IB Physics Sub-topic D3 Notes
The motor effect
The last concept in Topic D you need to understand is what happens when charges travel through magnetic fields, meaning it crosses flux lines. This causes the charge's magnetic field and the external magnetic field to interact. Michael Faraday described a visual analogy to explain how this affects the moving charge:
- The external magnetic field basically conforms around the charge’s magnetic field.
- This creates a build-up of magnetic force around one side of the charge and an empty space on the other side.
- So, the built-up force pushes the only movable component of this system, the charge, out of the system.
This phenomenon is called the motor effect because it is applied to motors to convert electrical energy into kinetic energy.
You need to remember that if the magnetic field is stronger, or the charge has a higher value, velocity, or angle of crossing, the deflection is stronger. The formula for this is:
Note that if a charge travels parallel to the external magnetic field, there is no interference of the magnetic fields and so there is no motor effect!
Now if the motor effect occurs with a single charge, it must also occur with multiple charges, ie a current! So, if a current-carrying wire crosses a magnetic field, it also undergoes the motor effect, just continuously. Understandably, the formula here changes:
It can often be daunting to keep track of the directions of the forces and fields in these situations. If you ever forget them, use Fleming’s left-hand rule. For this:
- Point your thumb, first finger and middle finger perpendicular to one another as shown.
- We can then remember that finger guns are used by the FBI, where F, B and I represent the symbols for force, field and current respectively. Thus:
- F is the motor effect force direction shown by the thumb.
- B is the magnetic field direction shown by the pointer finger.
- I is the current or charge direction shown by the middle finger.