Author Archives: Tech Time
Author Archives: Tech Time
FROM the baggage drop to the security line to the boarding gate, just getting through the airport these days can throw pitfalls that you never saw coming.
Here are nine typical airport mistakes you may be making, as well as some expert tips on making it out of the airport, and onto your plane, with as little hassle as possible.
1. NOT DOWNLOADING YOUR AIRLINE/AIRPORT APP
Using your carrier’s app is important not just at the airport, but before you get there, too. Most carriers have apps you can download on your smartphone that will alert you if your flight is delayed or cancelled, even before you leave for the airport.
Once there, the information on the app is often more up-to-date than the arrival-departure screens in the terminal. More and more airports have developed apps that help travellers navigate the terminals with maps, lists of services, etc. One particularly useful app is the US-based GateGuru, which covers more than 200 airports and, among other features, allows users to rate shops and restaurants as well as offer insider tips — it’s a Yelp for airports.
2. NOT CHECKING IN ONLINE
I was flabbergasted recently at the line snaking up to the ticket counter — just to check in. (And there were even check-in kiosks.) Unless you have some kind of problem that can’t be resolved ahead of time, there’s no good reason for not checking in online.
Just have the ticket sent to your phone (via text or email link), and if you don’t have any luggage to check, you can skip the counter and head straight to the security line. (If you have luggage you’ll need to drop it off, but if you’ve checked in beforehand, this goes quickly.) Also, some airlines only let you choose a seat when you check in; if you’re flying one of these, you’ll want to check in and choose your seat as soon as possible within the check-in time (usually 24 hours).
3. NOT BRINGING FOOD WITH YOU
It’s no secret that airport food, whether from a grab-and-go vendor or a sit-down restaurant, comes with a hefty price tag — and the only value-add is convenience, usually not quality.And that’s not the only reason to pack a snack in your carry-on luggage: If you get held up in the security line and get squeezed for time, a sandwich, chips, cookies, and fruit in your carry-on can save the day. Most food is allowed, except for liquids like salad dressings, soups, yoghurt, etc. If in doubt, check the your airport’s website for prohibited food items.
4. WEARING THE WRONG CLOTHING
I don’t just dress for comfort on the plane, I strategically dress to get through the line faster too. There will be times when those lines are closed and you end up in the regular lines, unpacking your laptop, taking off your shoes and belt, and digging out the liquids.
It pays to play it safe if at all possible. That means eliminating anything that could set off alarms when going through the body scanner, like chunky jewellery or a belt. Keep your footwear simple, too, with shoes that are quick and easy to get on and off (and don’t forget socks!).
5. NOT TAKING ADVANTAGE OF COURTESY CHECKED BAG AT THE GATE
If you’re a travel warrior who never checks a bag, this isn’t for you. But if you have to check a bag (that is, you’ll need to go to baggage claim anyway), you can often check your carry-on at the gate for no extra charge. I’ve run across this numerous times, especially on domestic flights that are full and when overhead space is at a premium.
Usually the gate agent will make an announcement asking for volunteers to check their carry-ons, but I’ve asked and been given the OK. I just make sure the things I need on the plane can fit in a bag under my seat, and I have one less bag to carry around — particularly helpful if you have a connecting flight and don’t want to lug it around the airport.
6. NOT PLAYING NICE
It’s not a matter of if, only when: you’re going to need someone’s help. It could be a problem of your own making, or the airline’s, or a force majeure, but it almost never pays to be angry, indignant, or whiny. Patience and a smile goes a long way when it comes to increasingly harried gate and flight attendants, security staff, and even your fellow travellers.
I’ve seen overweight bags given a pass (no punitive fee), seats changed, and special favours accommodated clearly because someone asked nicely. And even if you don’t end up getting what you want (or need), you know you went about it the best way possible.
7. NOT BUYING A PASS TO THE AIRPORT LOUNGE
If you’re not an elite flyer, or aren’t enrolled in a credit card that offers this perk, the world of airport lounges can seem like a pricey, exotic indulgence. But there are occasions — most notably if you have a long international layover — that it’s worth buying a day pass to your carrier’s airport lounge.
