# Physics - Magnestism

> Source: https://ollybritton.com/notes/a-level/physics/topics/magnetism/ · Updated: 2024-08-25 · Tags: school, physics, magnetism, a-level

## 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)??
![PHOTO BAR MAGNET](paste-308e9a507a1b588ac3f892374352c8afb57b7810.jpg)

#### Can you visualise what the magnetic field lines look like for two north poles pointing at one another??
![PHOTO TWO NORTH POLES](paste-308e9a507a1b588ac3f892374352c8afb57b7810.jpg)

#### ![PHOTO CHARGE OUT OF PAPER](paste-6e8cb7f1e115e6254de1951755cd2e74aedf4887.jpg) What does this represent??
Something coming out of the paper.

##### ![PHOTO CHARGE INTO PAPER](paste-09293e1313e47ea8df368555c3f4d874c5139077.jpg) 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??
$$
F = BIl\sin\theta
$$

##### 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 $E$)??
$$
F = Eq
$$

##### What is the formula for the force $F$ on a charged particle $q$ moving at $v$ through a magnetic field??
$$
F = BQv\sin\theta
$$

##### 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??
$$
Bqv = \frac{mv^2}{r}
$$

##### 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??
$$
Eq = Bqv
$$

### 2022-05-08
##### What is the symbol for magnetic flux??
$$
\phi
$$

##### 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 $A$??
$$
\phi = BA\cos \theta
$$

##### 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??
$$
\text{Wb}, \text{Webers}
$$

##### What are the units of flux linkage??
$$
\text{Wb turns}, \text{Weber turns}
$$

##### When do you use flux linkage instead of flux??
When there are multiple coils.

##### What is the formula for flux linkage??
$$
N\phi = NBA\cos\theta
$$

##### 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??
$$
\epsilon = -\frac{\Delta \phi}{\Delta t}
$$

##### 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 $v$??
$$
\epsilon = Blv
$$
### 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 $N_s$??
$$
\frac{V_p}{V_s} = \frac{N_p}{N_s}
$$

##### 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 $N_s$??
$$
\frac{I_s}{I_p} = \frac{N_p}{N_s}
$$

##### $$\frac{I_s}{I_p} = \frac{N_p}{N_s}$$ 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.

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