# Physics - Recall for Chapter 23: Magnetic Fields

> Source: https://ollybritton.com/notes/a-level/physics/recall-questions/chapter-23-magnetic-fields/ · Updated: 2022-04-20 · Tags: physics, a-level, school, recall-questions

## Flashcards
##### What is a magnetic field??
A region of space in which objects with magnetic properties experience a force.

##### What are three things with "magnetic properties"??
* Moving charges
* Electric currents
* Magnetic materials

##### What is shown by the direction of a magnetic field line??
The direction of force on a free north pole.

##### Which direction do magnetic field lines flow in??
North to south.

##### How is a uniform magnetic field represented by field lines??
Parallel lines.

##### What does the spacing of magnetic field lines show??
The magnetic field strength.

##### What's another way of saying "magnetic field strength"??
Flux density.

##### How do you find the direction of the field lines around a current??
Using the right-hand grip rule.

##### What is represented by the thumb in Fleming's left-hand rule??
Force.

##### What is represented by the first finger in Fleming's left-hand rule??
Field.

##### What is represented by the second finger in Fleming's left hand rule??
Current.

##### What is represented by $\theta$ in the equation $F = BIl \sin\theta$??
The angle between the current and the field direction.

##### What is the SI unit of flux density??
$T, \text{Tesla}$

##### What is magnetic flux density??
The field strength that would cause a wire carrying a unit current to experience a unit force per unit length.

##### What experiment can be done to find flux density in the lab??
Set up a current carrying wire between the poles of a magnet on top of a top-pan balance.

##### Why will the scale reading change for a current-carrying wire suspended past a magnet on a balance??
Because there are equal and opposite forces.

##### When is the equation $F = Bqv$ used??
To find the force acting on a particle moving at right angles to a magnetic field.

##### Why does a charged particle always demonstrate circular motion in a uniform magnetic field??
There is a constant force acting at right angles to the motion.

##### Why do only particles with the correct velocity travel in a straight line in a velocity selector??
The electric force is equal and opposite to the magnetic force at that specific velocity.

##### What is magnetic flux??
It is the total component of the magnetic flux density passing at right angles through a surface of area $A$.

##### What is the formula for magnetic flux??
$$
\Phi = BA\cos\theta
$$

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

##### What is the relationship between flux and flux linkage??
Flux linkage is flux multiplied by the number of turns on the coil.

##### What is represented by $\theta$ in the equation $\Phi = BA\cos\theta$??
The angle between the magnetic flux lines and the normal to the surface.

##### How is flux linkage calculated??
$$
N\Phi
$$

##### What is the relationship between flux and flux density??
Flux density is the flux per unit area perpendicular to the field.

##### How is flux density represented in a field line diagram??
The spacing of field lines.

##### What is Faraday's law in words??
The magnitude of an induced EMF is directly proportional to the rate of change of flux linkage.

##### What is Lenz's law in words??
The direction of an induced EMF or current is always in such a direction as to oppose the change that created it.

##### What principle leads to Lenz's law??
Conservation of energy.

##### $$\epsilon = -\frac{\Delta N\Phi}{\Delta t}$$ Why is there a negative sign in this equation??
It is due to Lenz's law and conservation of energy.

##### What are two ways in which energy losses are reduced in transformers??
* Low resistance coils
* Laminated core

##### Why does a laminated core reduce energy losses in a transformer??
It reduces the heating effect of Eddy currents.

##### What are Eddy currents??
Currents which circulate in conductors due to a changing magnetic field.

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