P2 Circuit Devices (page 182)

Question 1

As well as temperature, resistance can depend on what other things?

Answer 1

can depend on things like light intensity, which is how LDRs work. They’re really useful for circuity that sense changes in light levels.

Question 2

What is LDR short for?

Answer 2

Light Dependent Resistor

Question 3

What is LDR?

Answer 3

it is a resistor that is dependent on the intensity of light.

Question 4

In bright light, do the resistance fall?

Answer 4

Yes

Question 5

In darkenss is the resistance at its highest or lowest?

Answer 5

at its highest.

Question 6

LDR have lots of applications including what?

Answer 6

automatic night nights, ourdoor lighting and burglar detectors.

(see diagram 1 on page 182)

Question 7

What is a Thermstor?

Answer 7

it is a temperature dependent resistor.

(The resistance of a thermistor depends on temperature)

Question 8

In hot conditions, does the resistance drop or go up?

Answer 8

the resistance drops.

Question 9

In cool conditions, does the resistance drop or go up?

Answer 9

the resistance goes up.

Question 10

Ghermistors make useful temperature detectors, give examples?

Answer 10

e.g. car engine temperature sensors and electronic thermostats.

(see diagram 2 on page 182).

Question 11

What can you use in Sensing Circuits?

Answer 11

you can use LDRs and Thermistors in sensing circuits.

Question 12

What can sensing circuits be used for?

Answer 12

to turn on or increase the power to components depending on the conditions that they are in.

Question 13

The circuit (diagram 3 on page 182) is a sensing circuit used to operate a fan in a room.

Why would the fixed resistor and the fan always have the same potential difference across them?

Answer 13

because they’re connected in parallel - see page 184.

Question 14

The diagram 3 on page 182 sensing circuit to operate a fan. The pd (potential difference) of the power supply is shared out between what? explain your answer

Answer 14

The pd of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances. The bigger a component’s resistance, the more of the pd it takes.

Question 15

The diagram 3 on page 182 sensing circuit to operate a fan. explain as the room gets hotter, what happens regarding the resistance of the themistor, does it increase or decrease? then what happens next?

Answer 15

as the room gets hotter, the resistance of the thermistor decreases and it takes a smaller share of the pd from the power supply. So the pd across the fixed resistor and the fan rises, making the fan go faster.

Question 16

Regarding the sensing circuit on page 182, diagram 3, could you connect the component across the variable resistor instead? explain your answer?

Answer 16

Yes.

For example, if you connect a bulb in parallel to an LDR, the pd across both the LDR and the buld will be high when it’s dark and the LDR’s resistance is high. The greater the pd across a component, the more energy it gets. So a bulb connected across an LDR would get brighter as the room got darker. (see diagram 4 on page 182).

Question 17

Describe one everyday use for the following components:

a) a LDR
b) a thermistor (2 marks)

Answer 17

a) E.g. automatic night lights - a light automatically turns on when it gets dark (1 mark)

b) E.g. thermostats - the heating automatically turns on/off at a certain temperature (1 mark)