There have been some questions about closed chops. Some people have trouble going from open to closed because they don't relax.  To play open chops the face has to be tight, mouthcorners ect.
 
Trumpet players use either mouthcorner tension, compression or lip curl.

But NOT all 3 !!!
You are making the lips so stiff that they stop buzzing.

You are setting the mouthcorners, cheek muscles ect too tightly for the range that you are playing in.
 
Abs way too tense. (No need to tighten up to play low notes.)
There is also Too much facial tension.
Start at the bottom of the horn and learn to relax.
Play a low G 1-3. Relax the face and let the note fall well below pitch.
Now slowly bring it up to its proper pitch center. (Most players play low notes on top of the pitch center.)

Learn to use that relaxed feeling and slowly play a g scale. Once the bottom octave is relaxed and free. Start learning to do it higher. Using a lip set point will help because you set toward the top of the range of a piece. This setting toward the top note means that 90% of all notes are played by relaxing instead of contracting the muscles. Not only does it sound and respond better it is also less tiring. If the music goes from low C to G on the staff set for a fourth space E. Play THAT note and leave the chops set that way. The top note in the phrase is a minor third away and everything else is the E or lower.  This eliminates the need for increasing tension to go to higher notes. (A touch more support or tongue arch will take you up the third.) Almost every note should require relaxing to play it. Combine that with thinking about sending the notes out to travel ON the air column.

The higher the note the farther you want to sent it. Do this by slowly bringing your abdominals toward the spine. That will cause an ever decreasing cavity for the air (everything shifts around.) This creates easy and relaxed support. This will be fixed the day you do all of these things.

Work page 125 of the Arban setting for the top note. This will teach you to relax for the low ones quickly.  As far as all the lip questions. Just buzz. If you place the horn to the lips as you buzz you will already have the right aperture .... for that note. Play the embouchure that you buzz. It is way easier.

Relaxed playing gives a full relaxed sound. Close embouchure means barely touching NOT squeezed shut.

Information about my trumpet & embouchure books.
http://www.BbTrumpet.com
 
Best wishes
Clint 'Pops' McLaughlin