Not long ago I had a seven-hour layover in San Salvador, El Salvador, and I happily coughed up the $US25 fee just to have a quiet place to rest. It also included free Wi-Fi, surprisingly good food, and a generous array of beverages, including liquor. Not paying for food and drinks offsets a good part of the charge.
8. SENDING PERSONAL INFO OVER AIRPORT WI-FI
Thankfully, more airports are acknowledging that free Wi-Fi isn’t just a convenience for travellers, it’s a necessity. And that’s a good thing.
But never forget that “free” doesn’t mean “safe”. Public Wi-Fi networks aren’t secure, so whatever you do, don’t type in personal information — passwords, IDs, etc — or you could return from your trip only to find your Facebook has been hacked and your bank account drained.
9. NOT MARKING YOUR LUGGAGE
You’ve been there, done that, and now you’re almost home. All that’s left is to grab your luggage from baggage claim. And one by one here they come, an endless stream of suitcases that look more or less the same. Save yourself the hassle of looking at each bag as is it goes by marking yours with a brightly coloured tag (mine’s bright orange).
Not only will it have your contact info should — gasp — your luggage go missing, but if all goes as planned you’ll be able to spot yours in a quick second and on your way you’ll go.
Once upon a time, jetting off on an airplane used to be the preserve of the super-wealthy.Nowadays, thanks to relatively recent rise of budget airlines, increasingly competitive airfares and more flight paths, experiencing an overseas holiday has never been easier.
As a result, many of us probably think we know exactly what to do at check-in , as well as what to expect at 35,000 feet.But as with any work environment, there’s a lot more going on behind the sceneswhich we don’t know about. Flight attendants and airline employees shared some fascinating insights on Quora , revealing plenty of things most airline passengers are unaware of.
A mixture of the gross and the enlightening, they’ll (hopefully) change the way you fly. At the very least, you’ll certainly think twice before putting on a pair of headphones.
1. Music to the ear
Watching a film, listening to some music or simply drowning out the noise around you may seem like a great way of passing the time, but maybe ask for some new headphones before you tune out.Apparently, many “airlines who provide headphones hardly or even NEVER bother to replace the foamy-like ear parts.
“So please ALWAYS tear them once you use them just to make sure that they will replace them.”And the same applies to any pillows or blankets you’ve been handed for the flight. Ask for a new set if you can.
2. Working hours
“We are not paid during boarding or until the door to the aircraft is shut. This means it’s mandatory to show up to work about two hours early but not be paid for it.”
Another flight attendant pitied anyone having a water-based drink.“The water from the plane is disgusting. I feel truly sorry for our coffee and hot water drinkers. That water is in a tank under the plane and I’ve never seen that tank be cleaned out.”Another one even revealed the plane water was referred to as “Galley Springs”.Delightful.
4. Toilet break
No one likes the toilets on planes. The constricted space, the unflattering lighting. But even flight attendants avoid them if possible.One fight attendant admitted, “To be honest I only use the lavatories on the airplane if I need to wash my hands or if I absolutely have to go to the bathroom.“If it’s a short flight and I can leave the plane, I’m definitely waiting until I can use the bathroom in the airport.”
And Kelly Goodnuff added, “[…] Just how dirty the floor is. We think it’s disgusting when you do not wear shoes in to the toilet.”
5. Kindness pays
Being a good Samaritan and swapping seats can result in some VIP treatment according to Fatihah Sudewo.“If you’re one of those who have a big heart in giving up your seat for whatever reason without making a scene, we treat you ‘special’.
“We would give you two bread rolls instead of one, a whole can of soda instead of rationing it, basically we would compensate you for your kindness.”No mention of an upgrade to first class though.
6. DON’T ask us to help in the following scenarios
An anonymous user revealed the following: “We have to be diplomatic in situations to avoid discrimination lawsuits.“For example, that man that’s overweight and spilling into your seat on a full flight? I can’t tell him to suck in his gut or ask a thin person to switch with you.“However, you can, because the worst that can happen is you’ll get a no and maybe a glare.
“If I do, I am risking a possible lawsuit against my airline or at worst, my job.“With the age of social media, a lot of things get twisted. I never want to be the flight attendant that ‘harassed a mother and her crying baby’ or ‘not let allow a man with a medical condition sleep’ because he snores.
