Electromagnetism Flashcards
What is a magnetic field?
the region around a permanent magnet or a moving charge (an electric current) in which another body with magnetic properties will feel a force
What do magnetic field lines show?
- the shape of the field -the direction of the field line at a point shows the direction in which a plotting compass would point
What is a solenoid
a long coil of current-carrying wire
What does it mean if the field lines are drawn closer together?
that the field is stronger
True or false: magnetic field lines have a beginning and end
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
When is the right hand rule used?
when an electric current is flowing in a straight conductor
What is dot and cross notation
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
What does the north-south polarity of a solenoid depend on?
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
Describe the field inside the solenoid
the field is strong and very uniform - this is demonstrated by the closely spaced parallel lines
What would occur if a magnetic material was placed inside a solenoid?
the magentic field would increase considerably
Sketch the pattern of magnetic field lines around a solenoid
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
Sketch the pattern of magnetic field lines around a single coil carrying current
Sketch the pattern of magnetic field lines around a bar magnet
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
the magnetic field is created by moving charges in the wire , so no current means the magnetic field would dissapear
Define Magnetic flux ø
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)
Define magnetic flux density, B
Its a measure of the strength of a magnetic field and its a vector quantity
When will the flux be maximum/minimum?
max when theta = 0º
and zero when theta = 90º
Typical values for a variety of flux densities
Did you know?
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
What does Fleming’s Left hand rule show?
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
What is 1 tesla equal to?
Wb/ m² (no. of field lines per unit area)
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?
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
When is the force acting on a current carrying wire max?
when the wire and the field lines are at right angles to eachother →theta = 90º
What occurs if there is a relative motion between a conducting rod and a magnetic field?
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
How do you induce an e.m.f in a flat coil or solenoid?
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
State Faraday’s Law and when can you use it?
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
State Len’s Law
the direction of the induced emf (or current) is always such that it opposes the change that caused it
What is a transformer?
a device that either increases or decreases the size of an alternating voltage with little loss of power
What do transformers consist of?
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
Transformers operate using electromagnetic induction and so require a constantly changing magnetic flux, what source will then need to function?
an a.c source
How can you tell a step-up transformer apart from a step-down transformer?
step-up transformers increase the voltage by having more turns on the secondary coil
Real transformers arent 100% efficient- some power is always lost
How can you reduce loses?
use laminated cores
State the transformer equation
Ns/Np = Vs/Vp = Ip/Is
