P15 - Electromagnetism (PAPER 2) Flashcards
What is the relation between different poles
Opposite poles attract, like pols repel
How do magnetic field lines of bar magnets exist
hey curve around from the north pole to south pole of the magnet
what is induced magnetism
Magnetism created in an unmagnetised magnetic material when the material is placed in the magnetic field
Why is steel used for permanent magnets instead of iron
Steel doesn’t lose magnetism easily, whilst iron does
How do magnetic field lines around a wire exist
as circles centred on the wire in a plane perpendicular to the wire
How do magnetic field lines in a solenoid exist
parallel to its axis and all in the same axis
What is a uniform magnetic field
one where the magnetic field lines are parallel
How does increasing the current affect the magnetic field
it makes it stronger
How does reversing the direction of the current affect the magnetic field
reverses the magnetic field lines
What is an electromagnet
a solenoid that has an iron core - insulated wire wrapped around an iron bar
What is a solenoid
a long coil of insulated wire
Where are some places electromagnets are commonly used
scrapyard cranes, circuit breakers, electric bells, relays
How do electromagnets work in objects such as circuit breakers / relays / electric bells
By attracting an iron armature which opens a switch
How can a force be increased in the motor effect (2)
if the current or the strengh of the magnetic field or length of conductor is increased
How can a force be reversed in the motor effect
if the direction of current or magnetic field is reversed
How do coils in an electric motor turn
When a current is passed through
What is magnetic flux density
a measure of the strength of a magnetic field
What is the equation for calculating force on a current carrying conductor at right angled to the lines of a magnetic field
F = B I L
force(N, Newtons) = Magnetic Flux Density (T, Tesla) x Current (A, Amperes) x length (m, metres)
What is the generator effect
the effect of inducing a potential difference using a magnetic field
What happens when a conductor crosses through the lines of a magnetic field
a potential difference is induced across the ends of the conductor
What is the relation between the speed a conductor crossing through the lines of a magnetic field and the induced potential difference
the faster the speed, the larger the potential induced difference
What is the relation between the direction of an induced current and the original change that caused it
They are opposite
what is a simple a.c. generator made from
a coil that spins in a uniform magnetic field
When is the waveform of the a.c. generator’s induced potential difference at its peak value
when the sides of the coil cross directly through the magnetic field lines
When is the waveform of the a.c. generator’s induced potential difference at its zero value
when the sides of the coil move parallel tot he field lines
What is a visual difference between a dynamo and an alternater
the dynamo has a split ring commutator whilst an alternator has two slip rings
What is a dynamo and an alternater
dynamo - a d.c. generator
alternator - an a.c. generator
What are transformers used for
increasing or decreasing the size of an alternating potential difference
How are the sizes of alternating potential difference increased and decreased
increased - step up transformer
decreased - step down transformer
Why do transformers only work with a.c.
because a changing magnetic field is necessary to induce a.c. in the secondary coil
What are transformers made from
a primary coil, a secondary coil and an iron core
what is the transformer equation
primary potential difference (Vp) / secondary potential difference (Vs) = Np / Ns
- where Np = number of primary turns
- where Ns = number of secondary turns
What is the relation between the numbers of primary turns and secondary turns in step-up and step-down transformers
step-up - Ns > Np
step-down - Ns < Np
What is the equation for a 100% efficient transformer
primary potential difference x primary current = secondary potential difference x secondary current
What does a high grid potential difference do and why is it good
reduces the current needed, reducing power loss and making the system more efficient
