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Magnetism · Physics

Charge in a Magnetic Field Simulation

Send a charge into a magnetic field and see it circle, hop through crossed fields, pass a velocity selector and spiral. The helical path is shown in 3D.

Interactive virtual lab · no download · works on phones and laptops

Chapter
Moving Charges and Magnetism
For
Class 12 · JEE · NEET
Time
~15 min

How to use it

  1. 1Choose a situation from the tabs.
  2. 2Change charge, mass, speed and field.
  3. 3Turn on the free-body diagram.
  4. 4In 3D, drag to turn the helix.

What to notice

  • r = mv/qB, but the period 2πm/qB does not depend on speed.
  • The magnetic force does no work: the speed never changes.
  • Crossed E and B pass only v = E/B undeflected.
  • A velocity component along B gives a helix of pitch v cos α · T.

For teachers

Teach it in 22 minutes

A lesson plan, questions to put to the class, a quick quiz you can project or print, and the formulas behind Charge in a Magnetic Field.

  1. 1Hook

    ~2 min
    • Ask: “A magnetic force acts on the moving charge. Will it speed up?” Send the charge in and watch the speed readout.

  2. 2Demonstrate

    ~9 min
    • Choose a situation from the tabs.
    • Change charge, mass, speed and field.
    • Turn on the free-body diagram.
    • In 3D, drag to turn the helix.

  3. 3Pause and ask

    ~5 min
    • Have a student say this in their own words: r = mv/qB, but the period 2πm/qB does not depend on speed.
    • Have a student say this in their own words: The magnetic force does no work: the speed never changes.
    • Have a student say this in their own words: Crossed E and B pass only v = E/B undeflected.
    • Have a student say this in their own words: A velocity component along B gives a helix of pitch v cos α · T.

  4. 4Check

    ~6 min
    • Run the 6-question quiz below: project it and take answers by show of fingers (1–4), or print it as a worksheet.

About the Charge in a Magnetic Field simulation

A magnetic field pushes on a moving charge with F = q v × B, always at right angles to its velocity. So it changes the direction of motion but never the speed, and a charge moving across a uniform field goes round in a circle of radius mv/qB. The time for one turn, 2πm/qB, does not depend on the speed — the principle of the cyclotron.

This charge in magnetic field simulation has four views: the circular path with its free-body diagram; a charge that falls through an electric field into a magnetic one and hops along; a velocity selector that passes only v = E/B; and a 3D helix for a charge with velocity at an angle to B.

Also covers: lorentz force, radius of circular path, cyclotron frequency, velocity selector, helical path, crossed fields.

Frequently asked questions

Why does a charged particle move in a circle in a magnetic field?

The magnetic force is always perpendicular to the velocity, so it acts as a centripetal force: qvB = mv²/r.

Does a magnetic field change the speed of a charged particle?

No. The force is perpendicular to the motion, so it does no work; only the direction changes.

When does a charge follow a helical path?

When its velocity has a component along B. That component is unaffected, so the circular motion is drawn out along the field into a helix.

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