Charge and current

  • Created by: Andrew
  • Created on: 02-10-19 09:23

Charge and current

Current is the rate of flow of charge (Q = I / t) where the charge is given by any charged particle ie. electrons and ions.

The charge on an electron (e) is 1.6x10^-19 C and is equivalent to -1 in relative charge, the net charge of a particle is written as Q = +/- ne where n is the relative charge.

Current is formed when charged particles move when a potential difference is introduced, negatively charged particles move from a -ve to +ve potential, while positively charged particles move from +ve to -ve, in conventional current, current always flows +ve to -ve regardless of the charge on the particles.

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Kirchoff's 1st Law

Kirchoff's 1st Law states that for any point in a circuit, the sum of the current into that point is equal to the sum of the current out of that point.

  • Σ I(in)=Σ I(out)
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Drift velocity

Drift velocity is how quickly charged particles move in a substance, it is relatively slow but because all the particles move at the same time it appears fast.

Drift velocity gives another equation for current: I = Anev, where:

  • I - current
  • A - cross sectional area of the wire
  • n - number density (the number of charged particles per m^3)
  • e - elementary charge of the particles
  • v - drift velocity

This can be rearranged to find drift velocity: v = I/Ane

As the cross sectional area of the wire decreases, drift velocity has to increase to maintain a constant current, meaning drift velocity and area are inversely proportional.

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