silly question on volatge

[bobbybrown]

ok a potential difference is needed for current to flow,
if you measure PD across a live and switch live at say a light switch there's hardly any difference so how does current flow?

 

[chiplard]

The pd is across the whole circuit, within the CU its from the Main Switch Live to Main Switch Neutral.

You will read nothing across the switch if it is on (closed) - open the switch and the voltage will appear across L and S/L ( if lamp is in )

 

[Sparks-R-Us]

Difference in the electrical potential of two points, being equal to the electrical energy gained by a unit positive electric charge moving from one point to the other. Electrons (which are negatively charged) flow in a conducting material towards the part that is relatively more positive (fewer negative charges). The SI unit of potential difference is the volt (V). The potential difference between two points in a circuit is commonly referred to as voltage (and can be measured with a voltmeter).

One joule of energy is gained by each coulomb of charge when moved through a potential difference of one volt. Potential difference V may be represented by the equation V = E/Q, where E is the electrical energy converted in joules and Q is the charge in coulombs. Chemical energy from a battery is converted to electrical energy in a circuit, this energy being given by rearranging the above formula: E = Q × V.

 

[Sparks-R-Us]

How to Find Potential Difference?
Calculating potential difference is made possible by the Ohm's law. It says that the voltage (potential difference) between two points of a conductor is directly proportional to the current flowing through it as long as the physical properties remain constant. This is expressed in the form of the equation:

V = IR

where V is the potential difference, I is the current flowing and R is the constant proportionality factor which is called as resistance. Thus knowing resistance and current flowing through a conductor, calculating potential difference is possible. There are devices called voltmeters that can be used to measure the electric potential difference between two points. They measure the voltage by detecting the current flowing through that conductor.

 

[Chr!s]

ok a potential difference is needed for current to flow,
if you measure PD across a live and switch live at say a light switch there's hardly any difference so how does current flow?

The currents flowing, its just the voltage has been dropped.

 

[telectrix]

in your case ( switch) potential difference is only present when there is a resistance to current flow. the resistance in this case is the lamp ( + a negligible cable resistanc). with the switch closed, the resistance betwwen the terminals is virtually nil, so no PD across switch.

 

[rryles]

ok a potential difference is needed for current to flow,
if you measure PD across a live and switch live at say a light switch there's hardly any difference so how does current flow?

There is hardly any resistance too. I = V / R. If V and R are very small, I can still be significant.