Question

Which of the following statements is true about magnetic fields produced by current-carrying wires?

the larger the current, the larger the magnetic field
the smaller the current, the larger the magnetic field
the longer the wire, the larger the magnetic field
the longer the wire, the smaller the magnetic field

Answer 1

The larger the current flowing in a wire, the stronger the magnetic field
is that surrounds the wire. 

That's why, if you want to make an electromagnet stronger, one way to
do it is to add another battery.  By increasing the voltage, you'll increase
the current flowing in the coils of wire, and the electromagnet will be stronger.

Answer 2

Answer:

option (a) and (C)

Explanation:

The magnetic field strength due to a current carrying wire is directly proportional to the amount of current flowing through the conductor and the length of the conductor. By the Biot Savart's law, the amount of magnetic field strength produced due to a small length element is given by

dB = k dl x i / r^2

Where, dl is the small length element, i be the current and r be the distance from the conductor where the magnetic field strength is to be calculated.