Fig. 1 shows a charging circuit used in a science museum display. A hand generator is turned by visitors and transfers kinetic energy into electrical energy...
Question 1 Report
Fig. 1 shows a charging circuit used in a science museum display. A hand generator is turned by visitors and transfers kinetic energy into electrical energy. The generator charges a capacitor. When a button is pressed, the capacitor discharges through an LED, producing a brief flash. The display has a resistor in series with the LED to keep the current below its maximum value.
(a) What energy store decreases when the capacitor discharges? [1]
(b) Which component stores charge in this circuit? [1]
(c) Describe the effect of removing the series resistor on LED current. [1]
(d) Calculate the charge transferred when a current of 0.20 A flows for 10 s. [2]
Fig. 1 shows a solar-powered pond aerator. A panel supplies electrical energy to a small motor that turns a fan above the water. In bright sunlight the panel provides 12 V and the motor takes a current of 0.30 A. The fan increases oxygen in the water for fish during hot weather. A student suggests adding a second identical solar panel in series when there is less radiation from the Sun.
(a) Calculate the power supplied to the motor. [1]
(b) What energy store increases in the spinning fan? [1]
(c) Which change happens when a second identical panel is connected in series? [1]
(d) Use the equation energy = power x time to calculate the energy transferred in 10 minutes. [2]
Answer Details
Capacitor and LED circuit
(a) When the capacitor discharges, its electrical energy store decreases. [1]
(b) The capacitor stores charge. [1]
(c) Removing the series resistor makes the LED current increase, potentially to a value large enough to damage the LED. [1]
(d) \[Q=It=0.20\ \mathrm{A}\times10\ \mathrm{s}=2.0\ \mathrm{C}\]
The charge transferred is 2.0 C. [2]
Solar-powered aerator
(a) \[P=VI=12\ \mathrm{V}\times0.30\ \mathrm{A}=3.6\ \mathrm{W}\]
The motor receives 3.6 W. [1]
(b) The spinning fan has an increasing kinetic energy store. [1]
(c) Connecting a second identical panel in series increases, or doubles, the potential difference. [1]
(d) \(10\) minutes \(=600\ \mathrm{s}\).
\[E=Pt=3.6\ \mathrm{W}\times600\ \mathrm{s}=2160\ \mathrm{J}\]
The energy transferred is 2160 J. [2]
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