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5.4. Magnetic Effects of Electric Currents

Throughout this chapter so far we have talked a lot about electric fields and electricity. We remember that this is all part of the broader study of Electromagnetism - which describes how electricity and magnetism are inextricably linked. Here we will look further into the magnetic effects of electrical current.

This link between electricity and magnetism is described by Maxwell's Equations - a key part of any first year Physics degree. This goes beyond the scope of the IB, but these are a set of equations that essentially describe this relationship. They involve a bit of tricky maths, but take a look at the Crash Course video below if you want to learn a bit more.

 

Magnetic Fields

No doubt you are fairly familiar with the shape of magnetic field lines from GCSE, though it is worth having a bit of a recap about the key points.

  • Magnetic field lines go from North to South.

  • Magnetic field lines can never cross.

  • The closer the magnetic field lines are together, the greater the magnetic field strength.

 

The term magnetic field strength is also known as magnetic flux density (i.e. how densely packed the lines of flux/ field lines are) and this is measured in Tesla (T), named after the car (or Nicola Tesla, one or the other).

The magnetic field surrounding a bar magnet is as follows. 

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We should remember that these magnetic fields exist in 3 dimensions. This simple Geogebra applet shows us the magnetic in 3D.

In a similar way to with electric fields, the magnetic field line direction at any point is in the same direction as the force that would act on a magnetic North monopole (unlike + and - charges, North poles cannot exist without South poles - but we can use the idea of a North monopole to our construct field lines).

We have previously seen that the magnetic field around a conductor is given by the Right Hand Grip rule (Geogebra simulation to experiment with). In this case, if the is lined up in the direction of conventional current, then the fingers will wrap around in the direction of the magnetic field. It is worth remembering that the flow of electrons is in the opposite direction to conventional current. 

A moving charged particle (such as an alpha particle), will induce a similar magnetic field around it, given by the Right Hand Grip rule. In this case, we line the thumb up in the direction of the movement of positive charges (i.e.  in the same direction as an alpha particle) or in the opposite the direction of negative charges (i.e. opposite the direction of electron flow). 

Falstad have another nice simulation allowing you to visualise the magnetic field in different situations. Switch to Display Field Vectors or Field Lines from the drop down menu. Experiment with different currents and moving charges.

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Video Lessons

Chris Doner
Magnetic Fields
IB Specific
Khan Academy
Magnetism Intro
Field around a wire
Science Shorts
Magnetic Fields
Study Nova
Magnetic Fields
Study Nova
Magnetic Fields

Resources

IB Physics
Topic 5 Notes
IB-Physics.net
Chapter 5 Summary
IB Revision Notes
Isaac Physics
Magnetic Fields around a Current
First 2 sections applicable here
Isaac Physics
Magnetic Fields
Mr. G
5.4 Teaching Notes
5.4 Student Notes
Physics and Maths Tutor
Fields Definitions
Fields Key Points
Fields Detailed Notes
B Field Flashcards
A Level Resources - content slightly different
Physics and Maths Tutor
Magnetic Fields
p17 & p18 are relevant

Questions

Cambridge University Press
Topic 5: Add Qs
Topic 5: Add Qs MS
Topic 5: MCQs
CUP Website Link
Freely available online
Dr French's Eclecticon
Magnetic Fields
Magnetic Fields Solutions
Link to Dr French's Site
Extension: Pre-University Material
Grade Gorilla
5.4 (Magnetic Fields) MCQ
Topic 5 (E & B Fields) End Quiz
Quick IB Specific Mixed MCQs
Isaac Physics
Uniform Fields
Mr. G
5.4 Formative Assessment
Topic 5 Summary Qs
IB Specific Questions
 

Magnetic Forces

We have shown above that all moving charges have a magnetic field surrounding them, whether that is current through a length of wire, or an alpha particle fired from a radioactive source. We also know that magnetic fields are able to interact to produce a force:  when I bring two bar magnets together, the interaction between their magnetic fields is what causes the force of attraction/ repulsion. It therefore follows that moving charges (with their associated magnetic field) can experience a force when placed in a magnetic field. 

N.B. It is worth emphasising that this is only the case when a charge is moving. A stationary negative charge placed between two magnets will not experience a force.

 

This video illustrates what we call the 'motor effect'. The rolling bar carries a current, and is able to freely roll when it experiences a force.

When this conductor (or these moving charges) is in the presence of an external magnetic field, these fields will interact, causing a force. We looked at this for GCSE, but it's worth recapping the key points using the video below.

The motor effect is described by Fleming's Left Hand rule, which describes the relationship between the direction of the electrical current (I), the magnetic field (B) and the induced force (F).

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Video Lessons

Resources

IB Physics
Topic 5 Notes
IB-Physics.net
Chapter 5 Summary
IB Revision Notes
Isaac Physics
Lorentz Force
Mr. G
5.4 Teaching Notes
5.4 Student Notes
Physics and Maths Tutor
Fields Definitions
Fields Key Points
Fields Detailed Notes
B Field Flashcards
A Level Resources - content slightly different
Physics and Maths Tutor
Magnetic Forces
p1 & p8

Questions

 

Additional Resources

IB Questions

A question by question breakdown of the IB papers by year is shown below to allow you to filter questions by topic. Hopefully you have access to many of these papers through your school system. If available, there may be some links to online sources of questions, though please be patient if the links are broken! (DrR: If you do find some broken links, please contact me through the site)

 

Questions on this topic (Section 5) are shown in pale green.

Use this grid to practice past IB questions topic by topic. You can see from the colours how similar the question topic breakdown is year by year. The more you can familiarise yourself with the IB question style the better - eventually you will come to spot those tricks and types of questions that reappear each year.