Electricity - Voltage and Current - Teacher's Notes  
  Alessandro Volta
Voltaic Pile An Italian professor of physics. In 1791, Volta was experimenting with metals and acids. He touched a silver spoon and a piece of tin to his tongue (saliva is slightly acidic) and connected them with a piece of copper wire. He experienced a "sour" taste, and realized he was experiencing an electrical phenomenon. (Touch the two terminals of a 9 volt transistor radio battery to your tongue to experience this for yourself.)
 
After more experiments, Volta was able to assemble a pile of cells to form a battery. Each cell was a disk of zinc and a disk of silver, separated by a layer of brine-soaked pasteboard. We now refer to that original battery as a Voltaic Pile. In Volta's honor, we refer to electrical pressure as "Voltage" and we measure the amount of electrical pressure in units of "Volts".
 
 
  Andre Marie Ampere
A French mathematician, often referred to as "The Newton of Electricity". Ampere experimented with electricity and magnetism, and the relationship between the two. In the early 1800's he realised that electrons were moving through conductors in a flow. In his honor, we measure the amount of flow in units of "Amperes", often shortened to "Amps".
 
 
  What Causes Voltage?
Electrons and Protons are "charged" particles. Electrons have a negative charge, and Protons have a positive charge. Like-charged particles repel. Opposite-charged particles attract. A piece of material with more Protons than Electrons will have a positive charge. Likewise, a piece of material with more Electrons than Protons will have a negative charge.
 
Each charged particle that is out of balance exerts some electrical pressure as it tries to get back into balance. Electrons push, as they try to get away from each other. Protons pull, as they try to attract electrons towards them. The total amount of pressure between two points is measured as Voltage.
 
 
  How Does A Battery Work?
In a battery, a chemical reaction causes electrons to move away from one terminal and towards the other. The terminal that they move away from is becomes positively charged, and the terminal that they move towards becomes negatively charged.
 
When the battery is on the shelf, or in a circuit that is turned off, the electrical pressure builds up to the point where the chemical reaction essentially stops. When the battery is in a circuit and current is flowing, the chemical reaction is going on, moving new electrons into position as the previous electrons flow out through the circuit. As the chemicals that support the battery action become depleted, the battery gradually looses its ability to generate a charge at its terminals.
 
 
  What Causes Current?
Given a path, electrons will flow away from a negative charge and towards a positive charge. In our simple example (the single cell flashlight), current flows through the light bulb on its way from the negative battery terminal towards the positive battery terminal.
 
 
  Questions for Students:
Q: Consider a short piece of insulated wire. Is there current flowing in this wire?
 
A: Yes. Not really correct. There might be a few electrons moving about, but no organized flow, thus no current.
 
A: No. Correct. There is no voltage acting on the electrons, so there is no flow. And there is no complete circuit to provide a return path for the electrons.
 
Q: If the two ends of the wire are connected together to form a complete circuit, is there now current flowing?
 
A: Yes. Not really correct. Again, there might be a few electrons moving about, but no organized flow and no current.
 
A: No. Correct. There is no voltage acting on the electrons, so there is no flow, even though there is a complete circuit. A wire is only a conductor; it is not a voltiac cell.
 
Additional Note:
Actually, current can be caused to move in this loop of wire using a magnetic field (see AC sources). But in the absence of a magnetic field, there is no force acting on the electrons, so there is no current.
 
 
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