Practically, how can a transformer step up or down the voltage, if it does not step up or down the power?

satguru

Power is usually measured in Watts, which is the current over the flow. Therefore although voltage is one factor (the flow) ampage is the other (the current) so it is perfectly possible to alter one while the other remains the same. For example in a 240 volt household current you put in a transformer which basically sends the current in circles to slow it down (not very technical but a good illustration) until the windings reach the desired output, say 9 volts for a domestic appliance like a radio. I'll let the link explain the maths as that bit is way beyond me. http://www.allaboutcircuits.com/vol_2/chpt_9/1.html The main point to note is by winding wire at different amounts the current can be altered to either increase or decrease the voltage, the amperage being determined by the tolerance of the materials of the transformer itself rather than the winding ratio which is the crucial element. Aug 25 09, 5:25 PM

looney_tunes

Power in electric circuits can be calculated in a number of ways. One of them is Power = Current x Voltage. In a transformer, the input voltage and the output voltage are different (output is greater in a step-up transformer, smaller in a step-down transformer), but the power (ignoring losses due to reality, and talking about an ideal theoretical situation) is the same on both sides. This is an application of the Law of Conservation of Energy. Hence, the side with the higher voltage will also have a smaller current, and the two power ratings will be the same. See "Ideal Power Equation" at the following link: http://en.wikipedia.org/wiki/Transformer Aug 26 09, 2:21 AM