7. Perk of the job
Fatehah also admitted, “Our meals are slightly better than the passenger meals, and even though it depends on the airlines, we also get fresh fruits – like whole fruits and not the cut ones, pickles, bread rolls, desserts, beverages. “Basically we have at least a trolley dedicated for the crews.”
8. Getting tipsy
That free booze on the trolley? Go easy on it. According to more than one flight attendant, many passengers don’t realize how much more drunk they get at 35,000 feet, owing to the altitude.
And if you’re getting noticeably drunk or out-of-hand, the flight attendant is responsible for managing the situation.“Sometimes if we think you’ve had too much to drink, we’ll serve you, but not serve the whole mini-bottle of booze,” confesses one user named Ellen. ”“We may just dip the rim of the glass in enough vodka or gin and fill the rest with mixer.”
9. Missing a flight attendant?
Another anonymous user also explained an important point as follows.“If your boarding is delayed because they are missing a flight attendant and you see a flight attendant rushing on to the plane, that flight attendant is most likely not the flight attendant that caused the delay.
“At airports we have standby flight attendants (one or two at a time) who are dressed in uniform, bags packed and ready to go if a flight needs them or reserve flight attendants who get short notice to cover a trip when another co-worker cannot make it.”
“Please set your portable electronic devices, including any mobile phones, to flight mode.”
It is a plea heard by passengers at the beginning of thousands of flights around the world each day, and an instruction the vast majority comply with, even if they’re not 100 per cent sure why.
The general assumption is that we must disable our mobile phones – some more begrudgingly than others – as their signal interferes with navigation instruments, and could even cause a crash. But is that really true?
Technically yes, says pilot and author of Cockpit Confidential, Patrick Smith. But it’s more an exercise of caution.
“It depends on the gadget and how and when that gadget is used,” he said.
Taking the example of laptops, Smith says, though an old computer can emit harmful energy, the greater risk they pose is becoming “high-speed projectiles during a sudden deceleration or impact”.
But for phones, which is at the crux of a businessman or keen tweeter’s journey, Smith says: “Can cellular communications really disrupt cockpit equipment? The answer is potentially yes, but in all likelihood no, and airlines and the Federal Aviation Administration (FAA) are merely erring on the better-safe-than-sorry side.”
He continues: “Aircraft electronics are designed and shielded with interference in mind. This should mitigate any ill effects, and to date there are no proven cases of a phone adversely affecting the outcome of a flight. But you never know.”
A mobile phone’s potential to interfere does not just exist when it is being used, but also when it is dormant, which is why the flight mode must be activated even if passengers don’t intend to their phone.
“Phones do have the potential to interfere with the plane’s instruments, but only to a small degree”
But Smith reckons that despite the clear request at the beginning of each flight, “at least half of all phones, whether inadvertently or out of laziness, are left on during flight”. He adds that if mobiles were that great a concern, the policy would be more actively enforced.
However, there are at least two serious incidents in which mobile phones have been implicated: first, the unsolved crash of a Crossair plane in Switzerland in 2000, when spurious transmissions confused the autopilot, and second, a fatal crash in Christchurch, New Zealand in 2003. But these are the extremes.
A blog by a pilot on airlineupdates.net claims that the interference caused by mobile phone signals registers on the headsets of the flightdeck, in the same manner that one might have encountered on speakers affected by a nearby mobile: “dit d-dit d-dit d-dit…”
“I actually heard such noise on the radio while flying,” wrote the author.
“It’s not safety critical, but is annoying for sure. Of course, there is plenty of attenuation between phones in the cabin and the pilots’ radio. However, if say 50 people on board are inconsiderate enough who can’t be bothered to switch their cell radio off, there will be 50 phones constantly looking for cell towers at maximum power. That is a lot of radio pollution.”
So, in a technical sense, phones do have the potential to interfere with the plane’s instruments, but only to a small degree.
However, as Smith points out, there are also social implications.
“The minute it can be proven beyond reasonable doubt that phones are safe, a percentage of flyers will demand the right to use them, pitting one angry group of travellers against another, with carriers stuck in the middle,” he said.
“The airplane cabin is a last refuge of relative silence. Let’s keep it that way.” We’re inclined to agree.
