Magnetic Fields

Question 1

Field Lines

Answer 1

• x = fletching = current moving away, or into the paper
• . = arrow = current moving towards, or out of the paper
• Field lines travel from North to South (like a positive charge would
• Neutral Point = point where two opposing field lines cancel out

Question 2

Right Hand Rule and solenoids

Answer 2

• Current is thumb, field lines curl around in direction of fingers
• shape of a thumbs up
• right hand rule applies to positive charges e.g a proton, use left hand for negative charges e.g an electron
• solenoid = a coil of conductive wire that produces a uniform magnetic field inside it

Question 3

Field Strength

Answer 3

• Force = Magnetic Flux Density (perpendicular to current)currentlength of current carrying wire in field
• if a coil is in the field use the circumference of the coil as its ‘length’
• or Force = Magnetic Flux Density * current * length * sin(angle between wire and field), for use when B isn’t perpendicular
• magnetic flux density = B = Tesla
• earths magnetic flux density = 50 MicroTesla (at 65 degrees)
• F= Flux densitycharge on particlevelocity of particle

Question 4

Hall Effect

Answer 4

• electrons in a wire are pushed to one side of the wire or the other depending on the direction of the magnetic field due to the left hand rule
• this produces a measurable potential difference across the wire called a hall p.d
• can be used to measure magnetic fields strength with a Hall probe

Question 5

Electron Beams

Answer 5

• electron guns are given energy equal to charge*voltage = eV, gravity has negligible effect on electrons
• Thermionic emission leads to release of electrons with heat, and they can be accelerated towards electric fields and manipulated by changing magnetic fields to direct the beam in Cathode Ray Oscilloscopes, old TV’s etc
• in fixed magnetic fields BeV is centripetal force for circular motion of electrons where K.E and V remain constant

Question 6

Magnetic Flux

Answer 6

• magnetic flux density is the number of field lines in a given area of a conductor
• Phi = flux (Wb) = Magnetic Flux Density*Area (Tm^2)
• Voltage = Change in Flux/Change in time
• No. turns in coilflux= No. turnsFlux density*Area = magnetic flux linking the coil

Question 7

Electromagnetic Induction

Answer 7

• E.M.F induced in coil = No.turns in coil*-(change in flux linkage/change in time)
• change in flux linkage is generated by relative movement of the coil and the field
• Lenz’s Law = induced magnetic field from current produced will oppose e.m.f, so energy is conserved.

Question 8

Solenoids

Answer 8

• magnetic flux density inside solenoid = permativity of free space* no. turns per metre * current in solenoid

Question 9

Transformers

Answer 9

• Primary coil produces a magnetic field that is transmitted across the soft iron core and links the other coil
• Using alternating current makes the field constantly vary as it changes direction so flux linkage across secondary coil caries and e.m.f is induced
• Vs/Vp = Ns/Np (for ideal transformers where efficiency = 100%)
• IsVs = IpVp

Question 10

Alternating current and Volatges

Answer 10

• V = V(max)sin(2pialternating frequencytime)
• I = I(max)sin(2pialternating frequencytime)
• Average value of of V or I on a.c graphs is 0 as they have a sinusoidal shape
• root mean square of I or V = I(max)/sqrt2 or V(max)/sqrt2
• r.m.s is value given for actual value
• peak V or I = Vsqrt2 or Isqrt2