is an example?
Always flows in 1 direction eg - powered by battery
What is AC current? What is an example?
Swaps direction of flow periodically eg - household electricity
What is pulsed current? What is an example?
Switched on and off periodically eg – physio electrotherapy
What does currents look like?
What does Interferential Therapy (IFT) look like?
2 circuit currents
Control both independently
What is the superposition of oscillating currents (for IFT)?
Add currents oscillating at different frequencies → current oscillating at the average frequency, with amplitude modulation at “beat frequency” = difference in frequencies
