Resistance and Ohm's law
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 4 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
- Why do wires get hot?
- What resistance is
- Using the equation
Read the transcript of the teaching part
Last time
Before we start: last time, in Electric current, d.c. and a.c., you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Why do wires get hot?
A kettle element glows red hot, but the cable that feeds it stays cool. Both carry the same current. The difference is resistance. A thin, long wire of the right material resists the current and gets hot, while a thick copper cable hardly resists it at all.
What resistance is
Resistance is the opposition of a component to the flow of current through it. It is defined as the potential difference across the component divided by the current through it. R equals V over I. It is measured in ohms. One ohm is one volt per ampere.
Using the equation
R equals V divided by I. V equals I R. I equals V divided by R. A bigger resistance means a smaller current for the same p.d. Use the triangle: cover the quantity you want.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Resistance and resistance wires · Ohm's law and current–voltage graphs.
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.