Magnetic force on charges and the velocity selector
Practice Flashcards
Flip to reveal answersWhat is the magnetic force on a moving charge?
Track your progress — Sign up free to save your progress and get smart review reminders based on spaced repetition.
All 15 Flashcards — Magnetic force on charges and the velocity selector
Sign up free to track progress and get spaced-repetition review schedules.
Question
What is the magnetic force on a moving charge?
Answer
**F = qvB** when the charge moves at right angles to the field B (given as F = qvB sinθ). It is **zero** for a stationary charge.
Question
Which way does the magnetic force on a moving charge point?
Answer
**Perpendicular** to the velocity v (and to B). Because it is always sideways, it changes the charge's **direction** but never its **speed**.
Question
Why does a charge follow a circle in a uniform magnetic field?
Answer
The force F = qvB is always perpendicular to v, so it acts as a **centripetal force**, curving the path into a **circle** of radius r = mv/(qB).
Question
Formula for the radius of a charge's circular path in a magnetic field?
Answer
$r = \dfrac{mv}{qB}$ — a heavier or faster particle curves in a bigger circle; a stronger field or bigger charge curves it tighter.
Question
What is a velocity selector?
Answer
A device with **crossed** electric and magnetic fields (E and B at right angles). Only charges of one speed pass straight through; the rest are deflected.
Question
What is the condition for a charge to pass straight through a velocity selector?
Answer
The electric and magnetic forces **balance**: **qE = qvB**. The net force is then zero, so the charge is undeflected.
Question
What speed is selected by a velocity selector?
Answer
$v = \dfrac{E}{B}$ — from qE = qvB, the charge q cancels.
Question
Does the selected speed v = E/B depend on the charge or mass?
Answer
**No** — q cancels in qE = qvB, so every undeflected particle has the same speed v = E ÷ B, whatever its charge or mass.
Question
In a velocity selector, what happens to a charge moving SLOWER than v = E/B?
Answer
The magnetic force qvB is smaller, so the **electric force qE wins** and the charge is deflected the way qE points.
Question
In a velocity selector, what happens to a charge moving FASTER than v = E/B?
Answer
The magnetic force qvB is larger, so the **magnetic force wins** and the charge is deflected the other way.
Question
A selector has E = 2.0 × 10⁴ N C⁻¹ and B = 0.10 T. What speed passes through?
Answer
v = E ÷ B = (2.0 × 10⁴) ÷ 0.10 = 2.0 × 10⁵ m s⁻¹.
Question
Why does a magnetic field never change a charge's kinetic energy?
Answer
The force is perpendicular to the motion, so it does **no work** on the charge — only its direction changes, not its speed.
Question
How do you find a particle's charge-to-mass ratio from its path in a magnetic field?
Answer
From r = mv/(qB): **q/m = v/(Br)**. Measure r at a known v and B.
Question
Isotope ions (same charge, same speed) in the same field: which curves more?
Answer
The **lighter** one: r = mv/(qB) ∝ m, so the heavier ion moves on the **bigger** circle.
Question
A positive and a negative charge enter the same magnetic field. How do their paths differ?
Answer
They curve **opposite ways** round their circles; the radius also depends on each one's m/q and speed.
Read the notes
Full study notes for Magnetic force on charges and the velocity selector
Topic 4.3 hub
Motion in electromagnetic fields
More from Topic 4.3
All flashcards in this topic
Physics exam skills
Paper structures & tips
Track your progress with spaced repetition
Sign up free — Aimnova tells you exactly which cards to review and when, so you remember everything before your IB exam.
Start Free