A few questions regarding electric circuits (calculations)?

Question

Could you provide working out too please? Thanks :D

Q1. Find the current in the 20Ω and 5Ω resistors in the following circuit:
http://i28.lulzimg.com/22c6123200.jpg

Q2. In the circuit below, the reading on the ammeter is 3.2 A.
http://i28.lulzimg.com/77e1d039ac.jpg
Determine:
a) the reading on the voltmeter
b) the potential difference across the 40Ω resistor
c) the current in the 40Ω resistor

Q3. http://i28.lulzimg.com/7d19d5dcbc.jpg
For the circuit above:
a) Determine the total resistance.
b) Find the reading on the ammeter.
c) Draw a voltmeter in the correct place to measure the potential difference across the 0.3 resistor (you can just describe it)
d) Draw an ammeter in the correct place to measure the current in the 0.3 resistor (you can, again, describe the location instead)
e) Determine the readings on the meters mentioned in parts (c) and (d) above.

Thanks in advance!

Sayyed Hamid Reza · Accepted Answer

Q1 answer:

R(total) = 50 + (5 || 20) = 50 + 1/ (1/5 + 1/20) = 50 + 1/ (5/20) = 54 ohms

I (total) = V(battery) / R(total) = 12 / 54 = 2 / 9  Amp

i1 = current in 5 ohm      i2 = current in 20 ohm
we have : i1 + i2 = 2/9    and    i1 / i2 = 20 / 5 = 4
=>    i1 = 8 / 45  amp    and   i2 = 2 / 45 amp
        i1 ~ 0.178  amp    and   i2 ~ 0.044  amp


Q2 answer:

the exact value of the total current can be calculated as 3.182 amp.

if we consider the  I(total)=3.2
we have    V(voltmeter) = 3.2 * 10 ohm = 32 volts
=>  V (40 ohm) = 50 - 32 = 18 volts
=>  I  (40 ohm)  = 18 / 40 = 0.45 amp

with the more exact value of   I (total) ~= 3.182
we have     V(voltmeter) = 31.82 volts
=>  V (40 ohm) = 50 - 31.82 = 18.18 volts
=>  I  (40 ohm)  = 18.18 / 40 = 0.4545 amp



Q3 answer:

R(total) = 100 + (0.3 || 0.5) = 100 + ( 3 / 16 ) = 100.1875
I (ammeter) = 20 / 100.1875 = 0.19963 amp

c) just connect the voltmeter terminals to each end of the 0.3 resistor
    the plus terminal to the top end (in diagram) and the minus terminal to the bottom end.

d) put the ammeter in series with the 0.3 resistor, i.e. draw it on the same wire as the 0.3
    resistor

e)   V = 20 - 100 * 0.19963 = 0.037 volt
      I  =  0.037 / 0.3 ~= 0.125 amp

chrones · Answer

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A few questions regarding electric circuits (calculations)?

2 Answers