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Theme D — Fields

D.2.11 Magnetic Fields — IB Physics HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 26 of 39 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Lesson summary

D.2.11 Magnetic Fields — IB PHYSICS SL notes: Describe magnetic fields around magnets and currents and the forces they exert on moving charges and current-carrying wires.Magnetic field (B-field) — a region of space in which a magnetic pole, a moving charge or a current-carrying w…

  • Magnetic flux density (B)
  • Typical values
  • Two bar magnets
  • Uniform field
  • Similarity
  • Right-hand grip rule

Part 1 of 9Magnetic fields and their sources

1Magnetic field (B-field)
  • Magnetic field (B-field) a region of space in which a magnetic pole, a moving charge or a current-carrying wire experiences a force
2Sources
  • Sources a field is created by moving electric charge or by permanent magnets
  • Permanent magnets are materials that produce a field.
  • A stationary charge produces no magnetic field.
  • Currents in wires and coils create fields, and in magnets the motion and intrinsic magnetic moments of electrons do.
3Detecting
  • Detecting the field pulls on magnetic materials such as iron and moves the needle of a plotting compass
4Magnetic flux density (B)
  • Magnetic flux density (B) the number of field lines per unit area passing through a region of space, a vector measured in tesla (T)
5Tesla
  • Tesla the flux density that causes a force of 1 N on a 1 m wire carrying 1 A at right angles to the field
  • 1 T = 1 N A−1 m−1.
6Typical values
  • Typical values the Earth's field is about 5 × 10−5 T, a fridge magnet about 5 × 10−3 T and an MRI scanner about 3 T

Part 2 of 9Field lines of magnets

Direction
  • Direction lines leave a north pole and enter a south pole, showing the force on a free north pole
Strength
  • Strength closer lines mean a stronger field and the field around a bar magnet is strongest at the poles
Rules
  • Rules lines never cross and form closed loops with no beginning or end, since there are no isolated poles
Mapping
  • Mapping iron filings or plotting compasses show the pattern
Two bar magnets
  • Two bar magnets like poles repel and opposite poles attract
Uniform field
  • Uniform field equally spaced parallel lines, created when two opposite poles are held close together

Part 3 of 9Earth's magnetic field

Similarity
  • Similarity it acts like a bar magnet
Compass
  • Compass its north pole points to the Earth's magnetic south pole, which lies near the geographic north pole

Part 4 of 9Field of a straight wire

Pattern
  • Pattern circular field lines centred on the wire, in planes perpendicular to it
  • They are closer together near the wire where the field is strongest.
  • Reversing the current reverses the field direction.
Right-hand grip rule
  • Right-hand grip rule point the right thumb along the conventional current and the curled fingers give the field direction
  • For a current upward, the field is anticlockwise seen from above.
Magnitude
  • Magnitude for a long wire B = μ0I/(2πr), with μ0 = 4π × 10−7 T m A−1, so B falls as 1/r

Part 5 of 9Solenoids and coils

Solenoid
  • Solenoid a long coil of wire that gives a strong, nearly uniform field inside and a field like a bar magnet outside
  • More turns in a smaller region increase the flux density, which is how an electromagnet works.
Magnitude
  • B = μ0nI, with n the turns per metre and I the current
Poles
  • Poles curled fingers give the current direction and the thumb points along the field inside, towards the north pole
  • The north pole is at the end where the current appears anticlockwise.
Flat circular coil
  • Flat circular coil it equals one turn of a solenoid, with field lines emerging through one face (north) and entering the other (south)
Building a solenoid
  • Building a solenoid the fields of many circular coils combine to form a solenoid

Part 6 of 9Force on a moving charge

Equation
  • F = qvB sin θ for a charge q moving at angle θ to B
Direction
  • Direction the force is perpendicular to both v and B
  • For a positive charge use the right-hand rule, and reverse it for a negative charge.
Zero force
  • Zero force none acts on a charge at rest or one moving along B (θ = 0)
No work
  • No work the force is always perpendicular to the velocity, so it changes direction but not speed
Circular motion
  • Circular motion if v is perpendicular to B, the magnetic force is the centripetal force
  • qvB = mv2/r, so r = mv/(qB).

Part 7 of 9Force on a current-carrying wire

Equation
  • F = BIL sin θ for a wire of length L at angle θ to B
  • For a perpendicular wire F = BIL.
Direction
  • Direction the left-hand rule uses thumb for force, first finger for field and second finger for conventional current
Parallel wires
  • Parallel wires each lies in the field of the other, so they attract if the currents are in the same direction and repel if opposite
Applications
  • Applications the definition of the ampere, electric motors, loudspeakers and galvanometers

Part 8 of 9Uses of r = mv/qB

1Mass spectrometer
  • Mass spectrometer ions of known speed and charge enter a field and move in semicircles
  • Heavier ions follow larger circles, so the landing position reveals mass.
2Cyclotron and synchrotron
  • Cyclotron and synchrotron magnetic fields keep particles in circular paths while electric fields accelerate them
3Hall voltage
  • Hall voltage the force pushes moving charge carriers in a conductor to one side, giving a small transverse voltage used in field sensors
4Comparison with electric force
  • Comparison with electric force the electric force F = qE acts on charges at rest and changes speed,
    • whereas the magnetic force needs motion and does no work

Part 9 of 9More from the specification

1Magnetic flux density measurement
  • Magnetic flux density measurement measuring Lorentz deflection forces on current-carrying conductors provides an operational means to quantify magnetic field strength.
2Electrostatic Charge Sharing by Contact
  • When two identical conducting spheres carrying different quantities of electric charge are brought into direct contact, charge redistributes equally between them until both attain the same electrostatic potential.
3Coulomb Constant and Permittivity
  • The electrostatic constant k = 1 / (4πε₀) ≈ 8.99 × 10⁹ N m² C⁻² incorporates the electric permittivity of free space ε₀, scaling the magnitude of attraction or repulsion between charges.
4Capacitance and Plate Separation
  • The capacitance of parallel conducting plates depends on plate area A and separation distance d according to C = ε₀A / d;
    • storing charge Q establishes a potential difference V = Q / C.
5Electric Potential in Radial Fields
  • The electric potential V at distance r from an isolated spherical charge Q is V = Q / (4πε₀r), representing the external work done per unit positive coulomb charge brought from infinity.
6Permittivity
  • A property of a medium affecting the electric force within it.
7Coulomb
  • Charge passing a point when a current of 1 A flows for 1 s.
8Distance
  • Total length of path travelled, a scalar.
9Constant
  • Not changing.
10Potential difference
  • Work done by a unit charge passing through a component.
11Relative permittivity
  • Ratio of the permittivity of a material to that of free space.
12W = mg
What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)

Key terms

TermMeaning
Magnetic flux density BStrength of a magnetic field, in tesla (T).
Tesla1 T = 1 N A⁻¹ m⁻¹.
SolenoidA long coil of wire that produces a nearly uniform field inside.
Right-hand grip ruleGives the direction of the field around a current.
F = qvB sin θMagnetic force on a moving charge.
F = BIL sin θMagnetic force on a current-carrying wire.

Avoid these mistakes

1
Saying the magnetic force on a moving charge acts along the direction of motion — it is perpendicular.
2
Claiming a magnetic force changes the particle's speed — it changes only direction.
3
Using the direction of electron flow instead of conventional current in the hand rules.
4
Forgetting sin θ when the wire or velocity is not perpendicular to the field.
5
Believing a charge at rest in a magnetic field experiences a force.

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