Charge, current and drift speed
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 4 practice questions, 3 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Slow electrons, instant light
- Charge and current
- Charge is quantised
Read the transcript of the teaching part
Last time
Before we start: last time, in I-V characteristics and power in circuits, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Slow electrons, instant light
Flip a switch and the lamp lights at once. Yet the electrons in the wire creep along at less than a millimetre per second. The current is a flow of charge, and the whole wire is already full of free electrons, so they all start moving together.
Charge and current
Electric current is the rate of flow of charge: I equals delta Q over delta t. The unit of current is the ampere, a base unit. One coulomb is the charge passing in one second at one ampere. Conventional current flows from positive to negative. Electrons in a metal drift the opposite way. A steady current Q over t gives the charge Q equals I t.
Charge is quantised
Charge comes in lumps. Any charge is a whole-number multiple of the elementary charge e, one point six zero times ten to the minus nineteen coulombs. A current carries charge in whole multiples. It cannot carry one and a half times the elementary charge. To test a charge, divide by e. If the answer is a whole number, it is possible. Quarks have charges of a third of e, but they are never found alone.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Drift speed.
The whole chapter: Electricity, Electricity.
Chapter 9.1: Electric current
- Charge, current and drift speed 5 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.