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1. Do wires degenerate when electric current is passed through them? I mean, do they lose matter after continued use?

Wires made from metals all conduct electric current by the motion of electrons. Electrons move in (or out) of one end of a wire, and an equal number move out (or in) the other end. This balance is maintained by extremely powerful electric forces. No net amount of matter is ever transferred when current flows.

Two effects may occur during high current flow: 1) the wire may become overheated to the point that surface oxidation or even evaporation may take place, 2) at the connection points at each end of the wire, especially if the terminations are of a different type of metal than the wire, some atoms may migrate into or out of the wire.

Under normal current flow, there is no significant loss of matter in a wire. Only in the extreme case of overheating will the wire be degraded or even destroyed by melting.

The tungsten filament of an incandescent light is an example of a wire under extreme conditions. Eventually all filaments will fail due to the high temperature of operation. If the bulb is cracked, the filament will be exposed to oxygen and immediately be oxidized or 'burn out.'



2. The US Antarctic programme advised its researchers to take loads of extra batteries when they visit, because batteries run down faster in cold weather. But I can't figure out why this is. Surely chemical reactions slow down in cold temperatures. Could it be the reason?

ELECTRIC current is produced by a battery when a connection is made between the positive and negative terminals via a piece of electrical equipment. The current is the result of a chemical reaction in the battery, and all such reactions proceed more slowly at low temperatures. It follows that if a battery is used at a low temperature, less current is produced at a given state of charge than at a higher temperature. So as batteries run down they more quickly reach the point at which they cannot deliver enough current. If the battery were exposed to a higher temperature again it would operate satisfactorily.

When a battery is not used, it will go flat as a result of leakage between the two terminals. This reaction is also temperature dependent and proceeds more slowly at low temperatures. This can be seen in rechargeable batteries, which go flat in about two weeks at normal ambient temperatures, but last more than twice as long if kept in a fridge.



3. A lightning bolt recently struck the ground near my home, local telephones were damaged, fuses were blown and computers burned out, but other strange things happened. Some television sets switched on spontaneously and showed perfect pictures, yet those that were switched on normally after the strike had an odd color shift. Red became green, white became yellow and green became blue. If you switched them off for a minute, then on again, the problem was cured. Why did this happen? It might not be significant, but at the time of the strike, burglar alarms sounded spontaneously.

The peculiar results of the lightning strike can be explained by a mixture of electromagnetism and economics.

A TV picture is drawn on the screen by an electron beam. The beam is steered across the glass by two magnetic fields for horizontal and vertical positioning. It is translated into a visible glow by phosphors coated on the inside of the screen. The more intense the beam, the brighter the glow from the selected pixel.

Lightning strikes involve peak currents of approximately 100,000 amps. This abrupt current makes a strong magnetic field, which apparently magnetised the steel in your TV. The effect of having a magnetised TV is to add a constant offset to the steering of the electron beam. This might cause the beam to miss the spot intended and land on an adjacent site.

Each pixel on the TV is divided into three, one third for each primary colour, so along each horizontal line you could have blue-red-green, blue-red-green. In this case a shift of one place to the right would send red to green, and green to blue. White to yellow could be explained because the blue phosphors are relatively inefficient compared with the green and red phosphors. An electron beam capable of generating a bright green would only produce a dim blue, and the overall white would be tainted yellow.

The reason that your TV contains steel is economic because steel is comparatively cheap and easy to manufacture compared with non-magnetic counterparts. Because many things can magnetise your TV, manufacturers include a "degaussing" coil that demagnetises the bodywork each time the TV is turned on by producing a transient oscillating magnetic field. The "thunk" noise you hear when you turn on a TV is this degaussing. The TVs' colour returned to normal if they were switched off and switched on again after the strike because they automatically demagnetise themselves at power-on, wiping away the lightning's effect.



4. Walking through the countryside last weekend I noticed the overhead power lines were emitting a very loud hum. What causes this noise in high-voltage cables?

The hum is the result of the alternating electric field affecting water on the surface of the power line.

Possible mechanisms include vibration of the water surface caused by the field, spitting from the tips of water droplets (corona discharge), or the rapid expansion of the air at the tips, due to heating.

While the details of how the noise is actually produced are not clear, we do know a good deal about what affects the level of noise. Scientists have measured the spectrum of the noise, the effect of rain and the age of the power line, as well as the voltage and the design of the line.

