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ENGINEERING, PPT 2025 HSC 18 MC

A 9 V battery is used to power a simple series circuit. The circuit has one 2.1 V LED with a maximum allowable current of 50 mA.

Which resistor would be the most suitable option to ensure the LED operates at maximum brightness (without failure)?

  1. \(38\ \Omega\)
  2. \(42\ \Omega\)
  3. \(138\ \Omega\)
  4. \(180\ \Omega\)
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\(C\)

Show Worked Solution

\(V_R= V_{\text{supply}}-V_{\text{LED}}= 9-2.1= 6.9\ \text{V}\)

\(R= \dfrac{V_R}{I}= \dfrac{6.9}{0.05}= 138\ \Omega\)

\(\Rightarrow C\)


♦♦ Mean mark 39%.

ENGINEERING, PPT 2018 HSC 22d

To operate successfully, a laser diode requires 2.3 V while drawing a current of 25 mA.
 

For the circuit shown, calculate the required resistor value `R` needed to produce the correct operating voltage.   (3 marks)

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`268\ text{Ohms}`

Show Worked Solution

`V= V_(\text{total})-V_(\text{led})= 9-2.3= 6.7\ text{V}`

`R= V/I= 6.7\(25 xx 10^(-3))= 268\ text{Ohms}`


♦ Mean mark 43%.

ENGINEERING, PPT 2019 HSC 23b

An electric scooter is powered by a 12 volt rechargeable battery with a capacity of 18 Ah.

Calculate the energy stored in the battery. Use 1 Wh = 3600 J.   (2 marks)

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`777.6\ text{kJ}`

Show Worked Solution

Power = volts × current

12 V × 18 A × 1 h = 216 W in 1 h

`:.` Power stored = 216 Wh

  
1 W = 1 J/s

1 Wh = 3600 J  (1 h = 60 × 60 seconds)

`:.` Energy stored  = 216 Wh × 3600 J  
   = 777 600 J  
   = 777.6 kJ  
♦ Mean mark 46%.

ENGINEERING, PPT 2021 HSC 22c

An electric skateboard battery is rated at 0.078 watts at an operating voltage of 48 volts.

Calculate the capacity of the battery in amp hours, if it fully discharges in one hour of use. The power (`P`) is given by the formula  `P=V I`.    (3 marks)

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`text{Capacity} = 5.85\ text{Ah}`

Show Worked Solution

`text{Power = 0.078 W,  Volts = 48 V}`

`P` `=VI`
`I` `=P/V=0.078/48`

 
`:.\ text{Capacity}=(0.078×3600)/48=5.85\ text{Ah}`


♦ Mean mark 50%.

ENGINEERING, PPT 2021 HSC 18 MC

The diagram shows a simplified circuit of an electric scooter.
 

What would be the reading of the ammeter in this circuit?

  1. 3.0 A
  2. 6.0 A
  3. 12.0 A
  4. 26.9 A
Show Answers Only

`C`

Show Worked Solution

Find total resistance (`R_t`):

`1/R_t` `=1/R_1+1/R_2`  
`1/R_t` `= 1/(1+2)+1/3=2/3`  
`R_t` `=3/2 = 1.5\ Ω`  

 
Find current (`I`):

`I=V/R_t=18/1.5=12\ text{A}`

`=>C`

Mean mark 52%.