Magnetic Fields Flashcards
How is a magnetic field created
By moving electric charge
Permanent magnets
Define magnetic flux density, B, of a field
The number of magnetic flux lines passing through a region of space per unit area
What is the unit of magnetic flux density
Tesla
Define 1 tesla
The flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux
What happens to the strength of the magnetic field if flux density is higher
Stronger magnetic field
What do flux line represent
Direction and magnitude of a magnetic field
What direction are flux lines in a magnetic field
North pole to south pole
Equation for the force on a current-carrying conductor in a B field
F = BIL x sin theta
where F is the force
B = magnetic flux density of applied B field
I = current in conductor
L = length of conductor
theta = angle between conductor and B field
When will a current carrying conductor such as a wire experience max magnetic force
If the current through it is perpendicular to the direction of the magnetic field lines
When will a current carrying conductor such as a wire experience no magnetic force
When current is parallel to the magnetic field lines
At what angle is the force on a current carrying wire in a B field greatest
90 degrees as sin theta = 1
What is the direction of current flow
The flow of positive charge so from positive to negative.
This is in the opposite direction to the flow of electrons
Describe Fleming’s left hand rule
Point the thumb, first finger and second finger at right angles to each other
Thumb points in direction of force
First finger points in direction of B field
Second finger points in direction of current flow (+ to -)
What does a dot represent in magnetic fields
Magnetic field is directed out of the plane of the page
What does a cross represent in magnetic fields
The field is directed into the plane of the page
What does the direction of the magnetic force on a charged particle depend on
Direction of flow of current
Direction of the magnetic field
Describe and explain the path of a charged particle when it enters a uniform magnetic field
It travels in a circular path as the direction of the force is perpendicular to the velocity and directed towards centre of the path
What does a cyclotron do
Makes used of the circular trajectory of charged particles in a magnetic field to create a spiral path.
It accelerates charged particles such as protons to very high speeds
Medical applications of cyclotrons
Produce tracers for imaging
Create high energy beams of radiation for radiotherapy
What does a cyclotron consist of
2 hollow semicircular electrodes/ dees
Strong uniform magnetic field
Alternating electric field
Describe how particles moves in a circular path inside the dee
- There is a source of charged particles at the centre of the cyclotron.
- A B field is applied perpendicular to the path of particles.
- Magnetic force acts perpendicular to the field and path of particles.
- This causes particles to move in circular path
Describe how particles cross the gap between dees
There is a gap between each semi-circular dee.
An electric field is applied in the gaps between the dees.
An electric force acts on the charged particle, causing it to accelerate.
This causes them to enter next dee at higher speed and follow a path with a larger radius
Describe how particles leave the dee
The charged particles travel with constant speed in the dees.
Time spent in one dee is constant.
Direction of e field alternates each time particles reach a gap.
This means they always accelerate to the opposite dee.
Process repeats as particles spiral outwards till they leave
What is the role of the uniform magnetic field in a cyclotron
To supply the centripetal force needed to keep the particles moving in a circular motion
What is the role of the alternating electric field in a cyclotron
To accelerate the particles between the dees
What is electromagnetic induction
Process in which an emf is induced in a closed circuit due to changes in magnetic flux
When does electromagnetic induction occur
When a conductor cuts through magnetic field lines
Define magnetic flux
The product of the magnetic flux density and the cross sectional area perpendicular to the direction of the magnetic flux density
Units for magnetic flux
Webers (Wb)
Equation for magnetic flux
Φ = BA
where Φ is magnetic flux
B is magnetic flux density
A is cross sectional area
When is magnetic flux maximum
When magnetic field lines and the area they are travelling through are perpendicular
What is the relationship between coils in a wire and emf induced
More coils in a wire = larger emf induced
Define flux linkage
The product of the magnetic flux and the number of turns on the coil
Equation for magnetic flux linkage
magnetic flux linkage = ΦN = BANcos theta
Units for magnetic flux linkage
Wb turns
What is the voltmeter reading when the bar magnet is not moving through a coil
Voltmeter shows a zero reading.
