OCR A-Level Physics/Electrons, Waves and Photons NEW SPECIFICATION/Electric current

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OCR A-Level Physics/Electrons, Waves and Photons NEW SPECIFICATION
Electric current Resisitance

Charge and Current

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Current is the flow of charged particles around a circuit. The electrons are usually the particles that flow. Metals are good conductors because they have free electrons that are able to flow. The SI unit for current is Amperes (A)

Cross Sectional Area

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The cross-sectional area of the wire is the area of a circle of radius r: A=πr2=π(d2)2, where d is the diameter of the wire. The SI unit for area is ms^2.

Electron Flow

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Before it was known that it was electrons that were the charge carriers, it was believed that current flowed from positive to negative. This became convention and is still used under the name Conventional Current even after the discovery of free electrons. We now know that electrons have a negative charge. This means they are repelled from the negative terminal and is attracted to the positive terminal.

Remember this - Electron flow is always opposite to conventional current.

Elementary Charge

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Charged particles have a charge in coloumbs. An electron is a charged particle and has the charge of approximately . This is known as the elementary charge and is represented with an e symbol. Protons also have an approximate charge of e. The difference being is that electrons are negatively charged so strictly speaking they have a charge of -e and protons have a charge of +e.

Drift Velocity

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When electrons flow in a circuit, they have a velocity. we have an equation that relates this velocity and current together


From this equation we know that the current is affected by the cross-sectional area of the wire, A as well as the number density, n of electrons. This is just the number of electrons per unit volume. The number of electrons are as many as there are atoms. Current also is dependent on elementary charge, e and the mean drift velocity, v If we rearrange the top equation we can see a relationship between the values.


We can then deduce these three relationships.


The elementary charge, e is constant.

Kirchoff's First Law

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Kirchoff's First Law - The sum of the currents entering any point is equal to the sum of the currents leaving that same point.


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Charge - Current passing a point in a given time.
1 Coulomb - Current of 1 Amp passes a point in the time of 1 second.

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Current - Current of a cross-sectional area depends on the mean drift velocity of the charge carriers and on their number density in the material.