Capacitance (symbol C) is a measure of a capacitor’s ability to store
charge. A large capacitance means that more charge can be stored.
Capacitance is measured in farads, (symbol F). However 1F is very
large, so prefixes (multipliers) are used to show the smaller values:
μ (micro) means 10-6 (millionth), so 1000000μF = 1F.
n (nano) means 10-9 (thousand-millionth), so 1000nF = 1μF.
p (pico) means 10-12 (million-millionth), so 1000pF = 1nF.
In a way, a capacitor is a little like a battery. Although they work in
completely different ways, capacitors and batteries both store
electrical energy, inside the battery; chemical reactions produce
electrons on one terminal and absorb electrons at the other terminal.
A capacitor is a much simpler device, and it cannot produce new
electrons – it only stores them.
Like a battery, a capacitor has 2 terminals. Inside the capacitor, the
terminals connect to 2 metal plates separated by a dielectric. The
dielectric can be air, paper, plastic or anything else that does not
conduct electricity and keeps the plates from touching each other.
The plate on the capacitor that attaches to the negative
terminal of the battery accepts electrons that the battery is
The plate on the capacitor that attaches to the positive
terminal of the battery loses electrons to the battery.
Once it’s charged, the capacitor has the same voltage as the battery
(1.5 volts on the battery means 1.5 volts on the capacitor). For a
small capacitor, the capacity is small. But large capacitors can hold
quite a bit of charge.
Here you have a battery, a light bulb and a capacitor. If the capacitor
is pretty big, what you would notice is that, when you connected the
battery, the light bulb would light up as current flows from the battery
to the capacitor to charge it up. The bulb would get progressively
dimmer and finally go out once the capacitor reached its capacity.
Then you could remove the battery and replace it with a wire.
Current would flow from one plate of the capacitor to the other. The
light bulb would light and then get dimmer and dimmer; finally going
out once the capacitor had completely discharged (the same number
of electrons on both plates).
The unit of capacitance is a farad (symbol F).
A 1-farad capacitor can store one coulomb (Q) of charge at 1 volt (V).
A 1-farad capacitor would typically be pretty big. So you typically see
capacitors measured in microfarads (millionths of a farad).
These sub units are:
farads 1microfarad( F)10 F also 10 F 1nano Farad
1 6 9
microfarads picofarad pF F 12 1 ( )10
There is a direct relationship between the Voltage (V) placed across
the plates of a capacitor and the charge (Q) held by them. If the
voltage is doubled the charge is doubled, if the charge is halved then
the voltage is halved etc. This tells us that the ratio of charge to
voltage is constant and this is known as the capacitance (C) of the
Earlier it was demonstrated the existence of internal resistance in the
power supply such as in a battery, and the effect that this resistance
has on the voltage supplied to the load was discussed.
The load voltage is the actual voltage given out by the power supply
after it has dropped a percentage of its EMF voltage across its
How much voltage is dropped across the internal resistance depends
on the value of the internal resistance in relation to the value of the
The relationship between the values of load resistance and internal
resistance is also important for another reason. Maximum power can
be developed in a load resistance only when the values of the load
resistance and the internal resistance of the source are equal.
This statement is known as the maximum power transfer theorem.
Figure shows a 12V EMF source of internal resistance 3 ohms
connected to a load resistance of 1 ohm. The total resistance in the
circuit is 4 ohms and the circuit current is therefore 3 amperes. The
power developed in the load (I2R) is therefore 9 watts.
Figure B shows the same source connected to a load resistance of 3
ohms. The total resistance is now 6 ohms and the current 2
amperes. The power developed in the load is now 12 watts.
Figure C shows the effect of inserting a load of 9 ohms. The total
resistance is now 12 ohms and the current 1 ampere. The power
developed in the load is now 9 watts.
The above examples have used the power formula I2R, but any of the
2 formulae, V2/R and I x V could be used.
Example ‘B’ using V2/R would give the same answer by measuring
the volts drop across the load resistance and then dividing the square
of that by the actual load resistance. Try it, it works!
The graph shown below shows these and other results by plotting the
power developed in different values of load resistance. It shows that
maximum power is developed in the load only when the load
resistance is equal in value to the internal resistance of the source
and, thus, illustrates the maximum power transfer theorem.
