What is Back emf, and supply emf. (1 Viewer)

[DuhBoyChong]

What is Back emf, and supply emf. I have trouble understanding

 

[PhysicsMaths]

In terms of motors, a rotating armature (carrying the rotor coils) that moves through an external magnetic field will cut magnetic field lines, and will hence experience a changing flux. According to Faraday's law (which states EMF is proportional to change in flux/change in time), a changing flux will induce an EMF (known as the supply emf). This EMF will induce a current that creates a magnetic field to oppose the changing flux that induced it in the first place (Lenz's law). The opposition in motion caused by the induced current is known as back emf (because it opposes the supply emf).

To further explain, back emf is necessary to ensure the law of conservation of energy holds. If the induced current flows in a direction to support the motion of the rotor, then it would spin faster and faster (creating energy out of nowhere).

Also note that the net EMF created within the motor is defined as:
net emf = supply emf - back emf
and it's also worth mentioning that as a motor's armature rotates faster, it will experience a greater changing flux, so back emf will increase. Therefore, the net emf within the motor decreases!

*also note that Lenz's law will only apply to systems in a closed circuit. A current carrying rod moving through an external magnetic field will experience no back emf.

 

[DuhBoyChong]

Thank you sooo much

 

[someth1ng]

Just be careful with your wording, it's not correct to say that back emf is there to satisfy the conservation of energy. It's there because there is a change of flux through the ring which produces the back emf which then means the motor will satisfy the law of conservation of energy.

 

[Kaido]

What is emf

 

[Crisium]

What is emf

Electromotive Force

 

[Kaido]

lol, like the actual definition

 

[InteGrand]

lol, like the actual definition

http://en.wikipedia.org/wiki/Electromotive_force

 

[Fizzy_Cyst]

lol, like the actual definition

For HSC sake, EMF = Potential Difference = Voltage

We use all 3 interchangeably

 

[Crisium]

For HSC sake, EMF = Potential Difference = Voltage

We use all 3 interchangeably

But in real life:

Potential Difference = Voltage but there is a distinction between these and EMF, if I remember correctly

 

[Kaido]

But in real life:

Potential Difference = Voltage but there is a distinction between these and EMF, if I remember correctly

Hmm, just curious, but is this truth?

 

[Crisium]

Hmm, just curious, but is this truth?

What my teacher told us

Then again he doesn't know what he's talking about sometimes

 

[InteGrand]

Hmm, just curious, but is this truth?

Yeah. See here: http://en.wikipedia.org/wiki/Voltage#Definition

"Given two points in the space, called A and B, voltage is the difference of electric potentials between those two points."

(And this is what "electric potential" is defined as: http://en.wikipedia.org/wiki/Electric_potential (includes mathematical definition))