Electromagnetism

Question 1

What is a magnetic field?

Answer 1

the region around a permanent magnet or a moving charge (an electric current) in which another body with magnetic properties will feel a force

Question 2

What do magnetic field lines show?

Answer 2

• the shape of the field -the direction of the field line at a point shows the direction in which a plotting compass would point

Question 3

What is a solenoid

Answer 3

a long coil of current-carrying wire

Question 4

What does it mean if the field lines are drawn closer together?

Answer 4

that the field is stronger

Question 5

True or false: magnetic field lines have a beginning and end

Answer 5

False- they have no beginning or end , they always form a closed loop *the direction is from a north pole to a south pole but they do not begin at north and end at the south

*notice how the field lines continue in the solenoid , similarly magnetic field lines continue inside a bar magnet to close the loop

Question 6

When is the right hand rule used?

Answer 6

when an electric current is flowing in a straight conductor

Question 7

What is dot and cross notation

Answer 7

dot and cross notation indicates the direction of the current : a circle with a dot shows the current is coming out of the page and a cross is moving into the page

Question 8

What does the north-south polarity of a solenoid depend on?

Answer 8

it depends on the direction of the current

Using the right-hand rule :

in this case the curling of your fingers indicates the direction of the current flow around the solenoid and , your thumb will point towards the north pole of the magnetic field of the solenoid

Question 9

Describe the field inside the solenoid

Answer 9

the field is strong and very uniform - this is demonstrated by the closely spaced parallel lines

Question 10

What would occur if a magnetic material was placed inside a solenoid?

Answer 10

the magentic field would increase considerably

Question 11

Sketch the pattern of magnetic field lines around a solenoid

Answer 11

inside the solenoid , the field is strongest and very uniform but outside the coil ,the field is similar to the magnetic field produced by a bar magnet

Question 12

Sketch the pattern of magnetic field lines around a single coil carrying current

Answer 12

Question 13

Sketch the pattern of magnetic field lines around a bar magnet

Answer 13

Question 14

Electromagnets can be switched on and off by having an electric current flowing through them. Explain what happens to the magnetic field of a solenoid when the current stops

Answer 14

the magnetic field is created by moving charges in the wire , so no current means the magnetic field would dissapear

Question 15

Define Magnetic flux ø

Answer 15

its the product of magnetic flux density ,B, and the area at right angles to the flux : ø =BAcos(theta)

where theta is the angle between the field lines and the normal to the surface

*the unit for magnetic flux is Weber (Wb)

Question 16

Define magnetic flux density, B

Answer 16

Its a measure of the strength of a magnetic field and its a vector quantity

Question 17

When will the flux be maximum/minimum?

Answer 17

max when theta = 0º

and zero when theta = 90º

Question 18

Typical values for a variety of flux densities

Answer 18

Question 19

Did you know?

Answer 19

Unlike electric charge, which can exist as isolated positive or negative charges , magnetic poles cannot exist as isolated north or south poles - even if you were to slice a magnet into even smaller and smaller pieces you would always have a north and south pole together

Question 20

What does Fleming’s Left hand rule show?

Answer 20

it shows the direction of the force on a conductor carrying a current in a magnetic field

Force/Motion - thumb

Field - First finger

Current - Second finger

Question 21

What is 1 tesla equal to?

Answer 21

Wb/ m² (no. of field lines per unit area)

Question 22

Fleming’s left-hand rule allows us to determine the direction that a current- carrying wire will move when placed in a uniform magnetic field but how do you find the magnitude of the force (which causes the movement) acting on a current -carrying wire?

Answer 22

F= BILsintheta

B- magnetic flux density of the magnets magnetic field

I- current flowing in the wire

L- length of wire within the region of the magnet’s magnetic field

-sine of the angle ,theta, between wire and field lines

Question 23

When is the force acting on a current carrying wire max?

Answer 23

when the wire and the field lines are at right angles to eachother →theta = 90º

Question 24

What occurs if there is a relative motion between a conducting rod and a magnetic field?

Answer 24

the electrons in the rod will experience a force which causes them to accumulate on one end of the rod

this induces an electromotive force (e.m.f) across the ends of the rod exactly as connecting a battery to it would

*if the rod was part of a complete circuit then an induced current would flow through it

Question 25

How do you induce an e.m.f in a flat coil or solenoid?

Answer 25

moving the coil towards or away from the poles of the magnet

OR

moving a magnet towards or away from the coil

-in either case, the e.m.f is caused by the magnetic field (or magnetic flux) that passes through the coil changing

*if the coil is part of a complete circuit, an induced current will flow though it

Question 26

State Faraday’s Law and when can you use it?

Answer 26

the induced e.m.f is proportional to to the rate of change of flux linkage

: E = - Nø ÷ t

• the equation can be used for a coil or conducting rod
• for a conducting rod , think of the flux change as the field lines being ‘cut’ as the rod moves
• the minus sign is Len’s law

Question 27

State Len’s Law

Answer 27

the direction of the induced emf (or current) is always such that it opposes the change that caused it

Question 28

What is a transformer?

Answer 28

a device that either increases or decreases the size of an alternating voltage with little loss of power

Question 29

What do transformers consist of?

Answer 29

two coils of wire wrapped around an iron core

• one of the coils is connected to an alternating voltage supply , this is called the primary coil
• the second is connected to the output , this is called the secondary coil

Question 30

Transformers operate using electromagnetic induction and so require a constantly changing magnetic flux, what source will then need to function?

Answer 30

an a.c source

Question 31

How can you tell a step-up transformer apart from a step-down transformer?

Answer 31

step-up transformers increase the voltage by having more turns on the secondary coil

Question 32

Real transformers arent 100% efficient- some power is always lost

How can you reduce loses?

Answer 32

use laminated cores

Question 33

State the transformer equation

Answer 33

Ns/Np = Vs/Vp = Ip/Is