T7: magnetism and electromagnetism Flashcards
What are the 2 poles of a magnet?
North and South
What are the poles of a magnet?
The poles of a magnet are the places where the magnetic forces (and field) are strongest.
What do all magnets produce?
a magnetic field
What is a magnetic field?
The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.
What is the direction of magnetic field lines?
from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.
What is the direction of the magnetic field at any point is given by?
The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point.
How do magnetic field lines show a stronger magnetic field?
field lines that are closer together.
What does the strength of a magnetic field depend on?
The strength of the magnetic field depends on the distance from the magnet. (the further away fro the magnet the weaker the magnetic field is)
What is the force like between a magnet and magnetic material?
always attractive, no matter the pole.
What happens when 2 poles of a magnet are put near each other? (give 2 examples)
they will each exert a force on each other.
- 2 like poles will repel
- 2 unlike/different poles will attract each other.
What is inside a compass?
a small bar magnet
When a compass is near another magnet, how can the magnetic field be found?
The compass contains a small bar magnet. The North Pole of the bar magnet will be attracted to the South Pole of any other magnet it is near. So the compass points in the direction of the magnetic field it is in.
How does a compass act when there is no magnet near it?
The compass will always point north. This is because the earth generates its own magnetic field, which shows the inside (core) of earth is magnetic.
What are the 2 types of magnets?
permanent and induced
What is a permanent magnet?
A magnet that produces its own magnetic field.
What is an induced magnet?
A magnetic material that becomes a magnet when it is placed in a magnetic field.
What force does an induced magnet cause?
an attractive force
What happens when the magnetic field is taken away from the induced magnet?
the induced magnet quickly loses most/all of its magnetism and stops producing a magnetic field.
What happens when current flows through a wire?
When a current flows through a conducting wire a magnetic field is produced around the wire.
What is the magnetic field produced by the current in the wire made up of?
co-centric circles perpendicular to the wire, with the wire in the centre.
What happens if the direction of the current changes?
The direction of the magnetic field changes.
What is represented in the right hand thumb rule?
thumb = current (point in the direction of the current)
fingers = magnetic field
What does the strength of the magnetic field (produced by the current in the wire) depend on?
- current through the wire and the distance from the wire.
- the larger the current through the wire and the closer to the wire you are, the STRONGER the magnetic field.
How can the strength of the magnetic field of a wire increase?
By wrapping the wire into a coil called a solenoid.
How does a solenoid increase the strength of the magnetic field of a wire?
- the field lines around each loop of wire line up with each other.
- this results in lots of field lines pointing in the same direction that are very close together, the closer the filed lines the stronger the magnetic field.
What is the magnetic field like inside the solenoid?
strong and inform (it has the same strength and direction at every point in that region)
What are the 4 ways to increase the strength of the magnetic field?
1) increase the current flowing through the solenoid.
2) increase the number of coils (keeping the length of the wire the same)
3) decrease the length (keeping the number of turns the same)
4) place an iron core in the centre of the coil.
How does putting an iron core in the centre of the coil increase the strength of the magnetic field?
- the iron core becomes an induced magnet whenever the current is flowing.
- If you stop the current, the magnetic field disappears.
What is a solenoid with an iron core called?
A solenoid with an iron core (a magnet whose magnetic field can be turned on/off with an electric current) is called an ELECTROMAGNET.
Why are magnets that can be switched on and off so useful?
- it is so quick to turn on and off
- because they can create varying force (like in loudspeakers)
How are electromagnets used in cranes?
The electromagnets will attract and pick up any magnetic material (in scrap yards). Using an electromagnet means the magnet can be switched on when you want to pick something up and turned off when you want to drop it.
How can electromagnets be used within other circuits to act as switches?
-when the switch on one circuit is closed, it turns on the electromagnet, which attracts the iron contact on the rocker.
- The rocker pivots and closes the contacts, completing circuit 2 and turning on the motor.
What is the motor effect?
When a current carrying wire (or any other conductor) is put between magnetic poles, the magnetic field of the wire interacts with the magnetic field it has been placed in. This causes the magnet and the conductor to exert a force on each other, and can cause the wire to move.
How does the wire have to be positioned to experience the full force of the magnetic field?
the wire has to be at 90° to the magnetic field. If the wire runs parallel to the magnetic field it won’t experience any force at all; at angles in between, it will feel some force.
What does the force act at right angles to?
- magnetic field of the magnets
- direction of the current in the wire
How can the direction of the force be shown?
apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails which completes the circuit. This generates a force that rolls the bar along the rails.
What does the magnitude of the force increase with?
- the strength of the magnetic field
- the amount of current passing through the conductor
What does the force acting on a conductor in a magnetic field depend on?
- the magnetic flux density
- the size of the current through the conductor
- the length of the wire in the magnetic field
What is magnetic flux density?
how many field (flux) line there are in a region.
For a conductor at right angles to a magnetic field and carrying a current, what is the equation for force?
force on a conductor (at right angles to a magnetic field) carrying a current = magnetic flux density × current × length
F = B I l
(force, F , in newtons, N
magnetic flux density, B , in tesla, T
current, I , in amperes, A (amp is acceptable for ampere)
length, l , in metres, m)
What is Fleming’s left hand rule?
1) FIRST finger in the direction of the FIELD
2) seCond finger in the direction of the CURRENT
3) thuMb will then point in the direction of the force (MOTION)
What does Fleming’s left hand rule show?
if the magnetic field or the current is reversed then the direction of the force will also be reversed
On a basic dc motor where and how does the force act?
- forces act on the 2 sides of the arms of a coil of wire that’s carrying a current.
