Putting charge in motion - the circuit

Circuits 1


Thus far, we have only discussed "static" (stationary) charges.  Static charges alone are useful, but not nearly as much as charges in motion.  As you recall, electrons are the most easily moved particles. 

 Recall the Galvani experiments discussed in class:

 https://www.youtube.com/watch?v=o8zNSzbjRLI

 https://www.youtube.com/watch?v=sJifWqUa2pY
So, electrons are moved by the chemistry of the electrochemical/voltaic cell (originally called the "electric pile" and now simply, battery).  Here are pictures/diagrams of Volta's original batteries:




There is a chemical reaction between the electrolyte paste (often an acid) and the two metals (often zinc and carbon).  Electrons are given up by one metal (zinc) and accepted by another (carbon or copper).  The motion of electrons is called "current", but we usually imagine that it is positive charge moving (to keep the numbers positive).

We can't see electrons, but we can certainly see what they do when they pass through a filament in a light bulb:



For sake of ease in sign convention (positive vs. negative), we define the following:


Current (I) - the rate at which positive charge "flows"

I = Q/t

The unit is the coulomb per second, defined as an ampere (A).  Just as one coulomb is a huge amount of charge (nearly 6.3 billion billion protons), one ampere (or amp) is a tremendous amount of current - more than enough to kill a person.  In fact, you can feel as little as 0.01 A.  Typical currents in a circuit are on the order of mA (milliamperes).

Essentially, current is how quickly charge travels (or charge per time, q/t).  The unit (a coulomb per second) is called the ampere (or amp, A).  To keep things simple, we think about positive charge moving, even though it is really all about the electrons.



We need to define other new quantities in electricity:  voltage, resistance, power.

Voltage (V) - the amount of available energy per coulomb of charge.  The unit is the joule per coulomb, called a volt (V, in honor of Allesandro Volta, inventer of the battery).

V = E/Q

Batteries and other sources (such as wall sockets) "provide" voltage, which is really a difference between TWO points (marked + and - on a battery).  





Or in schematic form:




Resistance (R) - the ratio of voltage applied to an electrical device to the current that results through the device.  Alternately:  the amount by which the voltage is "dropped" per ampere of current.

R = V/I

You can also think of resistance as that which "resists" current.  Typically, resistors are made of things that are semi-conductive (they conduct current, but less well than conductors and better than insulators).  Resistors are often made of carbon, but can also be made of silicon and other materials.  The unit is the volt per ampere, defined as an ohm (Greek symbol omega)

A convenient way to relate all of the variables is embodied in an expression often called Ohm's Law:

V = I R



So, what exactly IS a circuit?

An electrical circuit can be thought of as a complete "loop" through which charge can travel.  Therefore, it actually has to be physically complete - there can be no openings.  That is, the current actually has to have a complete path to take.  I will demonstrate this in class with bulbs and wires; for now, see the image above.

Some folks like analogies.  Consider a water analogy.  Voltage is like a tank of water (how much water).  Resistance is provided by a drain or faucet.  The rate at which water comes out is the current.  It's only an analogy, but it gets the gist of circuit terminology ok.




What about power?
Also consider electrical power (P).  Power is the rate at which energy is used or expended:  energy per time.  Symbolically:  P = E / t.  The unit is the joule per second, called a watt (W).  In electricity, power is also given by:
P = I V
P = I^2 R


Power allows us to express the brightness of a bulb.  Consider that a 100-W bulb is brighter than a 60-W bulb.

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