The Naked Scientists

The Naked Scientists Forum

Author Topic: Why are batteries defined in terms of voltage, but wires in amperage or current?  (Read 3065 times)

Offline taregg

  • Sr. Member
  • ****
  • Posts: 168
    • View Profile
why we say for  primary batterys power ....its volt.........but in elecrcity wires  we say the power is amp
« Last Edit: 04/06/2015 08:00:04 by chris »


 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1879
  • Thanked: 145 times
    • View Profile
Re: power of batteries
« Reply #1 on: 02/06/2015 13:02:39 »
Electrical power is usually measured in Watts (volts times amps).
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4719
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: power of batteries
« Reply #2 on: 02/06/2015 19:51:22 »
Power is volts x amps = watts but this isn't a particularly useful characteristic as most small batteries (e.g. AA cells) can deliver a huge amount of power for a very short time, and you can get even more short-term power by using the battery to charge a capacitor.

The capacity of a battery is measured in amp-hours (Ah) and is a more generally useful quantity. For example a car battery may be rated at 12V, 45Ah. This means it can deliver roughly  12 x 45 = 540 watts for an hour, or 54 watts for 10 hours, or 5 watts for 108 hours, etc.

There are additional ratings depending on the intended use of the battery, for example an aircraft starter battery can deliver as much short-term current (say 200 amps) as a car battery of 3 or 4 times its weight, but doesn't have enough capacity to keep the lights on overnight.
 

Offline taregg

  • Sr. Member
  • ****
  • Posts: 168
    • View Profile
Re: power of batteries
« Reply #3 on: 03/06/2015 18:10:20 »
I mean why we say primary battery has 5 volt...why we dont say it has 5 amps
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4126
  • Thanked: 247 times
    • View Profile
Re: power of batteries
« Reply #4 on: 03/06/2015 21:01:52 »
Quote from: taregg
I mean why we say primary battery has 5 volt...why we dont say it has 5 amps
5V would be an unusually high voltage for a primary cell (but commercially attractive, I'm sure!).
Typically the voltage produced by a chemical cell is in the range 0.7-3V, with the familiar "cheap" carbon-zinc batteries being 1.5V.

The open-circuit voltage is determined by the chemicals used in the cell - there are two electrodes, which have a different affinity for electrons. You need some very reactive chemicals to be able to reach a 3V potential difference. I imagine that the chemicals required to produce a 5V cell would be positively dangerous!

The short-circuit Amps that the cell can produce is determined more by its construction. So you can have carbon-zinc batteries which are large and fat, which will deliver far more current than other carbon-zinc batteries which are small and skinny. In the larger batteries, there is more area to the electrodes, lower resistance and hence higher maximum current.
« Last Edit: 04/06/2015 12:11:28 by evan_au »
 

Offline PmbPhy

  • Neilep Level Member
  • ******
  • Posts: 2762
  • Thanked: 38 times
    • View Profile
Re: power of batteries
« Reply #5 on: 03/06/2015 23:43:18 »
Quote from: taregg
why we say for  primary batterys power ....its volt.........but in elecrcity wires  we say the power is amp
Where did you get this idea from? Power is defined as the rate of doing work. A battery is never rated in terms of power nor is household electricity. Both are rated in terms of volts. E.g. household electricity is 120 volts at 60 Hz in the USA and in Europe, most of Asia, most of South America and Australia it's 230 volts but 60 Hz. The voltage is given in RMS (root-mean-square).

What you might be thinking of is the current rating in household wiring. That's the maximum recommended current to use in that wire.
 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1879
  • Thanked: 145 times
    • View Profile
Re: power of batteries
« Reply #6 on: 04/06/2015 00:26:12 »
I mean why we say primary battery has 5 volt...why we dont say it has 5 amps

Volts and amps are different.

Imagine a circuit is like a waterfall. The voltage is the height of the waterfall and the amperage is how much water is going down it per second.
 

Offline chris

  • Neilep Level Member
  • ******
  • Posts: 5339
  • Thanked: 65 times
  • The Naked Scientist
    • View Profile
    • The Naked Scientists
Quote
Volts and amps are different.

Imagine a circuit is like a waterfall. The voltage is the height of the waterfall and the amperage is how much water is going down it per second.

Lovely analogy!
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4126
  • Thanked: 247 times
    • View Profile
Quote from: taregg
in electricity wires we say the power is amp
The thickness of the conductor (usually copper) plays a large part of the cost of the wire, so it is important to know whether you need cable rated for 10A, 50A or 100A continuous current. If you draw too much current, the wire will get hot, melt the insulation and potentially cause a short-circuit and/or cause a fire. So the current forms an upper limit on how a particular cable can be used.

The power dissipated by the wire is represented by P=I2R, where:
  • P: The power dissipated in the wire (that makes it get hot)
  • I: The average current flowing through the wire
  • R: The resistance of the wire
  • You will notice that the Voltage is not part of this equation; it doesn't matter whether the wire is drawing 100A at 1 volt, or 100A at 1000V, it will get just as hot. 
The thickness and type of insulation determines the maximum voltage that the wire will stand without electrical breakdown. When we are talking about electrical wiring for a house, we just assume the voltage that is to be applied to the wires: there is an assumed standard in each country. For a nominal 230Vrms AC household supply, the voltage peaks at 326V=230√2. However, lightning surges can increase that by 1000V or more, so the insulation is typically rated at over 1500V.

From the current rating and the (assumed) operating voltage, you can work out the maximum power delivered over the wire. A 100A cable operating at 230V can deliver 100x230 = 23000 Watts = 23kW.

If people could come in contact with the wire, it is normally covered with two layers of insulation, each rated at 1500V or more.

For High Voltages (11kV or more), it takes a very thick layer of insulation to resist electrical breakdown. If the wire is run in underground conduits, the conductors are laid together, separated by thick layers of insulation. If it is strung along poles, it is cheaper to leave it uninsulated - the air is a pretty good insulator, provided you leave a large enough gap between the wires.

People who order high-voltage insulated cable know that they have to specify it by both current (which determines the copper thickness) and voltage rating (which determines insulation thickness).
 

Offline jerrygg38

  • Hero Member
  • *****
  • Posts: 781
  • Thanked: 27 times
    • View Profile
why we say for  primary batterys power ....its volt.........but in elecrcity wires  we say the power is amp

  The answer to me is common usage. We need a 9 volt battery or a 1.5 volt battery. If we need more current we need a D battery or a C battery for less. When we have extension cords we need a 7.5 amp cord or a 15 amp cord. Usually the voltage is the same or 120 volts.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums