Electric current, d.c. and a.c.
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 5 practice questions, 2 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
- A river of electrons
- What current is
- Measuring current
- The formula
Read the transcript of the teaching part
Last time
Before we start: last time, in E.m.f., potential difference and cells, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A river of electrons
A lightning strike carries about 28 000 amperes for a fraction of a second. A phone charger carries about one ampere. Both are electric currents: charge that is flowing. We measure how much charge passes each second, and we must know which way it goes.
What current is
An electric current is a flow of electric charge. In a metal, the charge carriers are free electrons, which drift through the lattice. In solutions, the charge is carried by ions.
Measuring current
Current is measured in amperes, A, with an ammeter placed in series, so that all the current goes through it. Current is the charge that passes a point every second. The unit of charge is the coulomb, C. One ampere is one coulomb per second.
The formula
Charge equals current times time. Q equals I t. Charge in coulombs, current in amperes, time in seconds. Always convert minutes and milliseconds to seconds.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Electric current.
The whole chapter: Electrical Quantities, Electrical Quantities.
Chapter 4.2: Electrical quantities
- Charge and electric fields 5 min
- Electric current, d.c. and a.c. 5 min
- E.m.f., potential difference and cells 6 min
- Resistance and Ohm's law 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.