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.
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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.
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Magnetic field lines go from North to South.
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Magnetic field lines can never cross.
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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).
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The magnetic field surrounding a bar magnet is as follows.
We should remember that these magnetic fields exist in 3 dimensions. This simple Geogebra applet shows us the magnetic in 3D.
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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).
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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.
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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).
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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.
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 | |||||
Isaac Physics | Magnetic Fields around a Current | First 2 sections applicable here | ||||
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
Chris Doner | Magnetic Forces on a wire | Magnetic Forces on a Particle | IB Specific | |||
Khan Academy | Force on a Charge | Force on a proton 1 | Force on a proton 2 | |||
Khan Academy | Electric Motor 1 | Electric Motor 2 | Electric Motor 3 | |||
Khan Academy | Force on a wire | Force between wires 1 | Force between wires 2 | |||
Science Shorts | Motor Effect | Charges in B Fields | Practical Tips | |||
Study Nova | Hand Rules 1 | Hand Rules 2 | ||||
Study Nova | Hand Rules for B Fields | Hand Rules for B Fields 2 |
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 | Magnetic Forces | p1 & p8 | ||||
Physics and Maths Tutor | Fields Definitions | Fields Key Points | Fields Detailed Notes | B Field Flashcards | A Level Resources - content slightly different |
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 | Force on a conductor | Force on a charged particle | Circular motion of a charged particle | Circular motion is covered in Section 6.1 | ||
Mr. G | 5.4 Formative Assessment | Topic 5 Summary Qs | IB Specific Questions | |||
Physics and Maths Tutor | Charges in B Fields (AQA 1) | Charges in B Fields MS (AQA 1) | Charges in B Fields (AQA 2) | Charges in B Fields MS (AQA 2) | A-Level Qs: overlapping content |
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.