L8: Electrical Safety ((Physics Module Lecture 3)

Question 1

Electric charges encounter ______ to their motion, depending on the material they are trying to move through

Answer 1

resistance

Question 2

What is current (I)?

Answer 2

Rate of flow of charge through a circuit

Question 3

What is voltage (V)?

Answer 3

Change in energy per unit charge across part of a circuit

Question 4

I and V are related through _____ of part circuit

Answer 4

impedance

Question 5

______ is often used as a better measure of physiological impact

Answer 5

Current

Question 6

What is the effect of current on the Human Body?

Answer 6

Question 7

You place electrodes on each side of a person’s limb and set an operating voltage. If you transferred this setup to a smaller person, without changing any of the operating settings, the current that would flow across the limb would be?

1. smaller for the smaller person
2. the same in each case
3. larger for the smaller person

Answer 7

3. larger for the smaller person

• Length decreases (+ve and -ve are closer together) –> electrical field increases –> larger current
• Same voltage
• If l is smaller –> resistance is smaller
• Current = larger

Question 8

What are 3 considerations in Electrical Circuit?

Answer 8

1. A source of energy is required to initiate a current – provided by a battery, or other power supply.
2. Energy is transferred from source to other components (eg: resistances).
3. Rate of energy dissipation by circuit element with current, I, flowing through it, & voltage, V, existing across it:
   
   • P = VI

Question 9

The label on a toaster says “240 V at 50 Hz AC 800 W”. How much current flows through the toaster heating element?

Answer 9

P = Vi (power = voltage x current)

3 Amps

Question 10

The label on a toaster says “240 V at 50 Hz AC 800 W”. What is the resistance of the toaster heating element?

Answer 10

R = V/I = 240/3 = 80 ohms

Question 11

The label on a toaster says “240 V at 50 Hz AC 800 W”. What would happen if you stuck a knife in the toaster? (and you have bare feet) Rbody~ 10^5W

Answer 11

Toaster –> earth (get through resistances)

• Knife
• Body
• Feet
• Path of least resistance (will go through body)

Question 12

The label on a toaster says “240 V at 50 Hz AC 800W”.What would happen if you stuck a knife in the toaster? (and you are wearing rubber thongs) Rthong ~ 10^12 ohms

Answer 12

Toaster –> earth

• Body
• Thongs (has huge resistance = current will be significantly lower –> possibly dropped below burn level to threshold of perception) Earth

Question 13

What are 4 electrical safety devices?

Answer 13

1. Fuse: Protects equipment (not people)
2. Circuit Breaker: Can improve safety, but has relatively slow response
3. RCD (Residual Current Device): Has faster response and lower threshold
4. Isolating transformer

Question 14

What are 4 ways to minimise risks with electricity?

Answer 14

1. Where possible, avoid using electrical equipment near water, metal piping, etc.
2. Use settings as low as practical while achieving intended treatment effect.
3. Ensure all electrical connections are in good contact (esp. electrodes on patient).
4. Only connect one electrical device to a patient at any time.

Question 15

What are 2 characteristics of electrical testing (test equipment regularly to ensure safe operation)?

Answer 15

1. At least yearly by qualified medical electrician.
2. More often if device is high-risk, has history of poor performance/reliability, etc.