Electromagnets & EM induction

Question 1

Rules for magnetic field lines

Answer 1

• they never touch or cross

* goes from north to south outside the device

Question 2

Methods of magnetizing and demagnetizing steel

Answer 2

can be magnetized with stroking method and an electrical method, is ‘hard’ and therefore doesn’t demagnetize

Question 3

what is the domain theory

Answer 3

When the domains(tiny cells) in a material are all lined up in the same direction the material is magnetized, they are pointing different directions in an unmagnetised material.

Question 4

uses of permanent magnets

Answer 4

-automobiles

-

Question 5

What was oersted’s discovery

Answer 5

He discovered that around a wire carrying a current there is a magnetic field. He found this by holding a wire over and parallel to a compass needle which is pointing N and S, the needle moves when the current is switched on. Reversing the current reversed the way the needle went.

Question 6

what is an electromagnet

Answer 6

Coil wrapped around a shot iron core

Question 7

How can you increase the strength of an electromagnet

Answer 7

• increasing current
• increasing the number of turns
• inserting a soft iron core

Question 8

Uses of electromagnets

Answer 8

• Junk yard crane
• Circuit breaker
• Some motors
• generators
• loudspeakers

Question 9

What is the motor effect

Answer 9

When a current carrying wire is placed in an external magnetic field, experiences a force and interacts with each other.

Question 10

How does a dc motor work

Answer 10

The rotating magnetic fields interact with the magnetic fields of the magnets in the stationary part of the motor (stator) to create a force on the armature which causes it to rotate

Question 11

How to induce electromagnetic current

Answer 11

Current will be generated in a closed loop of conductor moving through magnetic field lines.

Question 12

How does an ac generator work

Answer 12

dd

Question 13

Characteristics of ac

Answer 13

• Direction of current changes periodically with time
• has a changing magnetic field
• When a conductor is placed this changing field and emf is induced in it.

Question 14

what are the two types of transformers

Answer 14

• step-up

- step-down

Question 15

Transformer formula

Answer 15

Vs/Vp=Ns/Np

Question 16

What does a transformer consist of

Answer 16

primary coil and a secondary coil mounted on a core(loop) of laminated cost iron.

Question 17

How does the transformer work

Answer 17

Alternating current flowing in the primary coil produces an alternating field which is conducted around the core causing an alternating e.m.f to be induced in the secondary coil.

Question 18

What are ideal transformers + formula

Answer 18

100% efficient

• power delivered by the transformer= power consumed
• Vp x Ip = Vs x Is

Question 19

What is the use of transformers in the national grid

Answer 19

The national grid supplies power to houses and factories, which have drastically different needs in terms of amount of voltage. To have the high voltage a factory may require in a house would be very dangerous. In addition when transporting a lower voltage we loose less energy.