175
votes
Accepted
Why do fans spin backwards slightly after they (should) stop?
The overshoot behavior you noticed is called cogging and occurs when the magnet arrangement in the motor "catches" the rotating magnetic core of the motor during shutdown and jerks it back ...
- 95.5k
101
votes
Why do we use AC for long distance transmission?
The first point to make is: We don't always use AC. There is such a thing as high voltage DC for long-distance power transmission. However its use was rare until the last few decades, when relatively ...
- 1,111
63
votes
Accepted
Is there a physical limit to data transfer rate?
tl;dr- The maximum data rate you're looking for would be called the maximum entropy flux. Realistically speaking, we don't know nearly enough about physics yet to meaningfully predict such a thing.
...
- 4,650
56
votes
How do computers store sound waves just by sampling the amplitude of a wave and not the frequency?
"Amplitude" is the wrong word. The amplitude of a periodic function is the difference between its greatest value and its least value. Cross out "amplitude" from your textbook, and ...
- 15.2k
53
votes
How is data transferred between devices?
It is less of a "sending electrons," and more of jiggling them. Think of a wave, done by the crowd at a sporting event. One person raises their hands high, and then sits back down. The ...
- 50.2k
44
votes
How do headphones and earphones produce good bass if tiny speakers can't produce low frequency sounds very well?
There are a few reasons why small speakers have trouble creating bass.
Bass is directly proportional to the amount of air the speaker can
move. So you want a large cone that can move a large distance....
- 3,989
43
votes
How do computers store sound waves just by sampling the amplitude of a wave and not the frequency?
A slight clarification on Solomon Slow's complete answer:
If you sample the original signal frequently enough, the instantaneous values you measure at each of those tiny time slices will actually ...
- 95.5k
33
votes
Is there a physical limit to data transfer rate?
The Shannon-Hartley theorem tells you what the maximum data rate of a communications channel is, given the bandwidth.
$$
C = B \log_2\left(1+\frac{S}{N}\right)
$$
Where $C$ is the data rate in bits ...
- 471
31
votes
Does a Mobius resistor have zero inductance? How would you calculate the inductance?
The inductance can be calculated, but it is first necessary to look at the behavior at very fast timescales of a ns or so. Clearly the two faces of the strip form a transmission line and so, at short ...
- 589
30
votes
How do computers store sound waves just by sampling the amplitude of a wave and not the frequency?
Your instinct is correct, there are two degrees of freedom here so we have to measure two quantities. The one you are missing is the timestamp. We are measuring both the voltage of the signal and ...
- 507
29
votes
How do electrical devices limit the current flow from a socket?
Your home circuit does not "know" how much current to deliver to each socket or appliance. The circuit supplies a constant voltage, and it is then up to each appliance to limit the current ...
- 56.4k
27
votes
How is data transferred between devices?
I have drawn a diagram to illustrate the mechanisms of how telecommunications work. This is a highly simplified cartoon of what happens. What I’ll be describing is in part often described in Telecom ...
- 948
26
votes
Why do we use AC for long distance transmission?
The reason we use AC is that the AC voltage is easily changed using a transformer. To change DC voltage requires complex and inefficient circuitry.
Suppose you are sending some power $P$ from the ...
- 358k
26
votes
Why does an MRI machine or other EMP generating machine not damage humans, but it will fry computers?
Y'know that spark that jumps between your finger and a doorknob on a dry day in winter? That spark is enough to break down the "gate oxide" insulator between the gate electrode and the body ...
- 15.2k
25
votes
Accepted
Why is it easier to raise AC current to high voltage than DC?
Changing the Voltage of AC can be done with a simple iron core transformer. That's a simple device without moving parts that only consists of a magnetic core, copper wire and some isolation (...
- 866
21
votes
How do electrical devices limit the current flow from a socket?
There is already a good answer here, but I would like to add that a so-called "15A socket" is not so called because it "contains" 15A. The power grid and the wiring in your walls ...
- 15.2k
18
votes
Gravity train with the help of batteries?
Re: charging a battery on the trip to overcome the friction.
No.
Friction will always oppose the direction of travel. So it slows the train on the way down, losing energy. And it slows the train on ...
- 502
18
votes
Why does an MRI machine or other EMP generating machine not damage humans, but it will fry computers?
The trouble with circuits is that you have both highly conductive wires and highly resistive insulators, often very thin. When the magnetic field induces a voltage around a loop, the electric field ...
- 21.8k
15
votes
How is data transferred between devices?
All I really have to add to the other answers is to provide some context to complete the picture.
You probably know that when you send something (like a text, or a web request) over a network, it is ...
- 2,573
11
votes
Why do we use AC for long distance transmission?
This (from a now deleted page) clarifies how DC transmission lines are used for bulk power transmission:
Transmission Options
Power can be transmitted using either alternating current (AC) or ...
- 234k
11
votes
How do electrical devices limit the current flow from a socket?
Electric supplies to homes and industries are standardized in countries. This allows devices which are used in homes or in factories to be accordingly designed. For e.g. if the standard single phase ...
- 2,695
10
votes
Accepted
Can a television detector van REALLY tell if you have the TV switched on?
No, they were not lying to you.
Televisions depend on electronic circuits which resonate with the signal coming in so as to decode it.
TV receivers are also small antennas. The incoming signal ...
- 234k
10
votes
Accepted
How come a mobile phone signal is blocked by aluminium foil, but Wi-Fi gets through?
A matter of thresholds
The reality of spread-spectrum is complicated but let's imagine that the WiFi router and cell phone tower have both allocated 1 Watt to transmit to your phone, and it in turn ...
- 6,273
10
votes
How do headphones and earphones produce good bass if tiny speakers can't produce low frequency sounds very well?
It's actually because headphones/earphones are placed in directly contact with your ear and so the sound waves do not need to travel far. The intensity of sound decreases as $(\frac 1r)^2$, and so ...
- 29.9k
10
votes
Accepted
What do I hear when listening to a computer-generated sine wave?
To generate harmonics, you need a nonlinear element. Loudspeakers are not perfectly linear, so yes, they generate weak harmonics. Air is generally very close to a linear medium for sound unless the ...
- 21.8k
9
votes
Difference between live and neutral wires
Live, neutral,and earth, are labels that are used to convey some information on the use of each wire.
You are correct in thinking that in a typical two wire AC loop/circuit, both wires carry the same ...
- 2,533
9
votes
Accepted
Why do I lose signal in a train but not in a car?
Could it be simply that there are a higher density of signals within a train carriage and fewer phone masts to service them at once?
That is, you often have a dense crowd of people in a small space, ...
- 2,689
8
votes
What is the name of the shape of the iron core in a transformer?
This mathematical shape is a particularly simple example of a toroidal polyhedron.
- 51.7k
8
votes
Accepted
Why does my TV screen stay dust-free while other screens do not?
It is unlikely that the electronics of the device is responsible for that. The times of high static voltage in TVs are long gone. The only electronic thing I think could be responsible for that is ...
- 7,514
8
votes
What is the "spring" and what is the "mass" in a loudspeaker system?
I will slightly enlarge upon these answers as follows.
Since the cone is well-coupled to the air surrounding it, its effective mass includes the mass of some of that air. A speaker cone operated in a ...
- 95.5k
Only top scored, non community-wiki answers of a minimum length are eligible