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Theme D — Fields

D.2 Electric and Magnetic Fields — IB Physics HL Revision Notes & Topical Questions

5 min read 13 parts Lesson 15 of 39 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

D.2 Electric and Magnetic Fields — IB PHYSICS HL notes: Describe electric charge, Coulomb's law, electric fields, potential, potential energy, equipotentials and magnetic field patterns.Charge — the property of matter responsible for the electric force, a scalar measured in coulo…

  • Conservation
  • Positive charge
  • Without a field
  • With a field
  • What the symbols mean
  • Separation

Part 1 of 10Electric charge

Electric charge
Charge
  • Charge the property of matter responsible for the electric force, a scalar measured in coulombs
  • One coulomb is the charge carried by a current of one ampere in one second
Particles
  • Particles electrons are negative, protons positive and neutrons neutral
  • Most objects are neutral with equal numbers of protons and electrons
Quantised
  • Quantised charge is always a whole number of elementary charges e = 1.60 × 10−19 C
Forces
  • Forces opposite charges attract and like charges repel
Conservation
  • Conservation the total charge of an isolated system is constant
  • Charge is transferred but cannot be created or destroyed, as in n → p+ + e− + antineutrino

Part 2 of 10Charging and static electricity

Friction
  • Friction rubbing insulators transfers electrons, leaving one positive and the other negative
    • (loss of electrons)
Contact
  • Contact charge flows between touching objects to reduce the potential difference, such as the shock from a door handle
Induction
  • Induction a nearby charged object separates charge in a material without contact
  • A sphere earthed in the presence of a charged rod keeps a charge when the earth and rod are removed
Earthing
  • Earthing a connection to the earth discharges a body to 0 V through a low-resistance wire
Positive charge
  • Positive charge it is always the loss of electrons and not the gain of positive charge
Sparks
  • Sparks a very large potential difference breaks down the air
  • Bonding lines to earth reduce the risk when refuelling aircraft

Part 3 of 10Coulomb's law

Statement
  • Statement the force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation
  • F = kq1q2/r2
    What the symbols mean: F = electric force (N) · Q1, Q2 = charges (C) · r = separation (m) · k = 1/4πε0 = 8.99 × 109 N m2 C−2
Constant
  • Constant k = 1/(4πε0) = 8.99 × 109 N m2 C−2 with ε0 = 8.85 × 10−12 C2 N−1 m−2 for a vacuum
Separation
  • Separation r is between centres, and the law applies to spheres much smaller than their separation
Compared with gravity
  • Compared with gravity the same inverse square form, but electric forces can attract or repel
Sign
  • Sign like charges give a positive product (repulsion) and unlike charges a negative product (attraction)
Permittivity
  • Permittivity air is taken as ε0 and other materials have higher permittivity
  • εr = ε/ε0 has no units and reduces k

Part 4 of 10Electric field strength

Field
  • Field a region in which a charge experiences a force
Definition
  • E = F/q, the force per unit positive test charge, in N C−1 or V m−1
    What the symbols mean: E = electric field strength (N C−1) · F = force on the charge (N) · Q or q = charge (C)
Direction
  • Direction away from a positive charge and towards a negative charge
Point charge
  • E = kQ/r2, an inverse square law
Charged sphere
  • Charged sphere outside it acts as a point charge at the centre and inside the field is zero
Combining fields
  • Combining fields add vectors, using the sum or difference along a line and Pythagoras at right angles
Parallel plates
  • E = V/d for a uniform field from the positive to the negative plate
  • It cannot be used for a point charge

Part 5 of 10Electric field lines

1Uniform field
  • Uniform field equally spaced parallel lines with constant strength and force
2Point charge
  • Point charge radial lines, outward from positive and inward to negative, spreading out with distance
3Conducting sphere
  • Conducting sphere lines are perpendicular to the surface and the field inside is zero
4Two opposite charges
  • Two opposite charges lines run from positive to negative
5Two like charges
  • Two like charges lines run away from positive charges or towards negative charges, with a neutral point at the midpoint
6Parallel plates
  • Parallel plates a uniform field between the plates and non-uniform at the edges
7Density
  • Density closer lines show a stronger field
8Rules
  • Rules lines have arrows, touch the source charge and never cross

