Physics Electromagnetism Flashcards
how to find the magnetic field around a bar magnet using iron filings?
- place a piece of people in top of a bar magnet
- sprinkle iron filings on top of paper
- tap the paper to show the magnetic field lines
How to find the magnetic field around a bar magnet using a compass?
- on a piece of paper, draw around the bar magnet
- place a plotting compass near the magnet and mark north and south. draw a line between north and south marks
- repeat to show the magnets field
3 rules for drawing magnetic field lines
- draw at least 3 lines
- field lines CANNOT cross
- field lines go from N —> S
3 magnetic metals?
iron, cobalt, nickel
what is a permanent magnet?
a magnet where all the domains align
What is the magnetic field between opposite poles?
Straight, parallel, uniform field lines
if a current goes into you, which direction do the field lines go around
anticlockwise
if a current goes away from you, which direction do the field lines go around
clockwise
what is current going into you equivalent for in a wire
around a wire carrying current out of the page
what is current going away from you equivalent to in a wire?
around a wire carrying current into of the page
When is an electromagnet formed?
When a current carrying wire is wrapped around an iron core
what are electromagnets used in?
scrap yards and washing machine doors
what does the iron core do to the electromagnet?
makes it stronger, due to the current flowing through the wire
how does a magnetic compass show that the earths core is magnetic?
as it contains a small bar magnet, and the needle of the compass points in the direction of the earths magnetic field
The ? magnet induces/? temporary poles in the magnetic ? and these align so the ? material is attracted to the permanent ?
permanent, creates, material, magnetic, magnet
if the North Pole of the magnet is next to the magnetic material, what will be induced in the part of the material closest to the magnet?
a south pole as it has to be attracted to the North Pole
what is magnetism?
the ability of magnets to attract and repel other magnets without touching them
what is magnetism?
the ability of magnets to attract and repel other magnets without touching them
what are induced magnets??
magnetic materials that become magnetic/magnets when they are in a magnetic field but when moved away, they stop being magnets
how can and can’t magnetic materials interact with a magnet
they can be attracted, but not repelled
what will never be attracted to magnets
non magnetic materials
2 features of permanent magnets
- are always magnetic
- produce their own magnetic field
2 features of permanent magnets
- are always magnetic
- produce their own magnetic field
what is a solenoid?
a loop of wife coiled into a cyclindrical shape
what is a solenoid?
a loop of wife coiled into a cyclindrical shape
what are the field lines like outside the coil of a wire?
like the field lines around a bar magnet - magnetic field lines go from north to south
what are the field lines like inside the coil of a wire?
- field lines are evenly spaced, parallel, straight lines
- magnetic field is strong and uniform (same strength and direction at every point)
what does strong and uniform mean in terms of magnets?
Strong field lines with the same strength and direction at every point
What is the motor effect
When a wire with the current flowing through it is placed in a magnetic field, the magnet and the conductor experiences a force
what is the direction of the motor effect determined by??
Flemings left hand rule
equation for force
F = BIl (force = magnetic flux * current *length)
what is magnetic flux measured in
Tesla (T)
why will a coil carrying a motor rotate in a magnetic field? ( what does it depend on )
provided the current going up one side of the coil is flowing in the opposite direction than the current coming back down the other side
how is a signal passed to a loudspeaker
has an electromagnet inside the coil which allows the signal to be passed onto it in the form of a changing current
what happens as the direction of current changes
the direction of the magnetic field induced in the electromagnet changes
how are sound waves created in a loudspeaker
- a stationary magnet repeatedly attracts and repels the electromagnet
- this causes the speakers to move backwards and forwards
- therefore creating sound waves
what is changed into sound waves from a microphone?
an electrical signal
how does a school bell work?
- DC turns on ?
- ? armature is attracted to the ?
- this makes the ? strike the ?
- as the armature ?, the circuit is no longer ?
- due to this, the spring causes the ? to return to the original ?
- this process then ?
- electromagnet
- iron, electromagnet
- hammer, bell
- moves, complete
- hammer, position
- repeats
What happens to the transformers between power stations and consumers?
Power stations —> step up transformers/ increase p.d —> pylons —> step down transformers/ decrease p.d —> consumers
what does an alternating current do to the primary, secondary coils and the soft iron core
generates an alternating magnetic field in the primary coil
induces an alternating magnetic field around the soft iron core
this then induces an alternating current in the secondary coil
what are the turns for primary and secondary called
Np turns and Ns turns
What is the turns ratio
Np/Ns
What is the p.d called for primary and secondary coils
Vp and Vs
how are coils and p.d related (equation)
Np/Ns = Vp/Vs
Why can’t you get electrocuted in a transformer
because no electrons are moving
why is the soft iron core needed in a transformer
to guide/channel the magnetic field
What is the relationship between the p.d and turns ratio
The ratio of p.d. between the primary and secondary is the same as the ratio of turns
difference between a step up and a step down transformer to do with primary and secondary p.d
In a step-up transformer Vs > Vp; In a step-down transformer Vp > Vs
what does a split ring commutator do?
in a D.C motor it switches the current every 180 degrees to ensure the motor rotates a full 360 degrees
Why are the poles of the magnet sometimes curved
As the coil is in the magnetic field for longer
what happens when a current carrying wire is put into a magnetic field and the power source is turned on?
the current carrying wire gains a magnetic field which then interacts with the magnetic field of the permanent magnet
How does a DC motor work -
The current in the left hand part - the ? magnet experiences an ? force
The current in the right hand part - the ? magnet experiences a ? force
Due to the current flowing from ? to south, the coil should rotate ?
