Magnetic Fields Flashcards
What is a MAGNETIC FIELD?
Region surrounding a magnet or current-carrying conductor where another magnet or current-carrying conductor experiences a force.
What DIRECTION do magnetic field lines go?
North to South.
What is the MAGNETIC POLE of a current in a wire going IN?
South.
What is the MAGNETIC POLE of a current in a wire going OUT?
North.
What is the RIGHT THUMB RULE for a WIRE?
Thumb = direction of current
Fingers wrapped around wire = direction of the field.
What is the RIGHT THUMB RULE for a SOLENOID?
Thumb = direction of magnetic field.
Fingers wrapped around coil = direction of current.
What affects MAGNETIC FORCE/MOTOR EFFECT?
- Angle of current is to the field
- Magnitude of magnetic flux density
- Length of wire
- Size of current
What is MAGNETIC FLUX DENSITY?
The force on a wire per unit current per unit length in a magnetic field. Also known as magnetic field strength.
What is TESLA?
Magnetic flux density that causes a 1 Newton Force to act on 1 metre of wire carrying 1 amp of current.
When is MAX FORCE experienced?
When current carrying wire is at right angles to the magnetic field lines.
Why does the COIL TURN in a MOTOR?
- Opposite vertical forces on either side of coil (Flemming’s left hand rule) due to magnets on both sides creating a magnetic field.
- Causes anticlockwise rotation
- Split ring commutator switches the currents direction, keeping constant anti-clockwise spin.
What is FLEMING’S LEFT HAND RULE for the MOTOR EFFECT?
Thumb = force
Index = Field
Middle = current
(CURRENT AND FIELD ARE AT RIGHT ANGLES)
How does a LOUDSPEAKER WORK?
- alternating current creates changing magnetic field.
- current creates electromagnet in core.
- Magnetic between electromagnet and permanent magnet exerts force on coil. which is constantly changing.
- AC current is same frequency as variation of sound to be emitted.
- Coil vibrates at frequency of AC.
How to derive F=BQv?
F = BIL
I = Q/t
F=B(Q/t)L
L/t = v
F= BQv
What directions do ELECTRIC FIELDS and MAGNETIC FIELDS act in a deflection tube.
- They act in opposite directions by adjusting the strengths.
- Use Helmholtz coils.
Ee = Bev.
Why does a charged particle in a magnetic field move in a circle?
Magnetic force will always be at right angles to the velocity of the charged particle.
What is the radius of the circular motion in a magnetic field?
F = BQv
F = mv^2 / r
Solve for r.
Why does the time period not depend on velocity in a magnetic field?
F = mv^/r
V=Wr
W=v/r
F=mWv
W = 2pi/T
F = bqv
Equate these two together.
Why do particle velocity paths become more spiral in a magnetic field?
Particles lose kinetic energy through ionising collisions in air. This makes the path more curved and thicker.
What is MAGNETIC FLUX?
The quantity of B lines passing through an area of Am^2
What is FLUX LINKAGE?
The total amount of flux through a loop of wire.
What is FARADAY’S LAW?
The magnitude of EMF induced in a conductor equals the rate of change of flux linkage of which the conductor cuts through the magnetic flux.
What two ways can EMF be induced?
- Moving a current-carrying through a B wire.
- Move a magnet through a coil of current-caryign wire.
Why is EMF proportional to velocity?
E = change in Φ / change in t
Φ = BA
E = Change in BA / change in t
A = Length x velocity x time
E = B x change in Lvt / change in t
E = BLv
E is proportional to v
What is FLEMING’S RIGHT HAND RULE for GENERATING ELECTRICITY?
Thumb = direction of force
Index finger = magnetic field
Middle finger = INDUCED current
What is the MAINS POWER SUPPLY in VOLTAGE?
230V
What is LENZ’s LAW?
Direction of EMF induced in a conductor is such that it opposes the charge producing it.
How does a VINYL PLAYER work?
- Stylus passes through grooves, causes magnet it is attached to to vibrate
- Electromagnet is created as current passes through the coil around the iron core.
- Two magnetic fields overlap.
- Vibrations cause magnetic flux to be cut, producing an EMF, opposing current in coil.
Explain what is happening when a magnet is dropped through a coil.
- E = N * Change in Φ / change in t
- N is constant so the area underneath the graphs are always equal.
- Increase in EMF when magnet gets closer to the coil.
- Negative gradient when magnet passes though the coil because of changing magnetic flux.
- Larger negative amplitude because E is proportional to v. Magnet is accelerated due to gravity.
Why does current decrease when a motor begins to spin fast?
E is proprtional to v, EMF opposes current so if EMF increases, current will decrease.
What are the properties of Transformers?
Soft - loses magnetism very quickly after being shut off.
Laminated - reduces heat transfer.
Iron core - produces magnetic field that wraps around.
