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9.1.2Electricity and D.C. circuits

Calculating Electric Current and Charge

5 min read 7 parts Lesson 5 of 26 1 quiz

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Lesson summary

9.1.2 Calculating Electric Current and Charge — AS PHYSICS 9702 notes: Use Q = It and I = Anvq to calculate charge, current and drift speed.Current — the charge passing through a circuit per unit time.Charge — measured in coulombs (C).Equation — Q = It.What the symbols mean: Q = …

  • Rearranged
  • Equivalent models
  • Definition
  • Why current seems instant
  • Unit conversion
  • Number density

Section 1 of 7Charge, current and time

Current
  • Current the charge passing through a circuit per unit time.
Charge
  • Charge measured in coulombs (C).
Equation
  • Q = It.
    What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
  • Q is charge in C.
  • I is current in A.
  • t is time in s.
Rearranged
  • I = Q/t.
Sign
  • Sign the conventional current is the flow of positive charge, so the current should always be given as positive.

Section 2 of 7Charge carriers in a conductor

Carriers
  • Carriers current is due to the movement of charge carriers, usually free electrons in conductors.
Direction
  • Direction conventional current flows from positive to negative, while negative carriers move the opposite way.
Equivalent models
  • Equivalent models the effect is the same whether positive charge flows one way or negative charge flows the other,
    • which is why the positive charge model works.

Section 3 of 7Drift speed

Drift speed
Definition
  • Definition the average speed of charge carriers travelling through the conductor.
Size
  • Size it is very slow, only about a cm per second.
Why current seems instant
  • Why current seems instant the number density of charge carriers is so large that current flows instantaneously.

Section 4 of 7The equation I = Anvq

Equation
  • I = Anvq.
  • I is current in A.
  • A is cross-sectional area in m2.
  • n is the number density of charge carriers in m−3.
  • v is the average drift speed in m s−1.
  • q is the charge of each carrier in C.
Sign
  • Sign the same equation is used for positive or negative carriers.
Rearranged
  • Rearranged v = I/(Anq).
Unit conversion
  • Unit conversion convert mm2 to m2 before substituting.

Section 5 of 7More from the specification

Elementary charge
  • Magnitude of the charge of a proton or electron, 1.60 × 10⁻¹⁹ C.
Terminal
  • Connection point of a cell, labelled + or −.
Ammeter
  • Component that measures current.
Series
  • Components in one loop, sharing the e.m.f. between them.

Section 6 of 7Key terms

TermMeaning
ChargeQuantity of electricity in coulombs, Q = It.
Drift speedAverage speed of charge carriers along a conductor.
Number densityNumber of charge carriers per unit volume.
CoulombCharge passing a point when a current of 1 A flows for 1 s.

Section 7 of 7Exam technique

1
Leaving area in mm² — convert to m².
2
Thinking drift speed is the speed at which the signal travels — drift speed is very slow.
3
Using time in minutes in Q = It — use seconds.
4
Taking the charge of an electron as positive in I = Anvq — use its magnitude 1.60 × 10⁻¹⁹ C.
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