In many circuits we are interested in transferring the maximum
possible amount of power to a load circuit. To do this we must
‘match’ the load resistance to the internal resistance of the source.
Matching is very important in electronic circuits that usually have a
fairly high source resistance. A typical example is the ‘matching’ of
an audio amplifier to a loudspeaker and we shall consider this and
many others later in the book.
Note however that batteries, generators and other power supply
systems cannot be operated under maximum power transfer
conditions. It can be seen from the previous Fig that to do so would
result in the same amount of power being dissipated in the source as
was supplied to the load. This is obviously extremely wasteful of
energy and power supply systems are always designed to have the
minimum possible internal resistance to minimize losses.
All cells contain an internal resistance caused by factors such as the
plate material and size etc. In primary cells this resistance is quite
high, with secondary cells due to the plates having a much larger
cross sectional area compared to primary cells the internal
resistance is considerably lower.
When cells and batteries are connected together the internal
resistance has to be taken into account so as to determine the
output characteristics, this is achieved in the following ways:
In order to obtain a high voltage from any cell type it is necessary to
connect the cells in series. In this arrangement the voltage from
each cell is added for example 6, x 2volt cells connected in series
will give an open circuit terminal voltage (EMF) of 12volts. The
disadvantage of a series arrangement will be that the internal
resistance of each cell is also added, this results in a high EMF
voltage but low current output, therefore an increase in current
drawn from the cell will result in a drop in the output voltage due to
the high internal resistance.
In this arrangement all the cells will be required to be of the same
hour rating, as this type of connection will take the lowest
ampere/hour rated cell as its output.
If cells are connected in parallel this will have the effect of the individual
cell voltage being the EMF voltage, due to the large surface area of the
all the cathode plates being connected together, and also all the anode
plates being connected together, the internal resistance is greatly
reduced, in this arrangement the current from each cell is added
together, therefore the current capacity from a parallel connected group
of cells is greatly improved, and as the EMF voltage is the same as the
cell voltage it is considerable more stable when a load is connected to
In this arrangement the cells ampere/hour rating does not need to be
considered, however the voltage rating of each cell needs to be equal
as this type of connection will reflect the lowest voltage of cell as its
From the 2 methods of connection above, it would make sense to
connect all cells in a battery in a series/parallel configuration, this
would provide the battery with a high voltage and also a high current
capacity with an overall lower internal resistance, in this
arrangement the power obtained from the battery would be at its
EMF (Terminal Voltage) and internal
Internal resistance in batteries is mainly due to the resistance of the
electrolyte and the cross sectional area of the plates. The voltage
that is measured across the terminals of the battery when it is off
load is known as the EMF voltage. When a load is connected to a
battery the current flows through the internal resistance and causes
a voltage drop proportional to this resistance, if the internal
resistance remains constant then the fall in the terminal voltage will
be proportional to the load current.
With switch “s” open, the Voltmeter V reads EMF (off load voltage).
With switch “s” closed, the Voltmeter reads Pd (the volt drop across
the load or on-load voltage).
There are several types of alkaline batteries, so to differentiate the
types, the metal used in the manufacture of the plates usually gives
them their name, for example the most common in use on aircraft is
the Nickel/Cadmium battery as it is these 2 metals that are used for
the plates. However in all alkaline batteries the electrolyte solution
used is Potassium Hydroxide which has a specific gravity of 1.24 to
1.30, dependant on the size and material used for the metal plates
this will determine the value of EMF of the cell, with some cells being
as low as 1.2volts.
The cells in these batteries are arranged in steel containers and fitted
with safety valves, this is due to the fact that the cells can be charged
at a high rate – and so can be easily overcharged, this overcharge will
make the electrolyte “bubble” gas and create a large amount of heat,
which can if not dealt with cause the battery to go into a state of
thermal meltdown, Under normal conditions these batteries require
very little maintenance and only a periodic capacity check.
These type of batteries are constructed similar to the semi sealed type,
but when on charge they are deliberately allowed to “gas”, this gassing
is allowed to vent to atmosphere, this aids the temperature control of
the cells and indeed is used to monitor the battery charge by switching
the battery charger on or off dependant on the temperature reached by
Due to the cells being allowed to gas during charge the recharge time
is considerably shorter than the semi sealed type however this has the
disadvantage that the electrolyte requires “topping up” more often, the
frequency of this topping up will be determined by the aircraft manual.
