A ground fault circuit interrupter (GFCI) is a common device, contained in most bathroom outlets and in the cords of all modern hairdryers, that senses when some of the current flowing out one side of an electric outlet isn't returning through the other side of that outlet. The only way such an imbalance can occur is if some current is returning to the electric company through the ground. Because that accidental current path might include your body, the interrupter shuts off current as soon as it senses trouble. Inside the interrupter, the two wires from the electric company pass together around the iron core of a single transformer, forming two identical primary coils.
1. If the AC current passing through each coil is equal in magnitude but opposite in direction, how does the magnetization of the transformer's core change with time?
Answer: There is no magnetization of the transformer's core.
Why: Since the two currents passing around the core are equal in amount but opposite in direction, the magnetic fields produced by two currents cancel one another perfectly.
2. If the AC current passing through each coil isn't equal (perhaps because some current is escaping from a shaver plugged into the outlet), how does the magnetization of the transformer's core change with time?
Answer: The magnetization reverses its direction rapidly with time (120 reversals per second).
Why: Since the two currents passing around the core aren't equal in amount, their magnetic fields don't cancel and the core acquires an overall magnetization. However, because the two currents reverse directions every 120th of a second, the magnetization of the core reverses its direction 120 times a second.
3. The transformer's core has a secondary coil wrapped around it. If all of the current flowing to the hair dryer through one wire returns from it through the other wire, current passing through this secondary coil will experience no change in voltage. But if some of the current flowing to the hair dryer doesn't return, current in the secondary coil will experience a change in voltage. Explain.
Answer: If the currents through the two primary coils are equal, there is no overall magnetic field in the core and no electric fields around it to push or pull on current in the secondary coil. That current experiences no change in voltage. But if the currents through the two primary coils are unequal, there is an overall magnetization of the core, a magnetization that changes with time and is thus accompanied by an electric field. This electric field pushes or pulls on the current in the secondary coil, changing its voltage.
Why: When the currents in the two primaries aren't equal in amount but opposite in direction, energy is transferred between those currents and the current in the secondary coil. The voltage of the current in the secondary coil changes as the result of this energy transfer.
4. The current from the secondary coil is used to trigger a switch that disconnects the outlet from the electric company. This switch has manual "test" and "reset" buttons. To test the ground fault interrupter, you press the "test" button. What can this button do to simulate a real current accident?
Answer: The test button allows current to flow through one of the wires in the ground fault interrupter without returning through the other wire.
Why: By connecting on side of the electric outlet to ground, typically through an electric resistor that limits the current flow to a small value, the interrupter demonstrates that it is very sensitive to currents that don't come back from an appliance.
An uninterruptable power supply (UPS) allows a computer system to operate during a brief power outage. Most personal computer UPS systems use an electronic device to convert 12 V DC power from a battery into 120 V AC power for the computer. The UPS circuitry first converts the 12 V DC into 12 V AC and then uses a transformer to increase the voltage to 120 V, as required by the computer. In Europe, the final voltage is 220 V.
5. Why can't the transformer directly convert 12 V DC into 120 V DC?
Answer: Direct current produces only a constant magnetic field around the transformer's primary coil. Without a changing magnetic field around that coil, there will be no electric field to push current through the transformer's secondary coil.
Why: A steady current through the transformer's primary coil will make that coil magnetic, but its magnetism will be constant. Since only a changing or moving magnetic field creates an electric field, there will be no electric field present in the transformer to accelerate charges in the transformer's secondary coil. Thus the transformer can't transfer power from a direct current in one circuit to a direct current in another circuit and can't convert 12 V DC into 120 V DC.
6. An electronic switching system converts the 12 V DC from the batteries into 12 V AC for the transformer. What is different about the current flowing through the two wires attached to the battery and the current flowing through the two wires attached to the transformer?
Answer: The current from the battery always flows in one direction--from the battery through one wire and back to the battery through the other wire--while current from the transformer reverses directions frequently.
Why: Alternating current changes directions or "alternates". The current flows out the first wire and back through the second wire and then, moments later, it flows out the second wire and back through the first wire.
7. How does power move from the 12 V AC side of the transformer to the 120 V AC side of the transformer?
Answer: Current in the 12 V AC side of the transformer produces a changing magnetic field, which creates an electric field, which pushes on current in the 120 V AC side of the transformer.
Why: While charge doesn't move from one side of the transformer to the other, the energy that charges carry does move. It is passed from the current in the 12 V AC side to the current in the 120 V AC side by way of a magnetic field and an electric field.
8. Energy is conserved. If there is an average current of about 20 A in the 12 V primary circuit of the transformer, what is the average current in the 120 V secondary circuit of that transformer?
Answer: 2 A.
Why: When a current of 20 A is flowing through the primary circuit and experiencing a voltage drop of 12 V as it flows through the transformer primary coil, it is delivering 20 A times 12 V of power, or 240 W of power to the transformer. That same 240 W of power should appear in the secondary circuit, where a current of 2 A experiences a voltage rise of 120 V as it flows through the transformer secondary coil.