# Evan's Space

## Wonders of Physics # N2010 P2Q12 Fuel gauge using variable resistor Solutions:

View the video on how the gauge works.

<p><a href=”https://vimeo.com/138991946″>fuel guage using ammeter and variable resistor</a> from <a href=”https://vimeo.com/user10931667″>evantoh</a&gt; on <a href=”https://vimeo.com”>Vimeo</a&gt;.</p>

As the fuel level drops, the float which stays on the fuel surface will descend. The rod which is attached to the float will turn clockwise about the pivot X. As the rod turns, the resistance on the variable resistor increases. This increases the resistance of the circuit. Hence the current flowing through the circuit will decreases, causing the needle to deflect more to the left, indicating towards E (empty). Thus the reading on the fuel gauge decreases. Solutions:

(i) Both the fixed resistor and sensor are in series.

Total effective resistance Re = 5000 + 1000 = 6000 ohms

V = IR
12 = I x 6000
I = 0.0020 A

Hence potential across Y, V = IR (where I is constant in a series circuit)
= 0.0020 x 1000
= 2.0 V

(ii) When the temperature increases and the resistance of sensor Y decreases, the total effective resistance of the circuit decreases. Current flowing through the circuit increases.

Since V = IR, where the R of the fixed resistor is a constant 5000 ohms, as current I increases, the potential difference across the 5000 ohms resistor will increase.

### 5 thoughts on “N2010 P2Q12 Fuel gauge using variable resistor”

1. Hi, I would like to consult you about a question on LDR. I know that LDR increases its resistance when it is dark. It is also said that LDR is used in automatic street lights. So how does LDR helps to light up street lights in dark when the resistance of LDR will be high which also means the current will be low? Thank you.

• LDR is not connected in series to a lighting circuit. It is actually parallel to the lighting circuit. LDR is used in a potential divider which helps to switch on the lighting circuit when it is done. Do you learn potential divider in yr syllabus?

• Yes, I learn potential divider, but I thought LDR is placed in a series format like this one. http://www.doctronics.co.uk/images/vdiv4.gif

• Sorry for the late reply. Yes the ldr is placed in series with a fixed resistor. The Vout in your diagram is across the fixed resistor. So when it is dark the resistance in ldr increase. Total resistance increases and current decreases. So V across fixed resistance = Vout will decrease. So light circuits in Vout will not light up

• If u want the lightning circuit to be switched on when dark, then u should swop the fixed resistor n ldr where Vout is across ldr