When bar magnet held still, the rate of change of flux is zero so no emf induced
What is the voltmeter reading when a bar magnet moves inside a coil connected to a voltmeter
As the bar magnet moves, its magnetic field lines cut through the coil, generating a change in magnetic flux.
This induces a emf within the coil and so there is a reading on the voltmeter
What happens to the voltmeter reading when a bar magnet is taken back out of a coil connected to a voltmeter
Emf induced in opposite direction.
As the bar magnet changes direction the direction of current chages.
Voltmeter shows reading with the opposite sign.
What if the effect of increasing the speed of the magnet through a coil
Emf induced with a greater magnitude.
As the speed of the magnetic increases, the rate of change of flux increases
What is the direction of the electric current and emf induced in the conductor
Such that it opposes the change that produces it
What factors increase induced emf when moving a bar magnet through a coil
Moving the magnet faster through the coil
Adding more turns to the coil
Increasing the strength of the bar magnet
Faradays Law
The magnitude of the induced emf is directly proportional to the rate of change in magnetic flux linkage.
Lenz’s Law
The induced emf acts in such a direction to produce effects that oppose the change causing it.
Equation for magnitude of induced emf
𝜀 = 𝑁 x (ΔФ) / Δt
where e is induced emf
N is the number of turns of coil
ΔФ is the change in magnetic flux
t is time interval
What does the gradient of a graph of magnetic flux against time represent
EMF
What does this equation : 𝜀 = -𝑁 x (ΔФ) / Δt show
When a bar magnet goes through a coil, an emf is induced within the coil due to a change in magnetic flux.
A current is also induced which means the coil now has its own magnetic field.
The coil’s magnetic field acts in the opposite direction to the magnetic field of bar magnet which is shown by the negative sign
Equation for the induced emf in a conductor as it moves through the magnetic field involving velocity
𝜀 =BLv
What is the emf and flux linkage when the plane of the coil is perpendicular to the field lines
Flux linkage = MAX
EMF = 0
What is the emf and flux linkage when the plane of the coil is parallel to the field lines
Flux linkage = 0
EMF = MAX
Define alternating current
A current which periodically varies between positive to negative value with time
How can the variation of current/pd with time be describe
Sinusoidal
Define peak current (Io) or peak voltage (V0)
Max value of the alternating current or voltage
What is seen on an oscilloscope when the time base is turned off
A vertical line on the voltage-gain axis
Uses of an oscilloscope
DC and AC voltmeter
To measure time intervals and frequencies
To display AC waveforms
What is a transformer
A device that changes high alternating voltage at low current to low alternating voltage at high current and vice versa
Why are transformers used
To reduce heat energy lost whilst electricity is transmitted down electrical power lines
What is a transformer made up of
A primary coil, secondary coil and a soft iron core
Why is the soft iron core necessary in a transformer
It focuses and directs the magnetic field from the primary to secondary coil
Why is the core made up of soft iron
It can easily be magnetised and demagnetised
How do eddy currents arise
A changing magnetic field and flux from the alternating current creates a changing magnetic field in the core that acts against the field that induced them.
An emf is therefore induced.
A current flows, as the core is made from a conducting material
How do eddy current cause energy loss in a transformer
They generate heat in the wires
How are eddy currents reduced
Laminating the iron core with layers of insulation so current does not flow between them
Having a core made from a high resistivity metal
Ways to reduce energy loss in a transformer
Make the core from soft iron to allow easy magnetisation and demagnetisation
Laminating the core
Using thick wires
Where do inefficiencies in the core appear from
Induced eddy currents.
Reversal of magnetism
Poor insulation between primary and secondary coil
Why are step up transformers used duirng transmission
To increase the voltage which decreases the current through transmission lines.
This reduces overall heat energy lost in wires during transmission
