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PracticeTest2dsolutions - Test 2 sample problems Section A...

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Test 2 sample problems Section A 1. The volume enclosed by the plates of a parallel plate capacitor is given by the equation Volume = Ad . If one doubles the area, A , of the plates and reduces the plate separation, d , by a factor of two so that the volume remains the same, the capacitance will a) decrease by a factor of 4, b) decrease by a factor of 2, c) remain constant, d) increase by a factor of 2, or e) increase by a factor of 4. (e). Because the capacitance is determined by Equation 26.3, C = e o A / d , doubling the plate area and decreasing the plate separation by a factor of two will increase the capacitance by a factor of 4. 2. A very large capacitor is in series with a very small capacitor. The equivalent capacitance of this combination will be a) slightly greater than the capacitance of the large capacitor, b) slightly less than the capacitance of the large capacitor, c) slightly greater than the capacitance of the small capacitor, or d) slightly less than the capacitance of the small capacitor. 3. A capacitor stores charge Q at a potential difference Δ V . If the voltage applied by a battery to the capacitor is doubled to 2Δ V : (a) the capacitance falls to half its initial value and the charge remains the same (b) the capacitance and the charge both fall to half their initial values (c) the capacitance and the charge both double (d) the capacitance remains the same and the charge doubles Answer: (d). The capacitance is a property of the physical system and does not vary with applied voltage. According to Equation 26.1, if the voltage is doubled, the charge is doubled. 4. Many computer keyboard buttons are constructed of capacitors, as shown in the figure below. When a key is pushed down, the soft insulator between the movable plate and the fixed plate is compressed. When the key is pressed, the capacitance (a) increases (b) decreases (c) changes in a way that we cannot determine because the complicated electric circuit connected to the keyboard button may cause a change in Δ V . (d). Using Equation 26.10 for two capacitors in series, Since C small / C big is a small number, the denominator will be just slightly greater than 1, and the equivalent capacitance will therefore be slightly less than the capacitance of the small capacitor. 1 1 1 or 1 small big big small small eq C C eq big small big small C C C C C C C C C = + = = + +
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Answer: (a). When the key is pressed, the plate separation is decreased and the capacitance increases. Capacitance depends only on how a capacitor is constructed and not on the external circuit 5. Two capacitors are identical. They can be connected in series or in parallel. If you want the smallest equivalent capacitance for the combination, you should connect them in (a) series (b) parallel (c) Either combination has the same capacitance.
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