In order to tell a river lock attendant that you wish to go through the lock, you should <span>sound one prolonged blast followed by one short blast.
You should wait about 400 feet away from the lock and wait for the flashing light signal that allows you to enter.
Also note that </span><span>commercial traffic always have the first priority in entering the locks.</span>
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Answer:
A moving electric charge creates a magnetic field at all points in the surrounding region.
An electric current in a conductor creates a magnetic field at all points in the surrounding region.
A permanent magnet creates a magnetic field at all points in the surrounding region.
Explanation:
Magnetic field can be produced by:
- moving charges (i.e. a moving electron, or a current in a conductor)
- A magnet
The strength of the magnetic field produced by a current-carrying wire is

where
I is the current
r is the distance from the wire
As we see from the formula, the magnetic field is produced at all points in the surrounding region, because B becomes zero only when r becomes infinite. The same is true for the magnetic field created by a single moving charge or by a magnet.
The following choices instead are not correct:
- A single stationary electric charge creates a magnetic field at all points in the surrounding region.
- A distribution of electric charges at rest creates a magnetic field at all points in the surrounding region.
Because they involve the presence of stationary charges, and stationary charges do not produce magnetic fields.