NAME
(1) A circuit contains two batteries and three resistors as shown. Rand VB are constants.
2R
(a) Which resistors can be considered to be connected in parallel?
(i) All three.
(iv) None of them.
r(i1) The two in the middle with value R.
(iii) The two at right: R and 2R.
(b) Determine the current flowing in each resistor.
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(2) Four capacitors are connected as shown, with the supply wires at 20 V and 50 V. Voltages
are supplied by an external circuit not shown.
(a) What is the effective capacitance of
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these four capacitors?
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(b) What is the charge on the upper plate of the lower 5 jiF capacitor? Is it positive or negative?
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(c) What voltage will appear at the middle junction, labeled “Vmjd”?
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(3) In this circuit, initially the switch is set to the left, connecting the battery to the capacitor.
Assume that VB = 9 V, C 60 mF, and R = 2.5 k2.
(a) What is the initial charge on the
capacitor?
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(b) At time t 0, the switch is thrown to the right, connecting to the portion of the circuit
containing the resistor. What is the instantaneous current in the resistor just after the switch is
thrown?
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(c) How much time is required for the capacitor charge to be reduced to one-third of the initial
value?
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(d) For this problem, before the switch is thrown, while the capacitor is still connected to the
battery, suppose the capacitor (a parallel-plate vacuum capacitor) becomes completely filled by a
sheet of polystyrene (K 2.6). How much additional charge must flow into the capacitor?
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NAME
(e) Continuing from problem 3, on the last page, in the situation of part (d), how is the electric
field in the capacitor changed in the process of inserting the polystyrene sheet?
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(4) A resistor as illustrated consists of a hollow tube of graphite. Current flows from the inside
to the outside of the cylinder, shown by the arrows in the figure. The white inner region is the
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), while the grey is the graphite (outer radius R
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hollow portion (inner radius R
(a) What point will have the largest current
densi2—-——- J= E
i the inner edge, at RJ
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2
(ii) the outer e ge, at R
(iii) The current density is uniform.
(iv) the current density increases from
right to left.
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(b) Describe briefly the direction of average electron flow, at points within the graphite.
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(5) A 300 W floodlight is connected to a 120 V AC power outlet. What is the pcii value of the
current that will flow in the floodlight?
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