The cells are fitted with the same type of vent as the lead acid battery
in that they allow the electrolyte gasses to escape but not the liquid.
Nickel Cadmium Alkaline
This is a common type of alkaline battery; the plates are formed on
a woven wire mesh by heating, the cathode plate is formed by using
Nickel salts and the anode by using Cadmium salts, a separator of
nylon cloth and a gas barrier of cellophane is used between them,
the electrolyte has a specific gravity of 1.3 and consists of
Potassium Hydroxide and distilled water.
The chemical reaction of alkaline batteries works on the same principle
as the lead acid battery:However as can be seen, the difference is that the electrolyte does not
change its specific gravity or chemical structure during
charge/discharge, it is used purely as a medium in which the electrons
can flow through between the plates. As the charge of the battery
cannot be determined from the electrolyte specific gravity as in the lead
acid type, the temperature of the cell has to be monitored to determine
the charge state, for this reason, switching on or off the charge supply
to maintain the correct ampere/hour rating
These batteries use an impact and acid resistant case for each
individual cell, which is made from polystyrene compounds. Each
cell case is moulded so that they provide outlets for the terminal
posts so that each cell can be connected with ease to the adjoining
cell, the case also houses a vent valve, which whilst the battery is
being charged allows gases to escape but does not allow the
electrolyte to leak out, the method of connecting the cells varies
dependant on the required use of the battery, and will be discussed
Lead acid batteries consist of cells with an electrolyte made of
sulphuric acid and water mixed to a specific gravity of 1.270 for a
fully charged battery, only distilled water is used in this mix, as the
impurities found in normal tap water will reduce the life and charge
of the cells.
As the cells are discharged and charged, the level of electrolyte will
decrease and so periodically the battery cells will require a “top up”
of distilled water, if this action is required it is to be noted that the
acid is always added to the water, as the reverse procedure is
extremely dangerous, because the water will react violently with the
acid and literally explode when added.
A fully charged cell will have a voltage in excess of 2.5volts after its
charge this will drop to 2.2volts after an hour standing, during
discharge the cell voltage will drop to 2volts and will remain at this
level for the majority of the cell charge life after this time, which is
dependant on the load connected to the cell, the voltage of the cell
will drop to 1.8volts and is considered to be discharged. Because the
voltage remains at a constant 2volts for the vast majority of the time,
this is considered to be the cells nominal voltage value.
The definition of capacity in terms of batteries is the quantity of
electricity that can be taken from a fully charged cell at a specified
discharged rate measured in amps, before the cells nominal voltage
of 2volts drops to a defined level of 1.8volts. Battery capacity is
therefore measured in terms of current and time i.e. ampere-hours,
and is expressed as a percentage against the maximum available
amps/hours for that specific type of cell. The factors, which affect
the battery capacity, are the area and number of plates, strength of
electrolyte and temperature.
/ discharge voltage versus time
It can be seen that during the discharge the voltage of each individual
cell remains constant for a considerable time at approximately 2volts,
however this is only true if the load connected has a small current
draw, If a larger current is drawn from the cell the discharge will
become more linear as the voltage drops more rapidly. What can also
be determined from the graph is that the voltage value cannot be used
to determine the amount of charge in the battery, consider this graph.
Electricity can be generated in 6 basic ways. These are by:
Magnetism and Motion.
We shall look at these methods briefly.
Chemical Action (Cell)
This is by use of 2 different metals placed in a liquid called an
electrolyte. We call this a cell and by grouping these cells we
produce a battery.
Friction (Static Electricity)
When a comb is passed through hair it acquires an electrical charge.
If a piece of tissue paper is then held close to the comb it is attracted
towards it. These effects are due to static electricity (electrostatics).
Aircraft in flight build up large amounts of static electricity and acquire
a charge potential much greater than that of the atmosphere. This
charging of the aircraft is undesirable but unavoidable, although the
effects can be minimized.
