Category: 17 Alternating Current

153 Transformer Electric Shock

I take a standard step-down transformer and flip it around to use it as a step-up.

Instead of feeding a AC voltage, as is standard for a transformer, I connect a 9-V DC battery across the lower-turn coil. There is no output voltage across the higher-turn coil, except at the instants when the 9-V battery is connected or disconnected.

The voltmeter seems to suggest a much larger (induced) output voltage upon disconnection. I am not sure if I can trust the >1000 V reading. But I usually make my students (typically about 13) join hands to take the place of the voltmeter. They sure enjoyed the electric shocks when I connected/disconnected the circuit.

152 Transformer

The solenoid is the primary coil while the green wire is the secondary.

The video shows clearly how the secondary voltage is proportional to the number of turns in the secondary coil.

Since each additional turn increases the secondary voltage by about 0.064 V, and assuming that the primary coil is 230 V, the number seem to be suggest that the primary coil has about 3600 turns. It does not look like the primary coil has that many turns. So probably there is a lot of flux leakage.

049 Lenz’s Law Hovering Ring

If the solenoid were powered by a DC current, the ring should only experience a changing magnetic flux only at the instant when the solenoid was turned on. The ring would then have jumped only once, before falling back to the bottom. The fact that it is able to hover tells us that it continuously experiences an upward magnetic force that balances its downward weight.

This is possible only if the solenoid is powered by an AC current. We can in fact thinking of the solenoid as the primary coil of a transfomer, and the ring as the secondary coil. The primary and secondary currents are always in-phase (or completely out-of-phase if you choose). So the current in the solenoid and the ring produce magnetic fluxes which are always opposing each other, resulting in a continuous upward magnetic force (a sine-square varying force, to be exact) on the ring, which keeps the ring in the air.

025 AC lighting

First of all, the street lamps are pulsating because they are powered by an alternating current. Singapore uses 50 Hz AC supply, so the street lamps pulsate 100 times per second.

Some people expect 50 pulsations per second. They have forgotten that a lamp lights up two times in each AC cycle: once during the positive half-cycle, and once more during the negative half-cycle.

003 Why do LEDs leave a trail of dashes?

The LEDs are powered by a 50 Hz AC supply. Since LEDs are diodes, they light up only during the positive half cycles of the AC supply, but do not light up during the negative half cycles.

This flashing of the LEDs at 50 Hz is revealed if we keep moving the LEDs, as shown in this video.