Magnetism questions.

[iroflmfao]

These may be stupid.

1. What the difference between the two EMF forumlas, like how do i know which one to use. For instance what if all the variables for both forumlas were given in the question. Which one do i choose ?
2. How do the differentiate between the primary and secondary in transformer. Eg there a lamp at 18 V and the supply at 240.
 Which ones the primary o.0?

[Lasercookie]

1. What the difference between the two EMF forumlas, like how do i know which one to use. For instance what if all the variables for both forumlas were given in the question. Which one do i choose ?

Remember that EMF=vbl is mostly useful for a straight wire in a field.

is much more general.
refers to the case of a coil of wire.

I always use EMF is the negative rate of change of flux though. I haven't yet come across a question where the EMF=vbl formula was required. If you need to find out velocity, you can normally do it with v=d/t. However, both formula should give you the same answer.

2. How do the differentiate between the primary and secondary in transformer. Eg there a lamp at 18 V and the supply at 240.
 Which ones the primary o.0?

I normally assume supply to be in the primary, and the load to be in the secondary. Exam questions usually worded clearly enough to be able to differentiate the two.