P4.2 - Uses Of Magnetism Flashcards
What is the motor effect?
When a current carrying wire, in a magnetic field, experiences a force.
What does the size of the force for the motor effect depend on?
The size of the force exerted, depends upon the strength of the magnetic field, the current in the wire and the length of the wire.
What is the equation for the motor effect?
- F=BIL
- F - Force on conductor - N
- B - Magnetic flux density - T
- I - Current - A
- L - Length of conductor (in field) - m
What can you use to calculate the direction of the force exerted on a wire?
- Fleming’s Left Hand Rule
- Thumb - Direction of the force.
- Index finger - Direction of the magnetic field (N to
S). - Middle finger - Direction of the current.
-The direction of the force exerted, is always
perpendicular to both the direction of the magnetic
field, and the direction of the current in the wire.
What does an electric motor use?
An electric motor uses the motor effect to transfer electrical energy to kinetic energy.
Describes what happens to a coil in an electric motor.
- An electric motor (in effect) consists of a
loop/length of wire, with a current, in a magnetic
field. - The wire will experience a force, given by F=BIL.
- As current flows in opposite directions in the
opposite sides of the coil, forces act in opposite
directions on opposite sides of the coil, causing
the coil to rotate. - Using Flemings Left Hand Rule - Left side of the
coil experiences a force up and the right hand side
experiences a force downwards as the current is
running in the opposite direction - The coil rotates
clockwise. - Once the coil rotates to the vertical position, the
coil experiences no force on its sides because the
current no longer runs perpendicular to the
magnetic field lines but it continues to move past
this position due to its momentum. - As the two sides of the coil are now switched over
and the current is back to running perpendicular to
the field lines, but in an opposite direction the coil
will experience a force again and turns in the
same direction with the help of a split ring
commutator.
Why do you need a split ring commutator in an electric motor?
- If the current continued to run in the same
direction, the direction of the force would keep
on being in the opposite direction every half
turn of the coil. - The coil would end up flip-flopping backwards
and forwards about the vertical position. - A split ring commutator is used to reverse the
direction of the current in the coil every half turn
of the coil.
This reverses the direction of the forces on the sides of the coil every half turn, ensuring the coil continues to rotate in the same direction
What do you need to do in order to increase the speed of a motor?
- Increase the size of the force acting.
- F=BIL
- More loops/Longer length of wire.
- Increase the strength of the magnet.
- Increase the current in the coil of wire
What is electromagnetic induction?
- Sometimes called the generator effect.
- When a conductor cuts the magnetic field lines
of a magnet, a potential difference is induced
across the ends of the conductor. - Opposite of the motor effect.
How does electromagnetic induction work?
- When a wire cuts magnetic field lines, electrons
are moved to one side of the conductor, which
generates a p.d. - If the wire is part of a complete circuit, a current
will flow.
What does the size of the induced potential difference in a conductor depend on?
The size of the induced p.d. in a conductor depends upon the rate at which you cut field lines.
What are the three ways to increase the size of the induced potential difference?
You need to increase the rate of cutting field lines by either:
- Increasing the number of coils in the wire.
- Using a stronger magnet.
- Move the wire quicker through the magnetic field.
What happens when you reverse the direction of the wire (electromagnetic induction)?
Reversing the direction of the wire reverses the direction of the induced p.d. in the wire.
Explain the terms of field lines why using a stronger magnetic field induces a larger potential difference in a wire or coil.
- A stronger magnetic had a greater magnetic
flux density/greater density of field lines around
it. - Thus, when a wire is moved at any given
velocity through the magnetic field of a stronger
magnet, the rate at which the wire cuts field lines
is greater, so a larger potential difference is
induced.
What is Lenz’s Law and what does it ensure?
- “The direction of the induced p.d. is such that it
opposes the change producing it”. - Lenz’s Law ensures that the law of conservation
of energy isn’t violated.
Give an example of how Lenz’s Law works?
- Magnet approaching a coil of wire with its north
pole. - The coil cuts magnetic field lines.
- A potential difference is induced in the coil of
the wire and will give rise to a north pole on the
end the magnet is approaching. - The north pole of the magnet and north pole of
the coil will then repel, opposing the motion
inducing the p.d.
You need to be able to slow down a falling lift if the cable breaks.
Suggest and explain how you can use electromagnetic induction to slow down a falling lift.
-The lift is fitted with permanent magnets, and made
to travel inside a metal tunnel.
