Lessons › 5 Electricity and Magnetism › 5.3 ElectromagnetismPrevious lessonNext lesson

5.3.2 Ampere's Law

By the end of this lesson you can:

  • Describe the magnetic field around a current-carrying wire and inside a solenoid
  • Use the right-hand rule to find field direction
  • Calculate the force between parallel current-carrying wires
Find your course: NSW Year 11: MagnetismNSW Year 12: Module 6, IQ2QLD Unit 3: ElectromagnetismVIC Unit 3: Fields and motionAP Physics 2: Unit 12IB: D.2 Electric and magnetic fieldsIB: D.3 Motion in electromagnetic fields
🎥 Watch📝 Practise🕹️ Explore🎯 Exam ready🍎 Teachers
🎥Watch
Learn the idea: a short explanation, then a check.

We now turn our attention to a fundamental principle that underpins much of electromagnetism: moving electric charges generate magnetic fields.

In 1820, Hans Christian Ørsted made the surprising discovery that an electric current flowing through a wire caused a nearby compass needle to deflect. Shortly after, Michael Faraday established that these magnetic fields formed circular continuous lines of force around the current-carrying conductor.

At a microscopic level, we now understand that it is the motion of the individual charge carriers—the moving electrons—that generates this magnetic field.

The video below explores the mathematical relationship between electric current and the magnitude and direction of the magnetic field it produces.

📝Practise
Worked examples first, then problems to try, then worksheets.

Check Your Understanding: Ampere's Law & Magnetic Fields

1. A vertical wire carries a conventional electric current flowing vertically upward. When viewed from directly above, in which direction do the magnetic field lines circle the wire?
2. Which diagram shows the magnetic field around a straight wire carrying a current into the page?
🕹️Explore
Test your predictions in a simulation.

​The following interactive is self explanatory (by Walter Fendt)

🎥Watch
Learn the idea: a short explanation, then a check.
Ampere's Law application - the solenoid
So how does a straight wire with a circular magnetic field get used to make an electromagnet, or a solenoid? This video looks at the physics.

Going Deeper


​The following activity is more related to the magnetic field around a wire, or more specifically the magnetic field between two wires.
This Desmos activity allow you to see qualitatively the strength of the field between (and on either side) two wires. The wires are the asymptotes of the graph represents the wires
By sharing the current (which can be reversed by making them negative) you effectively see if there are places that exist where the B field is zero, andwhee they are a minimum
the 'k' value represents µ/2π though( its value isn't that large) and can be left alone. Changing it does not change the trend of the graph
The derivative is shown to show that a minimum can exist ate certain points