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A student constructs a battery using a potato, a strip of copper and a strip of magnesium - Scottish Highers Physics - Question 11 - 2018
Question 11
A student constructs a battery using a potato, a strip of copper and a strip of magnesium. The student then sets up the following circuit with the potato battery co... show full transcript
Worked Solution & Example Answer:A student constructs a battery using a potato, a strip of copper and a strip of magnesium - Scottish Highers Physics - Question 11 - 2018
Step 1
What is meant by the term electromotive force (e.m.f.)?
Answer
The electromotive force (e.m.f.) is defined as the amount of energy per unit charge supplied by a cell or battery in driving charge through a circuit. It can be measured in joules per coulomb (J/C), which is equivalent to volts (V).
Step 2
Determine the internal resistance of the potato battery.
Answer
To find the internal resistance, we can analyze the given graph, which displays the relationship between terminal potential difference (V) and current (I).
Using two points from the graph:
- Point 1: (I = 0, V = 670 mV)
- Point 2: (I = 180 µA, V = 290 mV)
The gradient (r) of the graph can be determined as follows:
Gradient = (\frac{(V_2 - V_1)}{(I_2 - I_1)} = \frac{(290 \times 10^{-3} - 670 \times 10^{-3})}{(180 \times 10^{-6} - 0)} = \frac{-360 \times 10^{-3}}{180 \times 10^{-6}} = -2000 \Omega)
Using the formula for internal resistance: [E = V + Ir] Substituting the values: [670 \times 10^{-3} = E - 180 \times 10^{-6} \times r] Thus, using the determined gradient: (r = 3600 \Omega)
Step 3
Explain, using band theory, why the blue LED will not operate with this battery.
Answer
In band theory, the p-type semiconductor has holes in the valence band, while the n-type has electrons in the conduction band. The blue LED requires a certain energy level to allow electrons to move from the conduction band to the valence band, emitting light.
However, the battery does not supply sufficient energy for electrons to bridge the band gap in the blue LED. Therefore, the blue LED does not operate because the energy from the potato battery is inadequate to excite the electrons in the conduction band of the blue LED into the valence band.