When an aluminium line conductor is new, it is shiny and slightly greasy. Water beads up on it due to surface tension, but the shape of a water drop is also affected by the local electrical field, which can be fairly close to the ionisation point of air--about 3 megavolts per metre. The water drops have been seen to assume pointed, conical shapes because of the field, and their sharp points strengthen the field locally, causing the breakdown of air molecules at the tips.

With a new conductor and a strong field at the surface, the amount of rain has very little effect on the noise. However, if the conductor is several years old, its surface will be pitted and dull. Instead of beading on the surface, the water penetrates the bundle of wires. So rather than having water beads all over the surface, the conductor looks smooth. At a low surface field strength and in very light rain, the noise is much quieter than for a new conductor in the same conditions.

However, the noise level is very variable, and depends much more on the rainfall rate. The heavier the rain, the louder the noise, which doesn't stop getting louder until the rain is very intense. Its spectrum includes higher frequencies, so it sounds more like frying.

But water cannot accumulate indefinitely inside a conductor. Wires seem to absorb water up to a critical level, and then release it suddenly. I have actually witnessed jets that were more than a metre long emerging from the bottom of an aged conductor, though this process is apparently silent.



5. Considering the enormous amount of electrical energy contained in a lightning strike, why has no one tried to tap it for domestic use? Lightning energy is grounded by sending wired rockets into clouds, so could this energy be used?

It might sound like quite a nice idea, but there are several reasons why the answer is no. The first stems from the confusion of power with energy. Lightning strikes have a lot of power, but very little energy.

Although a typical lightning strike can have a power of several gigawatts, it lasts for only a fraction of a millisecond. Because energy = power x time, the energy contained in a lightning strike is in fact a mere few megajoules, which compares poorly with the 33.6 megajoules released by burning a litre of petrol.

The second problem is the unpredictable and intermittent nature of lightning. And even if you covered the whole of the British countryside with lightning rods, you would still be faced with the problem of converting 5000-ampere direct current DC pulses lasting less than half a millisecond into the 230-volt, 50-hertz alternating current supply we use in our homes.



6. The oil refinery near my home sports a constantly burning flame. Other refineries around the world do the same. Why is this energy apparently wasted and why can't it be used as a power source?

In the early days of the oil industry, gas was a waste product and was just flared off. Nowadays, however, gas is a valuable commodity and is rarely thrown away. The flare your correspondent sees is only burning a very small proportion of the gas produced in the refinery, and is there as a safety measure.

The plant contains reaction vessels, pipes, compressors and other equipment, all full of high-pressure gas. In an emergency this gas becomes a very real danger and safety valves will be operated by emergency shutdown systems to release it. But, just releasing the gas into the atmosphere would create as great a danger as the system is trying to prevent, and so these valves are connected to the flare.

The flare is kept continuously burning so that, in an emergency, the released gas would be ignited safely and burnt as it is released. Given this requirement, the flame is kept as small as possible.


Why does an electric bulb need to be vacuumed? Can we light up a bulb in the space without enclosing in the glass bulb?

An incandescent light bulb contains a thin wire filament that glows hot when an electric current is run through it. In the presence of oxygen, the filament would oxidise and burn up as a result of the high temperature. This occurs within a split second of switching on the bulb. So a glass bulb is used to cover the filament and extract the air away first, then keep oxygen away from the filament.

However if there is vacuum only and nothing else, the atoms of the filament will evaporate away as soon as it gets even the slightest energy. So to prevent these atoms from escaping the metal surface, they have to be subjected to high pressure. This time it should be some gas which does not support oxidation. So, any gas that inhibits combustion will work.

By the way, even without oxygen and with other gases enclosed under pressure, the filament eventually deteriorates as its atoms dissipate. The presence of a halogen gas actually inhibits this deterioration, allowing higher filament temperatures and brighter light bulbs. So the glass globe can also help enhance a bulb's capability.



8. I have noticed that household light bulbs sometimes develop a high-pitched hum before the filament fails. Can anyone suggest a mechanism for the humming?

When a filament breaks for the first time, the two severed ends may be so close together that electric current can still flow (arc) across the gap. So, the bulb will still function. However the path of the current now is very zigzag compared to the initial smoother path through the filament itself. So the disturbances produced by such randomly flowing and randomly striking electrons produces sound, which is being referred to as "Humming".

Eventually the arcing erodes the severed ends of the filament and the bulb ceases to function.



