Physics - Magnetism
Flashcards
2022-04-28
What two things can produce a magnetic field?
- Permanent magnet
- Moving charges
What is always true about magnetic field lines?
They go north to south.
What does the direction of a magnetic field line show?
The direction of a force on a north pole.
What is always true about field lines?
They never cross.
Can you visualise what the magnetic field lines look like for a north and south pole pointing at one another (like in a bar magnet)?
Can you visualise what the magnetic field lines look like for two north poles pointing at one another??
What does this represent??
Something coming out of the paper.
What does this represent?
What does this represent?Something going into the paper.
What is the right-hand grip rule?
If you point your right-hand thumb in the direction of conventional current and grip, the way your fingers curl shows the magnetic field lines.
What unit measures the strength of a magnetic field?
Teslas, T
What is Fleming’s left-hand rule used for finding?
The direction of the force when a current-carrying conductor is placed in a magnetic field.
What does the first finger represent in Fleming’s left-hand rule?
First finger: Field.
What does the second finger represent in Fleming’s left-hand rule?
SeCond finger: Current.
What does the thumb represent in Fleming’s left-hand rule?
ThuMb: Motion.
What is the formula for the force $F$ on a current-carrying conductor in a magnetic field?
What is the definition of one Tesla of magnetic flux density?
When a $1m$ wire carrying a current of $1A$ at $90^\circ$ experiences a $1N$ force.
How could you derive the SI units for magnetic flux density?
Rearrange $F = BIl\sin\theta$
When using Fleming’s left hand rule for an electron moving in a particular direction, what is important to remember?
Electrons count as negative current.
What is the formula for the force $F$ on a charged particle $q$ in a uniform electric field (strength $E$)?
What is the formula for the force $F$ on a charged particle $q$ moving at $v$ through a magnetic field?
Why won’t magnetism change the speed of a charged particle?
Because the force is 90 degrees to the motion.
Is the magnetic force on a charged particle is at 90 degrees to the motion, what could that force also be known as?
The centripetal force.
What two equations would you set equal to use simple harmonic motion equations for charged particles moving through a magnetic field?
What is a velocity selector?
A device that uses both electric and magnetic fields to select charged particles of a specific velocity.
When will a charged particle going through a velocity selector remain in a level position?
When the electrostatic force pushing it down is equal to the magnetic force pushing it up.
How could you write that the electrostatic force pushing a charged particle down in a velocity selector must be equal to the magnetic force?
2022-05-08
What is the symbol for magnetic flux?
What is induction?
Where any change of magnetic flux through a conductor results in an EMF being induced in the conductor.
What is the left-hand rule for?
Working out the properties of the motor effect.
What is the right-hand rule for?
Working out the properties of inductor.
What’s a mnemonic for remembering what the left and right hand rules are for?
Long MRI
What is the formula for magnetic flux $\phi$ in terms of magnetic flux density $B$ and area enclosing the field $A$?
How can you think about flux density in terms of flux?
Flux density is like flux per unit area.
What are the units of flux?
What are the units of flux linkage?
When do you use flux linkage instead of flux?
When there are multiple coils.
What is the formula for flux linkage?
What is Faraday’s law, in words?
EMF is proportional to the rate of change of flux.
What’s a useful mental model for Faraday’s law?
“The faster you cut flux lines, the larger the EMF induced”
What is Faraday’s law in equation form?
What is Lenz’s law?
The induced EMF in a conductor always gives rise to a magnetic field in opposition to the original change in magnetic flux.
What does Lenz’s law confirm in electromagnetism?
The conservation of energy.
What’s the equation for the EMF induced in a conductor of length $l$ moving through a field with magnetic flux density $B$ at a velocity $v$?
2022-05-11
Is magnetic flux density a scalar or a vector?
A vector.
What are the two induction questions that are typically asked?
- A conductor sliding across a magnetic field
- A coil rotating in a magnetic field
What is magnetic flux?
A measurement of the total magnetic field which passes through a given area.
How can the EMF induced in a rotating coil be zero when it experiences the maximum magnetic flux?
EMF induced is equal to the rate of change of magnetic flux.
2022-05-15
Why do you have to be careful thinking about the induced EMF in a loop of wire moving through a magnetic field?
EMFs may be induced in opposite directions and cancel out.
2022-05-17
What is the formula for the output voltage $V _ s$ of a transformer given the input voltage $V _ p$ and the turns on the coils, $N _ p$ and $N _ s$?
What is the formula for the output current $I _ s$ of a transformer given the input current $I _ p$ and the turns on the coils, $N _ p$ and $N _ s$?
\[\frac{I _ s}{I _ p} = \frac{N _ p}{N _ s}\]
What is important to remember about this formula?
The $\frac{I _ s}{I _ p}$ is upside down.
Why are transformers made out of laminated materials?
Because it reduces the heating effects due to Eddy currents.
Can you explain why having more coils on one side of a transformer leads to a larger output EMF?
There is a greater rate of change of flux linkage.
When a switch is closed in a circuit consisting of nothing but a power supply, switch and coil of wire, what does it become?
An electromagnet.
What are two big advantages of transformers?
- No moving parts
- They are very efficient
What happens to the current in a wire if you double its length?
It is halved.
2022-06-08
What will happen to the original induced EMF in a coil of wire if it is rotated at twice the frequency through a magnetic field?
It will also double.