Block 2 - lecture 7

Question 1

What are the atomic movements that make up temperature?

Answer 1

• vibration
• rotation
• collective behaviour

Question 2

atomic movements possible within a solid?

Answer 2

• vibration

- rotation

Question 3

valence electron?

Answer 3

the electron given up by each atom that’s free to move around the material
aka. conduction electron

Question 4

when can a valence electron carry current?

Answer 4

when the electrons have enough energy to be in the conduction energy band

Question 5

how are the energy bands in metals?

Answer 5

valence band overlaps the conduction band

Question 6

how are the energy band in semiconductors?

Answer 6

small gap between valence band and conduction band

Question 7

how are the energy bands in insulators?

Answer 7

large gap between valence band and conduction band?

Question 8

how do energy bands explain the properties of semiconductors?

Answer 8

the small gap between bands means a valence electron can be given enough energy to jump up to the conduction band

Question 9

2 main types of thermal sensors?

Answer 9

• resistance-temperature sensors

- thermistors

Question 10

metal resistance vs temperature?

Answer 10

almost linear

Question 11

thermistor resistance vs temperature?

Answer 11

very non-linear, negative slope

Question 12

self-heating in a thermistor?

Answer 12

temperature increase lowers the resistance, increasing the current and the temperature

Question 13

circuit for a thermistor?

Answer 13

voltage divider and gain buffer

Question 14

thermocouple?

Answer 14

device that measures temperature by the proportional emf produced

Question 15

advantages of a thermocouple?

Answer 15

• higher temperatures (1000s)
• no external power source required
• can be a more varied response time (depending on wire used; can be quicker or slower)

Question 16

how a thermocouple works?

Answer 16

Thermoelectric principle:

valence electrons get excited by the heat and move to the colder end of the wire, with less excited electrons moving back to take their place.

As one wire does this more than the other, there’s an overall current flow.

Question 17

extension wires on a thermocouple?

Answer 17

the the two metals at the measured point and extended away from the sensor, so the reference junctions are further away.

Question 18

thermocouple issues?

Answer 18

-susceptible to electrical noise due to small voltages (need shielding, twisting and amplification)

Question 19

other thermal sensors?

Answer 19

• bimetal sensor (two metals with different expansion rates)
• gas thermometers (use pressure)
• liquid-expansion thermometers (use expansion of liquid)