Finding current across a resistor in a DC circuit

[nchin]

Help with this confusing circuit! test tmrw! Find i across resistor.

http://imageshack.us/photo/my-images/202/circuitu.jpg/

my teacher lectures to fast and i don't understand the notes i put down. can someone explain this one to me please??

 

[CWatters]

This is the way I would approach the problem...

You can ignore the 12 Ohm resistor because it's across the 240V supply and has no effect on the current or voltage in the rest of the circuit.

To find the current in the 40 Ohm I would first aim to work out the voltage across it.

I note that the 60, 40 and 48 Ohm are in parallel and that 60//40//48 works out as ...

1/(1/60 + 1/40 + 1/48) = 16 Ohm

Then I note that the 8 Ohm and 16 Ohm form a voltage (aka potential) divider...

http://en.wikipedia.org/wiki/Voltage_divider

So the required voltage across the 40 Ohm is...

240 * 16/(8+16) = 160V

Therefore the current through the 40 Ohm is

I₄₀ = 160/40 = 4A