Physics - Recall for Chapter 23; Magnetic Fields
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?
What are the SI units of flux?
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?
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.
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\[\epsilon = -\frac{\Delta N\Phi}{\Delta t}\]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.