+ - A LED V Fig. 1 Fig. 1 shows an engineer measuring a low-energy LED marker on a harbour wall. The LED transfers 24 J of electrical energy when 8.0 C of c...
Question 1 Report
Fig. 1 shows an engineer measuring a low-energy LED marker on a harbour wall. The LED transfers 24 J of electrical energy when 8.0 C of charge passes through it. The marker is visible at night, when reflected light from boats can make navigation difficult. The engineer places a voltmeter across the LED and an ammeter in series. These meters measure different quantities, so they must not be connected in the same way.
(a) What is the potential difference across the LED? [2]
(b) Describe how the voltmeter is connected in Fig. 1. [1]
(c) Explain why an ammeter is connected in series with the LED. [2]
The diagram shows a drone engineer testing a camera battery before a coastal survey. The battery label states 11.1 V and 2.0 Ah. The drone draws 1.0 A while hovering. The engineer explains that an ampere-hour is a convenient battery capacity unit, although charge can also be measured in coulombs. Wind changes the drone velocity, but it does not change the electrical definition of current. Assume the current remains at 1.0 A.
(a) Complete the calculation of the maximum hovering time in hours. [2]
(b) Give the battery capacity in coulombs. [2]
(c) What is the unit used for charge in part (b)? [1]
Answer Details
LED circuit
(a) \[V=\frac{E}{Q}=\frac{24\text{ J}}{8.0\text{ C}}=3.0\text{ V}\] [2]
(b) The voltmeter is connected in parallel, across the LED terminals. [1]
(c) The same current must pass through an ammeter and the LED when they are in series. It therefore measures the current through the LED. [2]
Drone battery
(a) \[\text{time}=\frac{\text{capacity}}{\text{current}}=\frac{2.0\text{ Ah}}{1.0\text{ A}}=2.0\text{ h}\] [2]
(b) \[2.0\text{ Ah}=2.0\times3600=7200\text{ C}\] [2]
(c) The unit of charge is the coulomb, C. [1]
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