- this force is the usual force which acts on any current in a magnetic field.
How does a basic dc motor rotate?
As the coil is on a spindle and as the forces act one up and one down.
What does the split ring commutator do?
-swaps the positive and negative connections every half turn
- so the direction of the current swaps every half turn.
- so the forces action on the coil will always be acting in the same direction
- so the motor keeps rotating in the same direction
How can the direction of the motor be reversed?
by swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field).
How does loudspeakers and headphones use the motor effect to work?
Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
What are the steps that enable loudspeakers and headphones to work?
1) An alternating current is sent through a coil of wire to the base of a paper cone.
2) The coil surrounds one pole of the permanent magnet and is surrounded by the other so the current causes a force on the child, which causes the cone to move.
3) When the current reverse, the force acts in the opposite direction, which causes the cone to move in the opposite direction.
4) So variations in the current make the cone vibrate, which makes air around the cone vibrate and creates variations in pressure that cause a sound wave.
How can you alter the sound wave produced?
The frequency of the sound wave is the same as the frequency of the alternating current so by controlling the frequency of the alternating current you can alter the sound wave produced.
What is the generator effect?
The induction of a potential difference (and a current if there’s a complete circuit) in a wire moving relative to a magnetic field, or experiencing a change in magnetic field.
What does the generator effect create?
a potential difference in a conductor and a current if the conductor is part of complete circuit.
How can a potential difference/current be created?
by moving a magnet in a coil of wire or by moving a conductor (wire) in a magnetic field. (“cutting magnetic field lines).
What happens if the magnet is shifting from side to side?
it creates a little”blip” of current if the conductor is part of a complete circuit.
How can the potential difference and the current be reversed?
1) if the magnet or conductor are moved in the opposite direction.
2) If the polarity of the magnet is reversed.
What happens if you keep the magnet (or coil) moving backwards and forwards?
you produce a potential difference that keeps swapping direction, an alternating current.
How do generators work to produce an ac or direct current?
by turning a magnet end to end in a coil or turning a coil inside a magnetic field.
What happens as you turn a magnet?
As you turn a magnet, the magnetic field through the coil changes. This change in the magnetic field induces a pd which can make a current flow in the wire.
What happens when you’ve turned a magnet through a half turn?
the direction of the magnetic field through the coil reverses causing the pd to reverse so the current flows in the opposite direction around the coil of wire.
What happens if you keep turning the magnet in the same direction?
the pd will keep on reversing every half turn and you’ll get an alternating current.
What does an induced current generate?
An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.
How can you change the size of the induced potential difference?
to change the size of the induced pd, you have to change the rate that the magnetic field is changing; induced potential difference can be increased by:
- increased speed of the movement (cutting more magnetic field lines in given time)
- increasing the strength of the magnetic field (so there are more field lines that can be cut)
What does an alternator do?
An alternator rotates a coil in a magnetic field (or a magnet in a coil). The construction of an alternator is pretty much like a motor.
(Alternators generate Alternating current)
What happens in an alternator as the coil (or magnet) spins?
A current is induced. This current changes direction every half turn.
What do alternators have instead of a split ring commutator and what does this mean?
AC generators have slip rings and brushes so the contacts do not swap every half turn, which means they produce an alternating POTENTIAL DIFFERENCE.
Dynamos work in the same way as alternators expect from one important different, what is this difference?
Dynamos have a split ring commutator instead of slip rings.
This swaps the connection every half turn to keep the current flowing in the same direction (generates a dc current).
What do oscilloscopes show?
how the potential difference generated in the coil changes over time.
How is an ac pd shown on an oscilloscope?
a line that goes up and down, crossing the horizontal axis.
How is an dc pd shown on an oscilloscope?
it is NOT a straight line but it stays above the axis (pd is always positive) so it is still a direct current.
On an oscilloscope, what does the height at any given point show?
the height of the line at any given point is the generated potential difference at that time.
What does increasing the frequency of revolutions cause?
an increase in the overall potential difference but it also creates more peaks too.
What happens in microphones?
- sound waves hit the flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.
- this causes the coil of wire to move (vibrate) in the magnetic field which generates a current.
- microphones convert the variations of pressure in sound waves into variations in current in an electric circuit. (reverse of loudspeakers)
What does the movement of the coil and the generated current depend on?
the properties of the sound wave (the louder the wave, the further the diaphragm moves).
What do transformers do?
change the size of potential difference of an alternating current.
What is the structure of transformers?
A basic transformer consists of a primary coil and a secondary coil wound/joined on an iron core.
What happens when an alternating pd is supplied across a primary coil?
The iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating pd in the secondary coil.
What happens if the second coil is part of a complete circuit?
it causes a current to be induced.
What is the ratio between the primary and secondary potential difference equal to?
the ratio between the number of turns on the primary and secondary coils.
What do step up transformers do and how?
- increase the pd
- they have more turns on the secondary coil than the primary coil
What do step down transformers do and how?
- decrease the pd
- they have more turns on the primary coil than the secondary coil
If the input pd and the number of turns on each coil are know, how can the output pd be calculated?
potential difference across primary coil ÷ potential difference across secondary coil = number of turns in primary coil ÷ number of turns in secondary coil
Vp ÷ Vs = Np ÷ Ns
Transformers are almost 100% efficient, if we assume they are then what is equal?
the input power is equal to the output power
Using P=VI , how can we link output power and input power?
Vp x Ip = Vs x Is
( potential difference across primary coil × current in primary coil =
potential difference across secondary coil × current in secondary coil )
What is the difference of V s and V p in step up and step down transformers?
In a step-up transformer V s > V p
In a step-down transformer V s < V p
