ohms law, voltage field

[kjeldsmark]

At this page http://amasci.com/elect/vwatt1.html it says:

 

VOLTS x COULOMBS = JOULES It takes energy to push some charge against the voltage pressure

 

 

This should be pretty basic. However, I don't understand the sentence. How come it says 'against a voltage pressure'? Isn't the voltage pressure helping the charge move along (against resistance)?

 

Thank you

[studiot]

First here is the full quotation.

 

It would have been nice to have been told that this was nearly at the end of the webpage linked to.

 

But what about joules and watts? Whenever a certain amount of charge is pushed through an electrical resistance, some electrical energy is lost from the circuit and heat is created. A certain amount of energy flows into the "frictional" resistor every second, and a certain amount of heat energy flows back out again. If we increase the voltage, then for the same hunk of charge being pushed through, more energy flows into the resistor and gets converted to heat. If we increase the hunk of charge, same thing: more heat flows out per second. Here's how to write this:

  VOLTS x COULOMBS = JOULES       It takes energy to push some charge                                   against the voltage pressure

Charge flows slowly through the resistor and back out again. For every coulomb of charge that's pulled slowly through the resistor, a certain number of joules of electrical energy race into the resistor and get converted to heat.

The above equation isn't used very often. Instead, we usually think in terms of charge flow and energy flow, not in terms of hunks of charge or hunks of energy which move. However, thinking in terms of charge hunks or energy hunks makes the concepts sensible. Once you grasp the "hunks" concepts, once you know that energy is needed to push each hunk of charge against a voltage force, afterwards we can rewrite things in terms of amps and watts. Afterwards we can say that it takes a FLOW of energy (in watts) to push a FLOW of charge (in amps) against a voltage. Yet first it's important to understand the stuff that flows. Think in terms of coulombs of charge and joules of energy.

 

This is a good example of the difficulties you can encounter when trying to explain electric current by means of the water in a hosepipe model.

 

Electric current is not water in a hosepipe they are different.

 

 

You are quite right to question this passage, it is unhelpful.

I have also underlined a further unhelpful extract which almost implies you can arbitrarily change the voltage across a resistor without changing the current, in conflict with Ohm's Law.

 

What is happening here is the failure to properly explain what is meant by 'voltage'.

 

'Voltage' has more than one meaning,

 

Side note : This is not unique in Physics : The product force x distance means either work or moment depending upon the context.

 

Firstly a primary source of electrical energy offers electrical energy measured in volts (per coulomb).

We do not normally state the coulombs bit because that varies whilst the voltage remains fixed.

It is called the EMF (ElectroMotiveForce) of the circuit.

This could be a battery, of say 9 volts.

 

It is this EMF which 'drives the charge through the resistance'.

 

A primary source of EMF introduces electrical energy into a circuit.

 

A resistor (or conductor) is not a source, primary or otherwise, of EMF.

 

The resistor has a voltage difference or drop between its end terminals.

 

We call this a potential difference or potential drop. (PD)

 

Energy is dissipated in a potential drop.

 

That is energy leaves the circuit (in the case of a resistor as heat)

 

Does this help so far?