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35 Cards in this Set

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  • Back
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What is a magnetic field?

Region magnetic effect observed

What is the direction of a magnetic field?

Direction of the force on a magnetic north pole due to the field.

What happens to the angle of dip of a field line as latitude (of the Earth) increases?

The angle of dip increases with latitude (parallel to earth of equator).

How are magnetic fields produced?

By movement of charge.

What rule predicts direction of the field?

Corkscrew rule:

Difference between a flat coil & a solenoid:

A solenoid is one wire making lots of loops and it creates a single eleptical magnetic field. In a flat coil, single coils are piled on top of each other & each single coil makes its own circular magnetic field, and these fields add up to make overall field strength. Magnetic field moves clockwise around both coils.

What causes the Earths field?

Liquid iron & nickel spinning around= electric current

What causes a bar magnets magnetic field?

Motion of electrons in atoms. Effects of many electrons don't cancel out resulting in a net movement of charge.

What makes a bar magnets magnetic field stronger?

If electrons are all aligned in same direction.

Anomalies in earths magnetic field=

possible mineral bearing ores. Large variations= conductive metals

Reasons for variations in earths field:

Underground cables, electromagnets in motors & em waves (human).


Magnetic storms from solar flares.

What 4 things increase the magnetic field strength?

1)>current


2)<radius of loops, <distance of current from center


3)solid core instead if air


4)>turns

Limitations of magnetic field strength:

Thinner wire: >resistance


>current: wire could melt

Experiment to show force excerpted due to current in a magnetic field?

Precision of balance determines precision of results.

What does the Hall Probe do & how does it do it?


(Hint= diagram to improve understanding of answer.)

Hall probe allows measurements of magnetic flux density by measuring pd:


1) A small semiconductor slice carries steady current.


2) A magnetic field perpendicular to current results in a force being produced on the charge carriers.


3) Electrons are forced to one end due to the force- leaving the other end positive in comparison. (Therefore a pd exists due to B.)

Ignore labels on picture.


Blue arrow= force


Red line/arrows= steady current


Magnetic field can't be seen on picture but would be going into the page.

What does a proton magnetometer do & how does it do it?

Proton magnetometer measures magnetic field to a higher precision by measuring frequency of proton precession in a magnetic field:


1) protons have charge & process around field lines.


2) motion of protons induces emf into coil (in a similar way the hall probe works).


3) emf is amplified & freq is measured by a counter.


How does freq of procession allow B to be found?

Freq of procession= Lamour freq.


=very high & is proportional to B.


=0.04258xB in Hz (given in formula book).

How can induced emf only require a conductor?

Faradays Laws:


1) conductor cuts magnetic flux or total magnetic flux linking a coil changes- emf induced.


2) induced emf depends on rate of change at which flux is cut/ rate of change of flux linkage.

How is emf produced in a conductor by cutting through magnetic field?

1) conducting wire moves- electrons move in it= flow of charge.


2) current in a magnetic field so force induced.


3) force forces electrons to one side of the wire. Other end has less electrons so is positive in comparison (=p.d).


comparison (=p.d).4) build up of electrons creates force that balances magnetic force on electrons so prevents further movement.


4) build up of electrons creates force that balances magnetic force on electrons so prevents further movement.

What is the flux cut per sec?

Area of magnetic field wire moves through per sec = emf.

How is emf induced by total magnetic flux linking coil changing?

1) A magnet moves in/ out if a coil, and as it does so there is a change in B at the coil.


2) ∆B induces a force which induces PD which induces a current.


3) movement in opposite direction- emf in opposite direction.


4) >velocity of magnet, >∆B, so >∆total magnetic flux (=BA) so therefore >emf.



5) the charge flow due to the magnet produces a force that opposes the force from the magnet. (Magnet moves into coil, nearest end repels magnet & magnet decelerates. Magnet removed- force attracts magnet.

What is Lenz's Law?

Work done in moving magnet into coil is dissipated in the circuit connected to coil. (Due to conservation of E.)

How do Geophones work?

1) magnet with large mass mounted on soft springs (low spring constant) inside circular coil.


2) case inserted into ground to measure vertical movements.


3) when an earthquake: magnet moves relative to case causing a change in flux linkage through the coil producing an emf.


4) oscillates up & down so alternating emf.

How can we measure smaller changes in voltage using a Geophone?

Use more turns in the coil or a stronger magnet.

How can you create a change in flux that only requires a conductor without movement?

1) current turned on in primary coil, creates a change in magnetic flux (magnetic field from movement of charge) which produces a current in the other coil.


2) current turned off- change in magnetic flux in other direction- induced current in other direction.



=a voltage pulse. Therefore alternating current produces a continuous change in flux linkage resulting in an alternating induced emf.

>emf by?

1) >alternating current


2) >number of turns in secondary coil: primary coil ratio


3) using a magnetic iron core so >∆flux

<energy loss by:

1) <resistance


2) maximising flux linkage by winding coils on top of each other


3) laminate core to reduce eddy currents (can cause temp to rise).


To investigate how B varies with 1)coil structure & 2)distance from coil, what set up is used?

Ac supply is linked to a solenoid to produce a magnetic field.


Search coil must be small to measure small area, so many turns in solenoid are needed for measurable induced emf.


Search coil is linked to an oscilloscope.

What causes eddy currents?

Change in flux means charge carriers circulate in one way then the other in complete loops in solid objects.

Why is emf larger in larger Eddy current loops?

1) ∆flux linkage(=BA) (and so emf) is proportional to area (πr^2) .


2) current depends on resistance of material & resistance is proportional to the circumference (2πr).


So emf proportional to r.

How do metal detectors work?

How do pulsed induction metal detectors work?

1) Pulse of current- pulse of B- eddy currents.


2) eddy currents decay- induce emf in detector coil.


3) if the ground is resistant- E dissipated more quickly so emf detected is short.


4) therefore long pulse for conductive materials & short for non conductive.

What is magnetic flux density & what is it measure in?

Magnetic field strength in T (Tesla) or gamma (10^-9 T)

What is this symbol & its unit?

Total magnetic flux in Tm^2

What rule shows direction of force excerpted on a still wire with a current going through it in a magnetic field?

Flemming's left hand rule.