2022 A/L Physics Past Paper · Question 04 (English Medium)

Answer: (1) stationary electric charges.

What the question asks

Find what a stationary magnetic field does not act on.

What each letter means:

$B$ ($\text{T}$): the magnetic flux density.

$q$ ($\text{C}$): the electric charge.

$v$ ($\text{m s}^{-1}$): the speed of the charge.

$\theta$ ($^\circ$): the angle between the speed and the field.

$F$ ($\text{N}$): the force on the charge.

Relevant theory

No force acts on an electric charge which is at rest in a magnetic field. However if the charge is moving it is equivalent to an electric current and hence a force would act on it.

According to modern concepts, the forces which act in magnetic fields are a result of the interaction between two magnetic fields which intersect with each other.

When the current starts flowing through the conductor AB a magnetic field is created around it.

Reading the options

For example, think of a charged balloon and a fridge magnet.

Hold the balloon still near the magnet. Its charge $q$ is not moving, so $v$ is zero. Nothing pulls or pushes it.

Now swing the balloon quickly through the field of the magnet. Its charge $q$ is moving now, so $v$ is not zero. The field pushes on it sideways.

$$F = Bqv\sin\theta$$

In words: $$the\,force = the\,field$$ $\times$ the charge $\times$ the speed $\times$ the sine of the angle.

If $v = 0$, then $F = 0$. A stationary charge feels no force from a magnetic field.

The book says the same thing in words.

No force acts on a charge that is at rest in a magnetic field.

A force acts only when the charge is moving.

A wire carrying a current and a magnet each have a magnetic field of their own.

Two magnetic fields act on each other, so a force acts between them.

Only the stationary charge feels nothing.

Why the other options fail

A moving charge is like a current, so a force acts on it.

A wire with a current has its own magnetic field. The two fields act on each other.

A magnet has its own magnetic field. Two magnetic fields act on each other.

A magnet has its own magnetic field, moving or not. Two magnetic fields act on each other.

Conclusion

No force acts on an electric charge that is at rest in a magnetic field.

The answer is (1).

A magnetic field pushes only on things that are moving charges or that have a magnetic field of their own.

A still charge has neither.

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