Moving Charges and Magnetism: common misconceptions

If your child says… “The wire feels the same force at every angle.”

Ask together

If current runs along the field, how much of it runs across the field?

What’s true

A straight wire carrying current parallel to a magnetic field feels no magnetic force.

If your child says… “The speed across the field sets the spacing between turns.”

Ask together

Which motion carries the charge forward along the helix?

What’s true

Spacing equals speed along the field multiplied by the time for one turn.

If your child says… “Extra turns add nothing.”

Ask together

Would another matching turn oppose this field?

What’s true

Matching turns strengthen the field.

If your child says… “Twice as far means one-quarter.”

Ask together

Does the outer circumference double or quadruple?

What’s true

Doubling distance halves this field.

If your child says… “The same total turns always give the same field.”

Ask together

How many turns now fit into the same length?

What’s true

For a long air-core coil, fewer turns per unit length mean a weaker central field.

If your child says… “Matching currents repel.”

Ask together

With left index finger into paper and middle finger upwards, where does your perpendicular thumb point?

What’s true

Matching currents attract.

If your child says… “The field lies along the coil’s face, so torque vanishes.”

Ask together

When the field runs along the loop’s face, is its magnetic moment along the field?

What’s true

The loop’s magnetic moment points perpendicular to its face, so measure the angle from that direction.

If your child says… “The bypass carries all the current.”

Ask together

If the shunt carried everything, what current would remain to move the pointer?

What’s true

Incoming current equals coil current plus bypass current.