P7-Magnetism and Electromagnetism Flashcards
At which part of a magnet are the magnetic forces
strongest
The poles of the magnet
What happens when two magnets are brought close
to each other
They exert a force on each other
What type of force is exerted if two like poles of a
magnet are brought near each other
A repulsive, non-contact force.
What type of force is exerted if two unlike poles of a
magnet are brought near each other
An attractive, non-contact force
What is the difference between a permanent magnet
and an induced magnet?
● A permanent magnet produces its own
magnetic field
● An induced magnet becomes magnetic
when placed in a magnetic field
What type of force does induced magnetism always
cause?
A force of attraction
What happens when an induced magnet is removed
from a magnetic field?
The induced magnet loses most/all of its
magnetism.
What is a magnetic field?
The region surrounding a magnet where
another magnet or magnetic material
experiences a non-contact force.
Give four examples of magnetic materials
- Iron
- Steel
- Cobalt
- Nickel
What can always be said about the force between a
magnet and a magnetic material
It is always attractive.
How does the strength of a magnetic field alter as
you move further away from the magnet producing
it
The magnetic field strength decreases
the further you move away
In what direction does a magnetic field point
● In the direction that a north pole would
experience a force if placed in the field
● From the north seeking pole to the
south seeking pole of a magnet
What does a magnetic compass contain
A small bar magnet that points in the
direction of the Earth’s magnetic field.
What is produced when current flows
through a conducting wire?
A magnetic field is produced around the
wire.
What determines the strength of the magnetic field
around a current-carrying wire
● The magnitude of the current flowing
through the wire
● The distance from the wire
What is a solenoid?
What is a solenoid?
Describe the magnetic field found inside a solenoid
Strong and uniform.
What is an electromagnet
● A solenoid with an added iron core
● Adding the iron core increases the
strength of the magnetic field
What is the motor effect?
When a force is exerted between a
magnetic field and a current-carrying
conductor placed in that field.
What rule is used to determine the force experienced
due to the motor effect?
Fleming’s Left-Hand Rule
When using Fleming’s left hand rule what does the
forefinger represent?
The forefinger points in the direction of
the magnetic field
When using Fleming’s left hand rule
what does the second finger represent?
The second finger points in the direction
of current flow in the conductor
What factors affect the size of the force on a
current-carrying wire in a magnetic field
● The magnitude of the current flowing
through the conductor
● The strength of the magnetic field that
the conductor is placed in
If the direction of current in a current-carrying wire
placed in a uniform magnetic field is reversed, what
happens to the force
The direction of the force is reversed.
If the strength of the current in a current-carrying
wire placed in a uniform magnetic field is reversed,
what happens to the force?
The strength of the force is increased
What criteria must be met for the equation linking
force, magnetic flux density, current and length to
hold?
The conductor must be at right-angles to
the magnetic field it is placed in
What is the unit used for magnetic flux density?
Tesla, T
How does an electric motor work?
● A coil of wire, carrying a current, is placed
in a magnetic field
● The forces on the two sides perpendicular
to the field experience forces in opposite
directions
● This causes a rotational effect
How do loudspeakers make use of the motor effect?
The motor effect is used to convert
variations in the current of an electrical
circuit into the pressure variations which
produce audible sound.
Explain how a loudspeaker works
● A cone with a wire wrapped around it is connected to an
a.c power supply and is placed in a permanent
magnetic field
● When current flows through the wire, it creates a
second magnetic field, which interacts with the
permanent field
● This produces a force which causes the cone to vibrate
How is the pitch of the sound from a loudspeaker
changed?
● The frequency of the a.c current is
altered
● This creates a different frequency of
vibration in the cone
What happens when an electrical
conductor moves relative to a magnetic
field?
A potential difference is induced across
the ends of the conductor
What happens to an electrical conductor when there
is a change to the magnetic field that it is placed in?
A potential difference is induced across
the ends of the conductor
What is the requirement for an induced potential
difference to cause a current flow
The conductor must form a closed loop
or be part of a complete circuit.
What can be said about the direction of the magnetic
field produced by an induced current?
● The direction of the field is such that it opposes the change that induced the current ● This change is either the movement of the conductor or a change to the field
What are two ways that the generator effect is used
to generate different types of current?
● In an alternator to produce
alternating-current
● In a dynamo to produce direct-current
What electromagnetic effect does a microphone take
advantage of and how?
● The generator effect
● It converts the pressure variations in
sound waves into alternating current in
a circuit
Describe the makeup of a basic transformer
A primary coil and a secondary coil of
wire wrapped around an iron core.
