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SOLUTIONS – MW4:
Q268) Kirchhoff’s junction rule states that the sum of the currents which enter a
node is equal to the sum of all the currents which exit the same node. Current is
equal to the charge which flows over a certain time. Thus, the amount of charge
which flows into a node is equal to the amount of charge which flows out of the node
– conservation of charge.
Q269) An electric charge q passing through a closed loop will lose potential energy
of qV by passing over a resistor. For resistors in series, it will lose this potential
energy over each of the resistors. It gains energy by a battery which is connected to
the loop. Kirchhoff’s rule states that the losses and gains in energy are equal –
conservation of energy.
Q2611) In the big loop containing A, emf, R1, the combined resistance of R2 and
R3, the combined resistance of R4, R5, and R6, and R7, the voltage drop over the
resistors has to equal the voltage that is provided by emf (conservation of energy).
(a)
If we assume no resistance in the ampmeter and the battery, the voltage will
remain constant.
(b)
If we do assume a resistance, the voltage drop over R7 will be larger and the
one over the ampmeter and the battery will be smaller.
(c)
The voltage drop over the combined resistance of R4, R5, and R6 will
decrease as the one over R7 increases, and as the voltage is the same over
all three resistances, the voltage drop over R4 decreases.
(d)
If R2 decreases, the total resistance of the network decreases, thus the
current increases for a constant emf. The current through R1 increases.
(e)
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 Spring '08
 Jordan
 Charge, Current

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