Question

The magnetic field around a current carrying coil lasts.
A. For three hours
B. As long as current flows through it
C. Till its half period
D. Field is permanent.

Answer 1

Hint: Magnetic lines are lines which are produced around the conductor when current flows through it. When current flows through the coil, the coil gets magnetized and the magnetized coil produces magnetic lines that are responsible to produce the magnetic field. The magnetic field associated with a magnet will exist as long as the magnet has magnetism and magnetism depends on current flow.

Complete step by step answer:
Space around the magnet where we can observe the influence of a magnet on other magnetic materials is called the magnetic field. So the magnetic field is always associated with a magnet as long as it has its magnetism. As the permanent magnets can have magnetism for a very long time, the magnetic field associated with them is permanent.

We know that magnetic fields and electric fields are inseparable i.e. if one of two exists then the other one will also exist. The time-varying electric field will produce a time-varying magnetic field and the time-varying magnetic field will produce a time-varying electric field. The moving electrons constitute current and we know that moving charged particles produce a time-varying electric field, hence current will produce a time-varying electric field and thus time-varying magnetic field also. Therefore, the current flowing through a coil will produce a magnetic field around it.

As we have discussed the moving electrons produce an electric field, as soon as the motion of electrons stops (in a definite direction), and hence the current through a coil stops, the electric field and hence the magnetic field produced will vanish.

So, the correct answer is “Option B”.

Note: Carefully note that electrons never stop moving so you should be expecting that it will always produce an electric field and hence the magnetic field. But in reality, this is not observed because electrons move in a random direction within the metal and hence the net electric and magnetic field produced will be zero. When a certain potential difference is developed across the terminals of the wire, electrons will move in a particular direction and hence will constitute a current. As long as this potential difference exists, the current flowing through a wire will exist, and hence magnetic fields also exist for the same duration of time.