Physics Q's hlp ASAP

[ahmed]

I am currently stuck on these Q's and can't get my head around them , ANY help is appreciated.
All pics relating to the Q's are attached below.
1)
a) Use superposition method, find the magnitude and direction of the current flowing in R = 15Ω
for the circuit of figure 4.
b) Use any technique other than that used in (a) to check the validity of your answer.

2)
For the circuit as shown in figure 5, determine the Thevenin equivalent circuit.
If the terminal AB is short circuited, determine how much current is flowing through the 20Ω resistor.

3)
A particular electric circuit is described in the following terms. Three 150 ohm resistors are first
connected in parallel. This parallel combination is then connected across the terminals of a power
supply consisting of a 500 V ideal voltage source in series with a 10 ohm resistor.
Answer the following questions about this circuit.
a) Draw the circuit.
b) Calculate the total power dissipated in the three 120 ohm resistors.
It is now required that another resistor, L R , be connected in parallel with the three 150 ohm resistors.
With this aim in mind, answer the following questions.
c) Determine the Thevenin equivalent of the circuit which will connect to the resistor, L R . Sketch
the Thevenin equivalent circuit.
d) The resistor, L R , is to have a potential difference across its terminals of 350V. Determine the
value of L R .

4)
If 1MW is to be transmitted over two 0.0100-Ω lines, estimate how much power is saved if the
voltage is stepped up from 500 V to 50000 V and then down again, rather than simply transmitting at
500 V. Assume the transformers are each 98% efficient.

5)
A series RLC circuit has L=5.5 mH and R=4.0 Ω resistor
a) What value must C have to produce resonance at 5000 Hz?.
b) What will be the maximum current at resonance if the peak external voltage is 10 V?.