If you have a question that
- is of interest to a broad audience
- seeks a scientific or technical answer
please ask it below as a comment and I’ll try to answer it!
Lou Bloomfield
Author: Lou Bloomfield
In the book, you discussed pushing on a file cabinet that was resting on the sid…
In the book, you discussed pushing on a file cabinet that was resting on the sidewalk. Why doesn’t the file cabinet move when you push even a little — you’re making the net force greater than zero?
When you exert a small horizontal force on the file cabinet, it doesn’t move because static friction between the ground and the file cabinet exerts a second horizontal force on the file cabinet that exactly balances your force. If you push the file cabinet west, the ground will exert a static frictional force on the file cabinet, pushing it east. The file cabinet will thus experience a net force of zero. You’ll have to push very, very hard before static friction will be unable to match your force. One you do exceed the limit of static friction, the friction will no longer be able to balance your force and the file cabinet will experience a net force in the horizontal direction. The file cabinet will then accelerate in the direction of your force.
What are vortex rings?
What are vortex rings?
These rings (also called smoke rings) are moving portions of fluid that are moving relative to the surrounding fluid. They form a remarkably stable structure. The inner edge of the ring heads forward, while the outer edge head backward and the ring pulls itself through the air. Fluid dynamicists study these sorts of objects.
What is the difference between “thickness” and viscosity? Is viscosity just a …
What is the difference between “thickness” and viscosity? Is viscosity just a fancy word for thickness?
Viscosity is a measurable quantity—a liquid has a specific viscosity as measured in units of poise or pascal-seconds. Thickness refers to the same characteristic as viscosity, but isn’t a specific quantity. It’s certainly correct to say that a thick liquid is a liquid with a large viscosity.
When you were showing us water faucets during class, each faucet had a corner im…
When you were showing us water faucets during class, each faucet had a corner immediately preceding the opening through which the water came out. Does that corner help slow the pressure of the water?
Most faucets do have a turn just before the water comes out and that turn is there to slow the water down. Unless the faucet is opened for maximum flow, the pressure of the water emerging from the valve part of the faucet is pretty close to atmospheric pressure, so there isn’t any need to control that pressure. But the water emerging from the valve may be traveling very fast and it could easily spray across the room if there were nothing in its way. To prevent such sprays, most faucets are bent so that water spraying out of the valve will hit the bend and become turbulent. The turbulence will help it to convert its kinetic energy into thermal energy so that it will emerge from the faucet at low speed and atmospheric pressure. (Great question!—I’d never thought of this before).
What is the difference, if any, between appliances with a 2 prong plug and a 3 p…
What is the difference, if any, between appliances with a 2 prong plug and a 3 prong plug?
In the 2 prong system, current travels to the appliance through one prong and leaves through the other prong. The roles of the two prongs interchange every 120th of a second. In the 3-prong system, there is one extra prong and that connects the frame of the appliance to the ground (the earth). This extra connection is a safety feature. If a wire comes loose inside the appliance and touches the frame, the frame can deliver charge and current to you through your hand and you can deliver it to the ground through your feet or your other hand. The earth is very large and a large amount of charge can flow into it without repelling further charge. Moreover most electrical systems are actually connected to the ground at some point. So if current can travel out of the circuit feeding power to the appliance and travel through you and into the ground, it will. You’ll get a shock. The ground connection (the extra prong) allows this extra current to flow to ground so easily that a huge current is drawn out of the power source, causing the fuse or circuit breaker in that power source to break the connection. When that power connection is broken, no power can flow to the appliance at all and you can’t get a shock from it. Plastic appliances often omit the extra prong because they have nothing dangerous to touch on their exteriors.
What is the hum you hear when walking under large power lines?
What is the hum you hear when walking under large power lines?
The electric currents in those lines are reversing 120 times a second in the United States (60 full cycles of reversal, over and back, each second). That means that the electrostatic forces between the charges they carry and anything nearby reverse 120 time a second and the magnetic forces that they exert on one another when currents flow through them turn on and off as well. You hear all of the motions that are caused by the pulsating electric and magnetic forces.
What is the purpose of the iron core in a transformer?
What is the purpose of the iron core in a transformer?
The iron core of a transformer stores energy as power is being transferred from the primary circuit to the secondary circuit. This energy is stored as the magnetization of that iron. The transformer needs to store that energy for roughly one half cycle of the alternating current or about 1/120th of a second. The more iron there is in the transformer, the more energy it can store and the more power the transformer can transfer from the primary circuit to the secondary circuit. Without any iron, the energy must be stored directly in empty space, again as a magnetization. But space isn’t as good at storing magnetic energy as iron is so the iron increases the power-handling capacity of a transformer. Without the iron, the transformer must operate at much higher frequencies of alternating current in order to transfer reasonable amounts of power.
What makes alternating current alternate?
What makes alternating current alternate?
The pump for alternating current (usually an electrical generator) creates electric fields that reverse their directions 120 times a second (60 full cycles of reversal, over and back, each second). This reversal pushes the current backward and forward through the wires connecting to this power source. The currents direction of flow alternates and so does its voltage.
When going from 12 volts to 240 volts, is the point that with higher voltage the…
When going from 12 volts to 240 volts, is the point that with higher voltage the power transfer proceeds with fewer particles?
Yes. If you use higher voltages, you can transfer the same amount of power with a small current of charged particles. The energy lost in the transmission through wires increases as the square of the amount of current through those wires so reducing that current is very important.