A theatre technician is preparing low-voltage LED strips for a stage set. Fig. 1 shows three strips connected in parallel to a 24 V power supply. Each strip...
Question 1 Report
A theatre technician is preparing low-voltage LED strips for a stage set. Fig. 1 shows three strips connected in parallel to a 24 V power supply. Each strip has a resistance of 12 ohms. The technician chooses parallel branches so that a fault in one strip does not make the whole stage dark. During a performance, one strip becomes disconnected. The fan used to cool the power supply continues to make sound, showing that not all electrical energy becomes useful light.
(a) What is the potential difference across each LED strip? [1]
(b) Calculate the current in one LED strip. [2]
(c) Calculate the total current supplied when all three strips operate. [2]
(d) Describe what happens to the other two strips when strip 2 is disconnected. [2]
(e) Calculate the power transferred by all three strips when they are operating. [3]
Answer Details
(a) Each branch in a parallel circuit has the supply potential difference across it, so each LED strip has 24 V. [1]
(b) Use \(I=\frac{V}{R}\):
\[I=\frac{24\text{ V}}{12\,\Omega}=2.0\text{ A}\]
The current in one strip is 2.0 A. [2]
(c) The total current is the sum of the three equal branch currents:
\[I_{\text{total}}=2.0\text{ A}+2.0\text{ A}+2.0\text{ A}=6.0\text{ A}\]
The total current is 6.0 A. [2]
(d) The other two strips remain on. Each has its own complete parallel branch and still has 24 V across it. [2]
(e) Use the total current in \(P=VI\):
\[P=24\text{ V}\times6.0\text{ A}=144\text{ W}\]
All three strips transfer 144 W. [3]
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