Part 6 of 10Electric potential and potential energy (HL)

Potential
  • Potential the work done per unit charge bringing a small positive test charge from infinity to the point
  • V = kQ/r in volts, positive near positive charge and negative near negative charge
Scalar
  • Scalar potentials from several charges add algebraically
Direction
  • Direction potential decreases in the direction of the field lines
Potential energy
  • Ep = kq1q2/r, zero at infinity
  • Positive for like charges and negative for unlike charges
Work
  • W = qΔV and ΔV = kQ(1/rf − 1/ri), where q is the charge moved and Q the charge producing the potential
Graphs
  • Graphs the gradient of the Ep–r graph is the force and the area under an F–r graph is the energy

Part 7 of 10Potential gradient and equipotentials (HL)

Potential gradient
  • E = −ΔV/Δr, the field is the negative gradient of the V–r graph
Area
  • Area the area under an E–r graph is the potential difference
Graph shapes
  • V follows 1/r and E follows 1/r2, which is steeper
Equipotentials
  • Equipotentials lines of equal potential perpendicular to field lines
  • Concentric circles around a point charge that get further apart, and equally spaced lines in a uniform field
Along an equipotential
  • Along an equipotential no work is done
Dipole
  • Dipole a central line at 0 V between opposite charges
Like charges
  • Like charges a region of zero field between them

Part 8 of 10Magnetic fields

Source
  • Source a magnetic field is produced by moving electric charge or by permanent magnets
  • A stationary charge produces none
Direction
  • Direction field lines run from the north pole to the south pole outside a magnet and never cross
Flux density B
  • Flux density B the number of field lines per unit area, in teslas
  • One tesla gives a force of 1 N on 1 m of wire carrying 1 A at right angles to the field
Poles
  • Poles like poles repel and opposite poles attract
Uniform field
  • Uniform field equally spaced parallel lines between opposite poles close together
Earth
  • Earth a compass points to the Earth's magnetic south pole, which is the geographic north pole

Part 9 of 10Fields of currents

Straight wire
  • Straight wire concentric circles around the wire, closer near the wire
  • Reversing the current reverses the field
Right-hand grip rule
  • Right-hand grip rule thumb along the conventional current and curled fingers give the field direction
Solenoid
  • Solenoid a field like a bar magnet with a uniform field inside
  • The thumb points to the north pole when the fingers follow the current
Flat circular coil
  • Flat circular coil equivalent to one turn of a solenoid, with a north face and a south face

Part 10 of 10More from the specification

Electric field
  • Electric field force per unit charge, with magnitude E = kQ / r2 for a point charge, where k relates to the permittivity
Relative permittivity
  • Relative permittivity a dielectric reduces the field by a factor equal to its relative permittivity
Magnetic force
  • Magnetic force a charge moves in a field and feels a force F = qvB sin theta perpendicular to its velocity
Gravitational comparison
  • Gravitational comparison gravitational fields are always attractive,
    • while electric forces may attract or repel, with the same inverse square equation form
Relative permittivity
  • Ratio of the permittivity of a material to that of free space.

Key terms

TermMeaning
Coulomb's lawThe force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation.
Electric field strengthThe force per unit positive test charge.
Conservation of chargeThe total charge of an isolated system is constant.
Quantisation of chargeCharge exists only in whole-number multiples of the elementary charge e.
Electric potentialThe work done per unit charge bringing a positive test charge from infinity to the point.
Magnetic flux densityThe number of field lines per unit area, measured in teslas.
SolenoidA long coil of wire that produces a nearly uniform magnetic field inside it.

Avoid these mistakes

1
Saying positive charge is gained — an object becomes positive by losing electrons.
2
Using E = V/d for a point charge — it applies to the uniform field between parallel plates.
3
Confusing q (charge moved) and Q (charge producing the field) in W = qΔV.
4
Believing the electric field is zero where the potential is zero.
5
Drawing field lines that cross or have no arrows.

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