- If the cable breaks, the lift will fall, and the
magnets will move past the metal surfaces of the
tunnel. - The metal surfaces of the tunnel will cut magnetic
field lines, in inducing a p.d. and current in them. - The current carrying surface will now interact with
the magnetic field of the magnets, give rise to a
motor force. - This motor force will acts upwards, opposing the
motion of the lift, causing it to fall/accelerate at a
lesser rate than it would under the force of just its
weight.
How does an alternator work?
- Coil of wire spins between the poles of a
uniform magnetic field (equivalent to the
magnet moving in and out). - The brushes are not attached to the slip rings
but brush against the slip rings so that the
voltmeter is always connected to the ends of
the coil but the coil does not become tangled. - As the coil turns, each side of it cuts the field
lines from below then above and so on. - The induced potential difference is therefore
repeatedly changing direction (and so therefore
is the induced current).
How does a dynamo work?
- Instead of two slip rings, a dynamo has a split ring
commutator. - The split ring reconnects the coil the other way
round every half turn of the coil. - Therefore, the induced p.d. does not reverse its
direction but can only change in magnitude.
Compare an alternator and a dynamo.
- Both use coils and magnets to induce a p.d.
(and current). - Alternators use slip-ring commutators to give
rise to an a.c. current. - Dynamos use split-ring commutators to give
rise to a d.c. current.
How does doubling the speed of rotation of a dynamo change the potential difference induced?
- The size of the peak p.d. induced will double.
- The rate at which field lines are cut is doubles
which doubles the frequency as well the the
peak p.d.
Why is the potential difference zero when the coil is parallel with the poles of the magnet?
When the coil is parallel with the magnet, it will cut no field lines when it starts to move, thus inducing no p.d.
A) Explain why the ammeter point deflects?
B) Explain why is deflects only once and returns
back to its position.
A) When the switch is closed a current flows
through the wire which gives rise to a magnetic
field.
-Coil Y will cut the magnetic field lines and a p.d.
will be induced in the conductor.
-If the conductor is a complete circuit, a current
will flow inside the conductor which is why the
ammeter pointer deflects.
B) When the current is constant, the field lines
around the magnet stop growing.
-Coil Y no longer cuts field lines so there is no
p.d. induced which means that no current flows,
so the ammeter does not deflect
What are transformers?
- Transformers are devices that use an
alternating p.d. (current) in a primary circuit to
induce an alternating p.d. (current) in a
secondary circuit. - Transformers work by making use of
electromagnetic induction. - Transformers only work with an AC supply in
the primary circuit.
How do transformers work?
- An alternating p.d. across the primary coil
produces an alternating current in the primary coil. - An alternating current in the primary coil produces
a magnetic field in the iron core that is always
changing. - The secondary coil experiences a constantly
changing magnetic field.
- A p.d. (current) is continually induced in the secondary coil (in alternating directions).
What do step up and step down transformers do?
- Step up transformers - Increase the p.d. (higher
voltage induces in the secondary circuit), because
the secondary circuit has more turns. - Step down transformers - Decrease the p.d. (lower
voltage induced in the secondary circuit), because
the secondary circuit has fewer turns.
Explain why it is important for the laminated iron core of a transformer to be a complete loop with no gaps?
The magnetic field lines wouldn’t be transferred over the primary coil to the secondary coil efficiently, and power loss would occur in the secondary coil.
How does a microphone work?
- A microphone is like a generator.
- A sound wave is like a pressure wave.
- A sound wave hits a diaphragm and causes it to
move in and out - areas of high pressure,
compressions, push the diaphragm in, and areas
of low pressure, rarefactions, pull it out. - As the diaphragm moves in and out, so does the
coil. - The coil moves inside a permanent’s magnet’s
magnetic field and so cuts field lines. - A p.d. (and therefore current) in now induced in
the coil, producing an electric signal that is fed
to a speaker.
How does a loudspeaker work?
- A loudspeaker is like a motor.
- A changing p.d. in a coil of wire produces a
changing current. - The wire is inside a permanent magnet’s
magnetic field and so experiences a force. - A cone attached to the coil moves in and out.
- A sound wave is generated of the same
frequency at which the diaphragm vibrates
(moves in and out).
How can you make a simple loudspeaker?
- You can make a simple loudspeaker with a paper
cone, a piece of wire, and a pair of magnets. - If you connect the ends of the wire to an
alternating potential difference of a suitable
frequency, you will hear a sound.