Why is iron used as the core for a transformer?
It is easily magnetised.
Explain how a transformer works.
● An alternating current flows through the
primary coil
● This induces a changing magnetic field in
the core
● This changing magnetic induces a current
to flow in the secondary coil
Why must the current flowing through the primary
coil of a transformer be alternating
● For current to be induced in the secondary
coil, the magnetic field in the core must be
continuously changing
● For the magnetic field to be changing, the
current in the primary coil must be alternating
What can be said about the electrical power input
and output of a 100% efficient transformer
The electrical power input is equal to the
electrical power output.
Like poles ______, unlike poles ________
Like poles repel, unlike poles attract
What is the difference between a contact and a non-contact force?
Contact forces need to touch to act, non-contact forces do not
Is magnetism a contact/non-contact force?
Magnetism is a non-contact force.
What is a permanent magnet?
A permanent magnet produces its own magnetic field.
What is an induced magnet?
An induced magnet becomes a magnet in a magnetic field.
What are the four magnetic metals?
Iron, Nickel, Steel and Cobalt.
Where is the field of a bar magnet strongest?
At the poles.
What happens to the strength of a magnetic field the further away you go from a magnet?
The strength reduces the further away you go.
What is a magnetic compass made of? What direction does it point?
A magnetic compass is made of a small bar magnet. It points towards geographic north, but magnetic South.
What direction do magnetic field lines go in?
From North to South.
How can we use a compass to plot the pattern of the magnetic field lines around a magnet?
Place plotting compass near the magnet on a piece of paper. Mark the direction the compass needle points. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time. Join the points to show the field lines.
What causes a wire to produce a magnetic field?
A current flowing through it.
What is the scientific name for a long coil of wire?
A solenoid.
Describe the magnetic field inside a solenoid.
Strong and uniform
What kind of magnet has the same magnetic field as the long coil of wire?
A bar magnet
What is the main advantage of using an electromagnet vs a permanent magnet?
Electromagnet can be turned off by a switch.
Name three ways that we can make an electromagnet stronger.
Increase the current, increase the number of turns on the coil, add an iron core.
What rule can we use to draw the magnetic of the magnetic field pattern for a straight wire?
The right-hand rule.
What do the fingers stand for in Fleming’s left-hand rule?
Thumb –force/motion, First finger –field, second finger –current
What is the equation for the force on a current-carrying wire?
F = B × I × l
How does an electric motor work?
A coil of wire carrying a current in a magnetic field experiences a force that makes it rotate. A split ring commutator changes the current direction every half turn
What does a step-up transformer do
(it) decreases the p.d
Explain how a transformer works
any four from:
• insulation prevents electricity/current flowing through the iron/core
or ‘insulation so electricity/current only flows in the
wires/turns/coils’
• alternating current/a.c. in the primary (coil)
• produces a changing magnetic field (in the iron/core)
• (and hence magnetic) field in the secondary (coil)
• induces/generates/produces an alternating potential
difference/p.d./voltage across the secondary (coil)
• (and hence) alternating current/a.c. in the secondary (coil)
The number of turns of wire o the coil is increased. This increases the maximum induced potential difference.
State 2 other ways in which the maximum induced potential difference could be increased
any two from: • increase speed of coil • strengthen magnetic field • increase area of coil do not accept larger
There are two types of traditional transformer; step-up and step-down.
Describe the similarities and differences between a step-up transformer and a step-down
transformer.
You should include details of:
• construction, including materials used
• the effect the transformer has on the input potential difference (p.d.).
a (laminated) core
core is made from a magnetic material / iron
2 coils
the coils are made from an electrical conductor / copper
the coils are covered in plastic / insulation
the coils are (usually) on opposite sides
step-up transformer has more turns on secondary coil than (its) primary (or vice
versa)
step-down transformer has fewer turns on secondary coil than (its) primary (or vice
versa)
effect on input p.d. :
step-up transformer, the output p.d. is greater (than the input p.d.)
accept voltage for p.d.
step-down transformer, the output p.d. is lower (than the input p.d.)
The loudspeaker cone vibrates when an alternating current flows through the coil.
Explain why.
(the current flowing through the coil) creates a magnetic field (around the coil)
(this magnetic field) interacts with the permanent magnetic field
or
current carrying conductor is in a (permanent) magnetic field
it must be clear which magnetic field is which
1
this produces a (resultant) force (and coil / cone moves)
1
when the direction of the current changes, the direction of the force changes
to the opposite direction
Describe, in as much detail as you can, how this circuit breaker works.
pulls iron bolt down
plunger pushed
push switch opens
