http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Physics (9702)/9702_s08_qp_2.pdf
HELP NEEDED for no. 6 (b)
Its too confusing ! pff
the formula for Power = R^2 / V were gonna use this for each scenario
so just keep in mind that each resistor is 38.4 ohms and the V is gonna be 240 V no matter what.
1) S1 the control switch is off >> R = 0 ohms >> P = 0 W
2) R in B is zero because it parallel to the wire (when S2 is closed) and the wire has negligible resistance and so when resistors are in parallel the total R is lesser than the least, so R in B is o. and R in C is obviously 0.
so R total = 38.4 ohms (of A)
P = 240^2 / 38.4 = 1.5 kW
3) the R of A and B will be taken here...now its a series circuit with A, B and the power supply. eliminate C.
total R = 38.4 + 38.4
P = 240^2 / 38.4 x 2 = 3 kW
4) All the components resistance are to be taken...
r total :
38.4 + 38.4 = 76.8 ohms >> A + B, now these are in parallel with C so
78.6 x 38.4 / 76.8 + 38.4 = 25.6 ohms
P = 240^2 / 25.6 = 2.25 kW
hope that helps =)