Why does a guitar hum if standing by an amp but hums less when you turn to a different direction?

Answer 1

For an Electric Guitar, any disturbance in the air or stray noise that is 'picked up' by the pickups, is converted to an electrical signal then fed through the cord to the amplifier.

If the guitar is close to the speaker, and the pickup is facing the speaker, then the original noise gets amplified, and then fed back into the pickup (louder this time). This louder sound is then amplified again, and the speaker produces an even louder sound, which is picked up again at a stronger volume level, an so on...

This is known as a positive feedback loop amplifier. Positive feedback amplifiers are unstable - the sound will get as loud as the amplifier is capable of producing, unless the pickup is moved away from the speaker (which breaks the loop). If you are standing far from the speaker and point the pickup toward the speaker, you may notice little or no effect. This is because the sound takes time to travel from the speaker, so that the pickup is picking up the sound from the speaker (which is at a lower level than when you're close) an also it is delayed more from the original sound, so there are new stray noises that are mixed in and you are not just re-amplifying the same sound over-and-over, anymore.