6.1.10Chemical Reactions
Magnetic Force on a Charge — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 17 of 21 Unit quiz
Section 1 of 5Charged Particles in a Field
A current is a flow of charged particles, so the force on a wire is the sum of the forces on its moving charges.
Force on moving charge
- A charged particle moving through a magnetic field experiences a force, which deflects its path.
Perpendicular force
- The force acts at 90° to both the direction of travel and the field lines, so it changes the direction but not the speed.
Maximum force
- A particle moving perpendicular to the field lines experiences the largest force.
Zero force
- A particle moving parallel to the field lines experiences no force at all.
Section 2 of 5Deflecting an Electron
Fleming's left-hand rule is used with conventional current, which is opposite to the electron's motion.
1
An electron beam enters a magnetic field region moving at right angles to the field lines.
2
Conventional current is opposite to electron flow, so the second finger must point opposite to the electron's direction of travel.
3
Apply Fleming's left-hand rule with the first finger along the field and the second finger along the conventional current.
4
The thumb gives the direction of the force, and the electron curves in that direction.
5
The path of the electron is curved rather than straight for as long as it remains in the field.
Section 3 of 5Applications and Key Points
Magnetic deflection is used wherever a beam of charged particles must be steered.
Applications and Key Points
Cathode-ray tubes
- Magnetic fields deflect electron beams in older televisions and oscilloscopes to produce a picture or trace.
Particle paths
- Charged particles curve in a fixed magnetic field, and the direction of the curve shows which way the force acts.
Current direction
- Remember that Fleming's left-hand rule uses conventional current, which is opposite to the motion of electrons.
Speed
- Because the force is always perpendicular to the motion, a magnetic force changes the direction of a particle but not its speed.
Section 4 of 5Worked Example
Question
- An electron moves to the right through a magnetic field that points into the page.
- State the direction of the force on the electron.
- The electron moves to the right, so conventional current is to the left.
- The field points into the page, so the first finger points into the page.
- The second finger points left, so the thumb points down the page.
- The thumb gives the direction of the force, so the force on the electron acts downwards.
Answer Downwards, towards the bottom of the page, perpendicular to both the electron's motion and the field.
Section 5 of 5Exam Checklist & Tips
6.1.10 – Key Points for the Exam
- Conventional current The flow of positive charge, which is opposite to the direction electrons move.
- Deflection A change in the path of a charged particle caused by a force acting at right angles to its motion.
- Maximum force The largest force, produced when a particle moves perpendicular to the field lines.
- Electron flow The movement of electrons, which is in the opposite direction to conventional current.
Common Mistakes
- Using the direction of electron flow as the second finger, which gives the wrong force direction.
- Saying a magnetic field speeds up or slows down a charged particle; it changes only the direction.
- Stating that a particle moving parallel to the field lines is deflected.
- Drawing the path of a deflected particle as a straight line.
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