Determine magnetic force directions for positive and negative particle beams
Use particle motion and charge sign to predict initial magnetic deflection without treating electron velocity as conventional current.
Extended only (Supplement); Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Distinguish positive and negative charge.
- Use perpendicular motor directions.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
An electron moves rightward. Which conventional-current direction corresponds to it?
What you will learn
- Use particle motion and charge sign to predict initial magnetic deflection without treating electron velocity as conventional current.
- Determine beam deflection from motion, imposed field and charge sign under a fixed page convention.
First use the positive-charge motion
For a beam of positive particles, the conventional-current direction follows particle motion. Apply the motor directional relationship to that motion and the imposed field. For particles moving rightward through a field into the page, positive-charge force is upward in the fixed page frame.
A negative-particle beam moving rightward corresponds to conventional current leftward. Its magnetic force is therefore downward in the same inward field. Electron velocity must not be inserted into the conventional-current rule without accounting for its negative charge. Charge sign, motion and field are three separate pieces of information.
Read initial deflection rather than inventing a full path
The directional result gives the force at the specified instant and thus the initial bending sense when that force changes the beam’s velocity direction. Later force direction follows the later motion direction. A single arrow at entry does not specify an entire trajectory for an unknown field region.
At fixed motion and field, changing charge sign reverses force direction. Reversing particle motion alone or field alone also reverses the magnetic force. Force magnitudes for unlike beams require further information; opposite signs do not mean equal mass, speed or equal curvature. This is written beam analysis, not a particle-beam apparatus procedure.
Pause and explain
A rightward electron beam enters a field into the page. Which initial force direction follows?
Worked example
Positive and negative particle beams both enter rightward into a page-inward field. State initial deflection senses.
Show the worked solution
- For positive charge use rightward conventional current.
- The positive-charge force is upward.
- For negative charge reverse that force sense.
Answer Positive beam upward; negative beam downward.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the initial force on rightward positive particles in an inward field.
Give me a hint
Use positive-charge motion as the conventional-current direction.
Compare my reasoning
- Mark inward field by crosses.
- Use rightward positive-charge motion.
- The motor directions give force upward.
Upward.
Look for these in your work
- Use the given page convention.
- Name the positive charge assumption.
Find the initial force on rightward electrons in the same inward field.
Give me a hint
Reverse the positive-charge force result.
Compare my reasoning
- Positive-charge force would be upward.
- Electrons have negative charge.
- Their force is downward.
Downward.
Look for these in your work
- Keep the same motion and field.
- Account for electron sign.
Two opposite-sign beams bend in opposite senses. A report says their curvatures must be equal. Evaluate it.
Give me a hint
Direction and curvature size require different information.
Compare my reasoning
- Charge sign fixes the relative force sense.
- Mass, speed and charge magnitude may differ.
- The direction result does not establish equal curvature.
Opposite bending senses do not establish equal curvature.
Look for these in your work
- Distinguish direction from magnitude.
- Name missing beam properties.
Common mistakes
- Retain particle motion when charge sign changes.
- Reverse the negative-charge result.
- Do not infer equal curvature.
What can you explain now?
How does conventional-current direction relate to negative particle motion?
Compare with the explanation
It is opposite
A negative-charge beam corresponds to conventional current opposite to particle velocity.
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Make it stick
Determine beam deflection from motion, imposed field and charge sign under a fixed page convention.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.