# Trying to understand graph from simple LC circuit

Discussion in 'Homework Help' started by andy24691, Dec 9, 2010.

1. ### andy24691 Thread Starter Member

Nov 25, 2010
42
0
If you had a circuit consisting of a voltage source in parallel with a resistor which is in parallel with an inductor, the relationship between the voltage input and the total current looks like this.

Why does it look like this?
Also with true power up the y-axis instead of current the graph is almost identical in shape. Why is this?

I've looked into the maths and I can't explain why the graphs look like this, I'm sure its quite simple but I can't seem to get my head around it.

cheers

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Post the circuit.

3. ### andy24691 Thread Starter Member

Nov 25, 2010
42
0
Its just a voltage source connected to a resistor and inductor in parallel.

4. ### mik3 Senior Member

Feb 4, 2008
4,846
63
A DC voltage source?

5. ### andy24691 Thread Starter Member

Nov 25, 2010
42
0
Its a AC source.

6. ### tyblu Member

Nov 29, 2010
199
16
$Z_{eff.} = R || sL = \frac{sLR}{R + sL} = \frac{sLR}{R + sL} \cdot \frac{1/sL}{1/sL}
= \frac{R}{1+ \frac{R}{sL}} \\
i_{source} = v_{source}/Z_{eff.}
= v_{source} \cdot \frac{1 + \frac{R}{sL}}{R}
= \frac{v_{source}}{R} (1 + \frac{R}{sL})
$

Is this what you get? This follows Ohm's law, not the given graph.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
What is the frequency, the inductance and the resistance?

How did you get that graph?