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Voltage
When a body is falling through a gravitational field, it is losing gravitational potential energy.  
Similarly, when a charge is "falling" through an electric field, it is losing electric potential energy.  
 
   
In the situation shown in this diagram, the water has more gravitational potential energy at A than at B so it falls.  
   
   
The potential energy lost by 1kg of water in falling from level A to level B is the gravitational potential difference between A and B.  
   
   
After a time the water will, of course, all be in the lower container.  
   
If a water pump is added, as shown in the next diagram, a continuous flow of water can be maintained.  
The pump lifts the water back up, giving it back the potential energy it lost in falling.  
   
We can think of a battery or other source of electrical energy as a pump for electrons.  
The battery maintains an electrical potential difference (or voltage) between points A and B.  
It "lifts the electrons up" so that they can "fall back down" through the bulb, where their electrical potential energy is converted to light (and thermal energy).  
The electrical potential energy lost by each Coulomb of charge passing from A to B is called the electrical potential difference between those two points.  
To measure potential difference (or voltage) we use a voltmeter and the unit of potential difference is the Volt, V.  
A potential difference (p.d.) of 1V means that each Coulomb of charge loses 1Joule of energy.  
So 1V = 1JC-1  
A voltmeter is always connected in parallel with other components.  
The resistance of a voltmeter must be high compared with other components in the circuit being investigated.  
If this is not the case, when the voltmeter is connected to the circuit it will change the value of the voltage it is trying to measure... not a very desirable situation!  
   
The Electron-Volt (eV)  
The ideas expressed above can be "inversed" to define an alternative unit for energy.  
If one Coulomb of charge is moved though a p.d. of 1V then the quantity of work done (energy converted) is 1J.  
When considering the energy possessed by individual charged particles (electrons, protons etc) we can scale this down and propose the following definition:  
If one electron is moved through a potential difference of 1V, then the quantity of work done (energy converted) is 1 electron Volt (1eV)  
   
As the charge on one electron is 1.610-19C this means that 1eV = 1.610-19J  
   
Thus, the eV is a suitable unit when referring to the energy possessed by atoms or sub-atomic particles.  
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