9. How fast do electrons travel when moving as an electrical current through copper wire?

The actual velocity of electrons through a conductor is measured as an average speed called drift speed. This is because individual electrons do not continue through the conductor in straight line paths, but instead they move in a random zig-zag motion, changing directions as they collide with atoms in the conductor. Thus, the actual drift speed of these electrons through the conductor is very small in the direction of current.

For example, the drift speed through a copper wire of cross-sectional area 3.00 × 10-6 m2, with a current of 10 A will be approximately 2.5 × 10-4 m/s or about a quarter of a millimeter per second.

So how does an electrical device turn on near instantaneously? If you think of a copper wire as a pipe completely filled with water, then forcing a drop of water in one end will result in a drop at the other end being pushed out very quickly. This is analogous to initiating an electric field in a conductor.



10. When someone gets electrocuted what kills him or her? Is it the current or the voltage?

The simple answer is that itís the current. The current passing through your body has lots of gruesome effects that were used for executions on the electric chair in America. Because itís not very easy for electricity to get through a human body, the current has to work hard. This heats up the body, literally cooking it.

However most deaths associated with electricity happen because the electricity upsets the way the heart beats. Heartbeat is controlled electrically and stray electricity messes up the very delicate balance needed to keep your heart beating steadily. If there has been too much current for too long the heart can't get its act together again and goes into 'ventricular fibrillation' a sort of fluttering that is useless at circulating blood. Without blood your brain gets starved of oxygen and you die.

High voltages are more dangerous than low ones because they can drive a high current through your body. However some sources of high voltages are not dangerous. Devices such as a school Van de Graff generator or the spark used to light gas on some cookers can generate several thousand volts but there are relatively few electrons involved so only a small current can flow for a very short time.



11. Why donít we consider drift velocity of the positive ions in a conductor?
As the positive ions are heavily and tightly bound in the conductor, their movement due to force of applied electric field is nearly negligible. So, we don't consider drift velocity of the positive ions in a conductor.



12. It is dangerous to operate electrical appliances with wet hands. Why?

As our body behaves as a resistor having variable value of resistance. When we touch the electrical appliances by our wet hand then the resistance of our body decreases which results in the flow of large amount of current through our body than touched with our dry hands. Since wetness decreases the value of resistance and increases the rate of flow of current in our body according to the Ohm's law as given below:

where, I is the current flowing through resistor or resistance R and V iis the potential difference between the circuit.

Voltage remaining constant:

13. Why should an ammeter have low resistance?

As the purpose of ammeter is to measure the current flowing through the circuit, it is connected in series with the circuit. If the resistance of the ammeter is high then the net or effective resistance in the circuit is the sum of the resistances of the circuit and ammeter. This increases the value of the equivalent resistance of the circuit resulting in the decrease of the current. Hence, measured current will be less than actual current which cause the default result. Hence, an ammeter have low resistance.


14. Why should a voltmeter have high resistance?

The voltmeter is connected in parallel with the part of the circuit of which the potential difference is to be measured. If its resistance is low then the current of the circuit is distributed to both of the braches. Hence, measured p.d. of the circuit will be less than actual p.d. Hence, a voltmeter has high resistance.


15. Is Ohmís law applicable to all the conductors?

No, Ohm's law is applicable or valid for only ohmic conductors like metal, electrolytes etc. But it is not valid for non-ohmic conductors like semiconductors, ionized gases.


16. Can the resistance of metals become zero?

Generally, at higher temperature the resistance of metals can not be zero. But, the resistance of metal may become zero at very low temperature. The state of the metal in which the resistance becomes zero is called their super conducting state.


18. What are the values of current and p.d when a galvanometer reads zero?

At null point the galvanometer shows no deflection i.e. it reads zero. Hence, the value of current becomes zero as well as the value of potential difference becomes zero at the null point according to the Ohm's law:

where, V is the potential difference, I is the current flowing and R is the resistance of galvanometer.


19. Which has higher overall resistance, an ammeter or a galvanometer?
A galvanometer has higher overall resistance than an ammeter. As the ammeter is made from the galvanometer by adding one resistor having some resistance in parallel with the load. Hence, due to this arrangement the equivalent resistance of ammeter decreases due to which a galvanometer has higher overall resistance.
20. The electron drift speed is very small but an electric bulb lights up instantly when switched on, why?
The electric bulb lights up instantly when switched on because the electric impulse reaches the bulb instantly. The velocity if propagation of electric impulse is the same as that of the light.


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