L7: Physics of Electricity (Physics Module Lecture 3)

Question 1

Which of the 3 are ‘physical’? (ie. follows the laws of physics). Why?

EXAM QUESTION

Answer 1

Answer: C

• Tissue absorbs very little
• Muscle absorbs more
• Bone absorbs everything

Question 2

What does absorption = heating look like in a graph?

Answer 2

• 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

Question 3

What is electrcity?

Answer 3

All electrical phenomena are due to electric charge.

Question 4

The basic element of charge is that carried by a _____ (+) or an ______ (-).

Answer 4

proton; electron

Question 5

Atoms are _____(neutral/positive/negative). equal numbers of ____ and _____.

Answer 5

neutral; protons and electrons

Question 6

What are insulators?

Answer 6

In some materials, charges are not free to move

Question 7

What are 2 examples of insulators?

Answer 7

1. Rubber
2. plastic

Question 8

What are conductors?

Answer 8

charge carriers are free to move electrons in metals

Question 9

What are examples of conductors?

Answer 9

ions in fluids in human body

Question 10

What are semiconductors?

Answer 10

(in computer chips, etc) have behaviour in between conductors and insulators

Question 11

Which would be a ‘good’ conductor of electricity?

1. Copper
2. Air
3. Pure water
4. Salty water
5. Dry skin
6. Moist skin

EXAM QUESTION

Answer 11

1. Copper (good)
2. Air (not good)
3. Pure water (not good conductor)
4. Salty water (able to conduct)
5. Dry skin (not very well)
6. Moist skin (good if its due to sweat)

Question 12

How does voltage make charge move?

Answer 12

1. Need something to “push” charge to make it move.
2. Voltage (eg. battery) generates an electric field
3. Need path for charge to move along:

Question 13

How do we need something to “push” charge to make it move?

Answer 13

• Voltage (eg. battery) generates an electric field
• Between 2 electrodes
• Electric field
• Positive ions go toward negative electrode (vice versa)

Question 14

How can we make a path so that charge can more along it?

Answer 14

Wire

Question 15

What is the electric current?

Answer 15

Rate of movement of charge over time

Question 16

What is the net flow of charge?

Answer 16

Question 17

How many charges flowing through area?

Answer 17

1 Amp = huge amount of flow from one direction to another

Question 18

What is convention?

Answer 18

Current has direction of flow of +ve charges.

Unit: Ampere (A) 1 A = 1 C/s

Question 19

What is impedance (less generally resistance)?

Answer 19

measures “difficulty” of moving charge along a path.

Unit: Ohm

Related to number of free particles to flow (ions/electrons)

Question 20

What is the units for impedance/resistance?

Answer 20

Ohms

Question 21

What is Ohm’s Law?

Answer 21

The current that flows through an object is proportional to the voltage between the ends of the object

Question 22

What are 4 characteristics of resistance?

Answer 22

1. Material itself (resistivity)
2. Length and CSA (related)
3. Small length = small resistance
4. Large area = small resistance

Question 23

What is resistivity?

Answer 23

p (“rho”) is a property of the material.

Question 24

What is the individual resistance of the electrode?

Answer 24

Small l ==> small R

Question 25

What is the individual resistance of the skin in contact with electrode?

Answer 25

Larger p, smaller l ==\> small R

Question 26

What is the individual resistance of the subcutaneous tissue between electrodes?

Answer 26

Smaller p, larger l ==\> larger R

Question 27

Where would be the electrodes be place if following this circuit?

Answer 27

Apply both side of arm

Question 28

Where would be the electrodes be place if following this circuit? What is the benefit of this placement?

Answer 28

Path of least resistance E g. apply only one side of arm * Diffuse area * Stronger electric field close to surface

Question 29

Where would be the electrodes be place if following this circuit?

Answer 29

Small electrode and large electrode

Question 30

What is the purpose of TENS nerve signalling?

Answer 30

* 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

Question 31

What are the 4 steps in an action potential?

Answer 31

1. Usually, net –ve charge inside → -ve voltage across membrane. 2. At stimulus, potential rises. 3. At threshold voltage, Na+ channels open → influx of Na+ → potential rises. 4. At peak voltage, Na+ channels close; K+ channels open → outflow of K+ → potential drops.

Question 32

What are the 3 types of currents?

Answer 32

1. DC 2. AC 3. Pulsed

Question 33

What is DC current? What is an example?

Answer 33

Always flows in 1 direction eg - powered by battery

Question 34

What is AC current? What is an example?

Answer 34

Swaps direction of flow periodically eg - household electricity

Question 35

What is pulsed current? What is an example?

Answer 35

Switched on and off periodically eg – physio electrotherapy

Question 36

What does currents look like?

Answer 36

Question 37

What does Interferential Therapy (IFT) look like?

Answer 37

2 circuit currents Control both independently

Question 38

What is the superposition of oscillating currents (for IFT)?

Answer 38

Add currents oscillating at different frequencies → current oscillating at the average frequency, with amplitude modulation at “beat frequency” = difference in frequencies

Question 39

When do you put all 4 electrodes on the body (2 circuits at the same time)? What are the 2 reasons?

Answer 39

1. Electric field is oscillating (AC) (back wards and forths) from #1 2. #2 trying to take path of least resistance (AC)

Question 40

What is the aim of IFT with 4 electrodes and 2 circuits?

Answer 40

* interference between the 2 (single point) * 2 signs (oscillating) --\> occasionally will fall in phase but they are different * There is a point where its constructive * Beat frequency --\> when 2 combine (mixing electric fields)

Question 41

What is beat frequency?

Answer 41

Difference in frequencies