However due to these forces the coil will ? rotate ? degrees one way and then ? 180 degrees the other way
However due to the motor going against the ?, and the split ring ? changing the current every half ?, it works
Therefore it can turn a full ? degrees and makes it ? and therefore function
north, upwards
south, downwards
north, clockwise
only, 180, rotate
(metal) brushes, commutator, turn
360, rotate
Where does the energy come from to generate the electric energy in the coil?
Kinetic energy —> electrical energy
How does a transformer work
The a.c supply generates an alternating current around the primary coil, which induces an alternating magnetic field around the iron core. This then creates an alternate current around the secondary coil.
How to tell what is a step up or a step down transformer and examples
If it’s a step up, the no of coils on the primary coil < no of coils on the secondary coil , eg: Np = 5, Ns = 10
If it’s a step down, the no of coils on the primary coil > no of coils on the secondary coil , eg: Np = 10, Ns = 5
National grid paragraph
To transfer ? from the ? station to my home, we could either use 1- a ? voltage and a ? current
OR 2- a ? voltage and ? current
The first method is inefficient because the ? current heats the power lines and wastes the ?
The second method is better - to get the high ? , a step up ? is used. At the other end of the grid, near my ?, a step ? transformer is used.
electricity, power, low, high, high, low, high, electricity, voltage, transformer, house, down
What current is used with transformers and why?
alternating current, because transformers don’t work with direct current
NEED TO REVISE CIRCUIT SYMBOLS!!!
3 ways the force on a current carrying wire can be increased
- increasing the magnet strength
- increasing the current
- increasing the length of wire in a field
2 ways the direction of force on a current carrying wire can be reversed?
- change direction of the current
- change direction of magnetic field
How can a magnet and a wire be arranged so there’s no force in the wire?
Have a magnetic field parallel to direction of current
Info about transformers?
Are almost 100% efficient
Power output is same as power input
If the p.d is stepped up by a given ratio, the current is stepped down by the same ratio
What is national grid
National grid is a network of cables and transformers that transfer electrical power from power stations to consumers
5 steps of a microphone
1. Sound ?, longitudinal cause diaphragm to ? which causes coil to ?
2. The coil cuts through ? field lines of the permanent ?
3. A ? is induced across the ends of the ?
4. Is part of a ? circuit so a current flows
5. An ? Signal is created as the coil ? , changing direction ?
- Waves, vibrate, oscillate
- Magnetic, magnet
- P.d, coil
- Complete
- A.c, oscillates, continuously
Moving coil loudspeaker steps:
- current flows through coil
- this creates a magnetic field
- this magnetic field interacts with the permanent magnetic field
- this exerts a force on the coil causing it to move
- however due to the a.c current, the current is constantly changing so the coil moves constantly up and down
- this produces vibrations which then produces sound waves in the air
What is lenz’s law?
When an electric current generates a magnetic field that opposes the original change, either of move ent or magnetic field
The north side of a magnet is put into the left side of a coil, then pulled out - using lenz’s law which pole would appear out of the coil?
South Pole
4 steps of how an alternator works
The coil of wire ? relative to the ?, which induces a magnetic ? in the coil, which then induces a voltage and ? in the coil
Then the slip ? and brushes ensure the contacts don’t swap every half ?
This means that they produce an ? p.d, and an alternating ?. A visualization of this can be ? using an ?
As the coil rotates ?, the peaks of the ? get ?, and more ?
rotates, magnet, field, current
rings, turn
alternating, current, produced, oscilloscope
faster, oscillation, larger, frequent
4 steps of how a dynamo works
The coil of wire ? relative to the ?, which induces a magnetic ? in the coil, which then induces a voltage and ? in the coil
Then the split ring ? ensure the contacts do swap every half ?
This means that they produce a ? p.d, and a direct ?. A visualization of this can be ? using an ?
As the coil rotates ?, the peaks of the ? get ?, and more ?
rotates, magnet, field, current
commutator, turn
direct , current, produced, oscilloscope
faster, oscillation, larger, frequent
how to tell difference between dynamos and alternators
dynamos carry direct current while alternators carry alternating current
what does a dynamo having direct current mean for the graph?
it only has positive values as it is a constant/direct current
Function of loudspeaker vs function of microphone
Loudspeaker converts electrical signals into sound waves while a microphone does the opposite - converts sound waves into electrical signals
when is a potential difference generated?
When a wire experiences a chance in magnetic field
what two examples would generate a p.d in a wire?
moving the wire
moving the magnet
A wire moving up and down repeatedly perpendicular to a magnetic field will:
Generate an alternating p.d
A wire moving parallel to a magnetic field will:
NOT generate a p.d
if the current and magnetic field are in the same direction what happens to the force?
there is no force
what 3 things can you do to increase the p.d
Increase the magnetic field strength
Move the coil through the magnetic field faster
Add more turns to the coil
2 steps when a wire experiences a changing magnetic field?
When a wire experiences a changing magnetic field, a potential difference will be induced across the wire.
If the ends of the wire are connected to make a closed circuit, then a current will flow around the circuit.
