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QuizPower sources and safety · 8 min

Quiz: power sources and safety

8 questions. Answer each one, then open Show the answer to check yourself. In Litmux Academy the quiz is marked and counts towards your certificate.

Question 1Three 1.5 V cells in series give:

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Answer: 4.5 V

In series the voltages add.

Question 2Two identical 3.7 V 2000 mAh cells in parallel make:

  1. 7.4 V 2000 mAh
  2. 3.7 V 4000 mAh
  3. 7.4 V 4000 mAh
  4. 3.7 V 2000 mAh
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Answer: B. 3.7 V 4000 mAh

Parallel: voltage stays, capacity adds.

Question 3A battery reads 12.0 V unloaded and 11.4 V while supplying 3 A. Its internal resistance is:

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Answer: 0.2 Ω

r = 0.6 V / 3 A = 0.2 Ω.

Question 4Why can a battery that reads 12.5 V on a meter fail to start a motorbike?

  1. The meter is wrong
  2. Its internal resistance is high: under the starter's big current the voltage collapses
  3. It is AC
  4. Starters need 24 V
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Answer: B. Its internal resistance is high: under the starter's big current the voltage collapses

A voltmeter draws almost no current, so it cannot reveal r.

Question 5A bench supply is set to 5 V with a 50 mA limit and connected to 10 Ω. What voltage does it show?

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Answer: 0.5 V

10 Ω would take 500 mA: CC at 50 mA, V = 0.05 × 10 = 0.5 V.

Question 6What harms a person in an electric shock?

  1. The voltage alone
  2. The current through the body
  3. The power rating of the supply
  4. The frequency only
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Answer: B. The current through the body

Current through the body does the harm; voltage drives it through the body's resistance.

Question 7A fuse is there mainly to protect:

  1. the person
  2. the wiring, from overheating
  3. the battery's capacity
  4. the switch
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Answer: B. the wiring, from overheating

It melts before the cable overheats. An RCD protects people.

Question 8Where should a battery's fuse go?

  1. Anywhere in the circuit
  2. As close as possible to the battery's + terminal
  3. In the − lead only
  4. After the load
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Answer: B. As close as possible to the battery's + terminal

Then all the wiring after it is protected.