Pressure (Crystal Controlled Oscillators)
Certain crystalline substances, notably quartz, exhibit a piezoelectric
effect which results in PD appearing between the opposite faces of
the crystal when it is mechanically deformed and vice versa. The
Crystal can be shown to
Light (Photovoltaic Cells)
These devices utilize the energy from a light source to produce
These devices can be used power battery chargers when connected
as solar panels.
At the point of contact between 2 different metals there exists an
electrical potential difference, which depends on the temperature of
the junction. When we complete the circuit with a second junction at
a different temperature, a current flows in the circuit.
This thermoelectric effect is called the Seebeck effect after the man
who discovered it, and the junction is called a ‘thermocouple’. Either
the net EMF in the circuit or the resulting current may be used to
Primary cells are called so, as the chemical reaction used to
produce the output voltage is extremely difficult, or impossible to
reverse i.e. they cannot be recharged, an example of this is the dry
primary cell, a standard type of dry primary cell is the Zinc carbon
cell, these are constructed by having 2 poles, one of them being a
carbon rod which is the + ve (cathode) and the other pole being
made of zinc, this being the –ve (anode) connected to a steel disc at
the bottom of the cell to aid connection to a circuit. In a dry cell the
zinc pole also acts as the container and surrounds the carbon rod,
and also holds the electrolyte, which is constructed from ammonium
chloride (sal-ammoniac). With this basic arrangement there is a
very distinct disadvantage in that as the cell is being discharged the
chemical reaction that takes place produces hydrogen bubbles
which accumulate around the carbon rod and effectively insulate it
from the electrolyte. To overcome this reaction – known as
polarisation, manganese dioxide is added to the electrolyte during
construction, Another problem that occurs in a dry primary cell is
that because the case is made of zinc this has a tendency to
corrode rapidly, this leads to the case leaking the electrolyte that it is
holding, to prevent this the electrolyte is mixed with wheat flour so
that it forms a thick paste and hence the term “dry” cell. The EMF of
a dry cell is approximately 1.5 volts.
We have seen that a current of electricity ( ) is a flow of electrons but
the electron itself is too small to be of use as the unit of electrical
quantity and therefore a more practical unit consisting of many
millions of electrons has been chosen. It is called the Coulomb (C)
and is 6.28 x 1018 electrons.
Note: This is a Quantity of electricity (Q) not a measure of current,
but it is used to define the unit of electrical current the
AMPERE (A). When a current of one ampere is flowing in a
conductor, 1 coulomb of electrons pass any point in the
conductor every second. In other words the size of an
electrical current is dependent upon the rate of flow of
electrons not a number of electrons.
We can write this in equation form.
Thus1 ampere of current flowing in a conductor for 1 hour is
equivalent to 3600 Coulombs and this is called an amperehour.
Now we have to look at what makes the electrons flow in a conductor
to form an electric current. Consider the diagram in which 2 bodies
with opposite charges on them are fixed in their position and not
If the bodies were free to move they would be attracted to one
another so clearly there is potential mechanical energy between
them. There is also electrical potential energy between them since
we know that if a conductor joins them, electrons will flow from the
negative body to the positive body until the bodies are equally
Therefore the oppositely charged bodies are producing the energy
required to move the electrons, i.e. to produce a current of electricity.
The oppositely charged bodies are said to have a potential difference
(PD) between them and the size of this PD is measured in the unit of
the Volt (V).
Conventional theory, also known as hole theory, states that current
flows from positive to negative. Protons or the lack of electrons (the
holes) move towards the negative. (Current flow direction in hole
theory is the opposite of that in Electron Theory.)
Having studied electricity at the atomic level we have met a number
of words, which need to be defined and explained before we move
The laws governing the behaviour of the different units are dealt with
in the relevant section rather than including them in these definitions.
Is the difference between charge values, which exists at the atomic
level in materials with free electrons.
The unit of potential difference (PD) is the Volt, which is defined as:
‘The difference of potential across a 1 ohm resistor carrying a
current of 1 ampere.’
This is the ability to cause current to flow in a complete circuit.
The unit of EMF is the Volt (V).
Note: It should be noted that both EMF and PD are measured in the
same units, they are, in fact, both differences in charge potential.
However, it is important to realize that an EMF is the force to do work,
i.e. cause current to flow, whereas PD is the volts drop as a result of
the current flow. Another way to look at it is that the EMF is off- load,
the PD is on-load.