PAG 4.3 - Using Non-ohmic Devices As Sensors

Question 1

How does the resistance of a NTC thermistor vary with temperature

Answer 1

As temp increases, resistance of NTC thermistor decreases

Question 2

Why should you avoid heating the water too quickly?

Answer 2

Thermistor and thermometer will have thermal inertia. The reading will not be the same as the actual temp of the water

Question 3

What is the advantage of heating the water very slowly when carrying out this experiment

Answer 3

By heating the water very slowly, you are allowing the thermistor and thermometer to become as close in temperature as possible

Question 4

What safety precautions should be taken when using a Bunsen burner?

Answer 4

• hair and loose clothing should be tied back
• heatproof mat should be used to protect work bench
• safety flame when not directly heating the water
• avoid touching metal regions near the flame

Question 5

What is the added risk of using wires near a Bunsen burner?

Answer 5

Must ensure that the wires do not get caught on the Bunsen burner or touch any hot components

Question 6

What does a potential divider do?

Answer 6

Allows a desired output voltage to be produced, desired ratio of resistances

Question 7

How could you cool a thermistor down to 0 degrees in this experiment?

Answer 7

Thermistor placed in beaker of ice, and heat using a Bunsen burner to get 0 degrees up until 100

Question 8

What should you do when you add water to ensure the temperature is constant throughout?

Answer 8

Use a stirrer to stir the water before taking temp readings

Question 9

How should you take readings from a thermometer

Answer 9

Eye level to reduce parallax error

Question 10

What graph could be plotted to obtain a calibration curve for temperature

Answer 10

Output PD against temperature

Question 11

How does the resistance of an LDR vary with light intensity?

Answer 11

Resistance of LDR increases as light intensity decreases

Question 12

How can light intensity be measured?

Answer 12

Digital light sensor

Question 13

Suggest a method for varying the light intensity incident on an LDR

Answer 13

Lamp connected to a dimmer switch can be shone at the LDR
Darker lab to achieve a good range for light intensities

Question 14

How should the light sensor be positioned in this experiment?

Answer 14

Light sensor should be as close to the LDR as possible, without covering it
This is so that it is exposed to the same light intensity as the LDR itself

Question 15

What graph could be plotted to obtain a calibration curve for light intensity?

Answer 15

Potential difference against light intensity

Question 16

How can your calibration curve be used to determine an unknown light intensity?

Answer 16

Move your LDR into a region of known light intensity and record the output potential difference. Draw a line across from this value and determine corresponding light intensity

Question 17

When may it not be possible to accurately determine an unknown light intensity?

Answer 17

Beyond the range of values you have, extrapolation may not be appropriate

Question 18

What safety precautions should be taken in a dark Labratory?

Answer 18

Ensure all tripping hazards are out of the way

Question 19

Why is it important the fixed resistor value isn’t too high?

Answer 19

If the fixed resistor is significantly larger than the thermistor resistance, changes in temperature will not result in a significant enough change in output voltage

Question 20

Why is it important that the fixed resistor value is not too low?

Answer 20

It may result in the output voltage changing across too big a range of results, may exceed the range measurable using a voltmeter