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  • A-Level Physics

Energy, Power & Resistance

Potential Difference

Potential Difference (p.d.) is the work done per unit charge – the amount of energy converted from electrical to another form per coulomb of charge passing through a component. 

W = VQ work done = p.d. × charge

Electromotive Force

Electromotive Force is the amount of energy converted into electrical energy per coulomb of charge through the source:

W= εQ work done = e.m.f x charge

Essentially, p.d. is voltage used up in a circuit, whereas e.m.f. is the voltage provided to the circuit by a cell or power supply.


Both p.d. and e.m.f. are often referred to as voltage, because they both shave the Volt as their unit.

The Volt is 1J of energy per 1C of charge.



Accelerating Electrons

A cathode-ray tube (also called an electron gun) is used to accelerate electrons. A hot metal filament releases electrons by thermionic emission, and then these are accelerated in a vacuum between the filament and a metal plate with a small hole in it. The two are connected and a very high p.d. is applied, making the filament a cathode and the plate an anode. The electrons released from the filament are attracted to the positively charged anode plate, and pass through the hole. This creates a concentrated beam of high velocity electrons.

Cathod-ray tube diagram, accelerating electrons diagram. EngineeringNotes

When an electron is accelerated in this way, the energy transferred equals the wok done on the electron, which equals the kinetic energy:

eV = 1/2 m v^2 work done on electron = gain in kinetic energy of electron



Resistance

All conductors have resistance that obstructs the flow of charge through them. The higher the resistance of a conductor, the more energy is required to push the charge through, as more energy is lost in the process.


Ohm's Law

Ohm's Law states that the current flowing through a conductor is directly proportional to the p.d. across it, provided that the physical conditions (such as temperature) remain constant.

V = IR p.d. = current x resistance

The units of resistance are Ohms, Ω, defined as the resistance of a conductor when 1 volt produces a current of 1 ampere through that conductor.