Should you use low or high voltage for long distance transmission of electricity?Power plants are often located far away from the houses they power. What voltage should be used to transmit the electricity? Suppose you wanted to deliver 120 kW of power (enough to supply around 100 houses) through a low-resistance (0.1 Ω) power line. Should you transmit it at 120 V or at 2,400 V? The low-voltage line carries a lot of current, which results in 83% of the power being lost in the transmission lines. This is not good! The high-voltage line performs much better, only losing 0.2% of the power during transmission. Read the text aloud Show More on transmission losses
This is why high-voltage lines are used for long distance transmission of electricity: to reduce power losses in the lines. But how do we change the voltage of the AC transmission line? To do this requires a transformer. Read the text aloud
How transformers work
A transformer uses mutual induction to transfer electrical power from one transmission line to another. The changing magnetic field produced by the primary coil induces a changing current in the secondary coil. The efficiency of a transformer is increased by winding the coils around a ferromagnetic core, because the magnetic domains in the core align to the magnetic field. A transformer exploits induction by introducing a twist: using a different number of loops in the primary and secondary coils. The induced voltage increases with each additional loop in the secondary coil. But energy (and power) must be conserved! When the induced voltage in the secondary is larger than the primary, the induced current must be smaller. Read the text aloud
(19.3) V s V p = N s N p
Vp  = primary coil voltage (V)
Vs  = secondary coil voltage (V)
Np  = number of primary coils
Ns  = number of secondary coils
Transformer voltage
The ratio of the voltages of a transformer is given by the ratio of the number of coils in the primary and secondary, as shown in equation (19.3). A “step-up” transformer increases the voltage, whereas a “step-down” transformer decreases the voltage. Transformers only work with varying input voltages and currents—such as AC circuits—because only a changing magnetic field will induce a current in the secondary. Read the text aloud
Shazi learned in physics class that constant voltages do not induce currents in transformers. She connected a constant +5 V power supply to the primary side of a transformer. To her surprise, when she turned on the power supply, she saw a current briefly induced in the secondary coil, followed by no current. When she turned the power supply off, she also saw a current briefly induced in the secondary coil—but this current flowed in the opposite direction. What's going on? Show
In a “step-up” transformer, which coil has more loops, the primary or the secondary? Show

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