Theme D — Fields
Theme D — Fields — IB Physics SL Revision Notes & Topical Questions
5 min read 11 parts Lesson 1 of 21 Unit quiz
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.
Lesson summary
Theme D — Fields — IB PHYSICS SL notes: Summarise gravitational and electric fields, Coulomb's law, charged particle motion in fields and Millikan's experiment at SL level.Newton's law of gravitation — the force between two point masses is proportional to the product of the masse…
- Field strength
- Earth's surface
- Line density
- Continuous fields
- Kepler's third law
- Permittivity
Part 1 of 8Newton's law of gravitation
Newton's law of gravitation
Newton's law of gravitation
- Newton's law of gravitation the force between two point masses is proportional to the product of the masses and inversely proportional to the square of their separation
- F = Gm1m2/r2, with r measured between centres.What the symbols mean: F = gravitational force (N) · G = 6.67 × 10−11 N m2 kg−2 · m1, m2 or M, m = masses (kg) · r = distance between their centres (m)
Planets
- Planets they are treated as point masses because their separation is much greater than their radii
Size
- Size forces between everyday objects are tiny, while planetary values run to 1020 N and beyond
Part 2 of 8Gravitational field strength
Field strength
- Field strength g = GM/r2What the symbols mean: g = gravitational field strength (N kg−1) · G = 6.67 × 10−11 N m2 kg−2 · M = mass of the body (kg) · r = distance from its centre (m)
Weight
- W = mg, so weight varies from place to place while mass is a fixed property
Moon
- Moon g at its surface is about 1.6 N kg−1
Units
- Units g can be written in N kg−1 or m s−2
Part 3 of 8Using the law of gravitation
1
Identify the two bodies and sketch the line between centres.
2
Convert distances to metres and masses to kilograms.
3
Treat the bodies as point masses and substitute.
4
State the direction, towards the other body.
- Changes
- use ratios, with distances inverted and squared
Part 4 of 8Field lines and Kepler's laws
Point mass
- Point mass field lines are radial and inward
Earth's surface
- Earth's surface field lines are parallel and evenly spaced, a uniform field
Line density
- Line density the number of lines through a sphere of area 4πr2 falls as 1/r2, so density shows magnitude and arrows show direction
Continuous fields
- Continuous fields lines only guide the eye
Kepler's third law
- T2 ∝ r3, found from gravity providing the centripetal force, v = √(GM/r)
Part 5 of 8Charge and Coulomb's law
Charge
- Charge it is conserved and quantised
Sharing
- Sharing identical spheres that touch share their total charge equally
Coulomb's law
- F = kq1q2/r2What the symbols mean: F = electric force (N) · Q1, Q2 = charges (C) · r = separation (m) · k = 1/4πε0 = 8.99 × 109 N m2 C−2
- k = 1/(4πε0) = 8.99 × 109 N m2 C−2.
- ε0 = 8.85 × 10−12 C2 N−1 m−2.
Permittivity
- Permittivity air is taken as vacuum, and all other materials have higher permittivity and so lower k
Difference from gravity
- Difference from gravity electric forces can be repulsive (like charges) or attractive
Part 6 of 8Electric fields
1Electric field strength E
- Electric field strength E force per unit positive charge, in N C−1
2Point charge
- E = kQ/r2
3Parallel plates
- E = V/dWhat the symbols mean: E = uniform electric field strength (V m−1) · V = p.d. between the plates (V) · d = plate separation (m)
4Field lines
- Field lines they point from positive to negative
- For a point charge they are radial outward (positive) or inward (negative).
- They become parallel where they meet a flat plate.
Part 7 of 8Charged particles in electric fields
Charged particles in electric fields
Uniform field
- Uniform field a charged particle feels a constant force
Path
- Path a particle entering across the field follows a parabola
- A positive charge goes towards the negative plate and a negative charge towards the positive plate.
- A neutron is undeflected.
Mass
- Mass deflection is smaller for greater mass
Charge-to-mass ratio
- Charge-to-mass ratio q/m is 1.76 × 1011 C kg−1 for the electron and 9.58 × 107 C kg−1 for the proton
Part 8 of 8Magnetic force
Magnetic force
Current-carrying conductor
- F = BIL sin θ, maximum when perpendicular and zero when parallelWhat the symbols mean: F = force on the wire (N) · B = magnetic flux density (T) · I = current (A) · L = length of wire in the field (m) · θ = angle between wire and field
Directions
- F, B and I are mutually perpendicular
- The left-hand rule uses conventional current and is reversed for negative charges.
Circular motion
- Circular motion a charge in a magnetic field moves in a circle of radius r = mv/(Bq), so q/m = v/(rB)
Crossed fields
- Crossed fields the speed is found from perpendicular E and B fields as v = E/B = V/(Bd)
Thomson's method
- Thomson's method it gives q/m = V/(rB2d)
Key terms
| Term | Meaning |
|---|---|
| Gravitational field strength | Force per unit mass at a point, g = GM/r². |
| Coulomb's law | The force between point charges is F = kq₁q₂/r². |
| Electric field strength | Force per unit positive charge. |
| Quantised | Existing only in discrete multiples of a basic unit. |
| Charge-to-mass ratio | The charge of a particle divided by its mass. |
| Velocity selector | Crossed E and B fields passing only particles with v = E/B. |
Avoid these mistakes
1
Using the distance from the surface instead of from the centre in the gravitational law.
2
Saying astronauts are weightless because there is no gravity at that height.
3
Reversing the direction of the force on a negative charge.
4
Treating the magnetic force as able to change a charge's speed.
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