Simple Phenomena of Magnetism – IGCSE Physics (0625)
Learning Objectives
- Explain that magnetic forces arise from the interaction between magnetic fields.
- Describe the behaviour of permanent magnets, induced (temporary) magnets and current‑carrying conductors.
- Apply the relevant equations to calculate magnetic forces, fields and flux densities.
- Connect magnetic concepts to the wider IGCSE syllabus (motion, thermal physics, waves, light, sound and electricity).
1. Motion, Forces & Energy (Core)
1.1 Key Quantities & Units
| Quantity | Symbol | Unit |
|---|
| Displacement | s | metre (m) |
| Velocity | v | metre s⁻¹ (m s⁻¹) |
| Acceleration | a | metre s⁻² (m s⁻²) |
| Force | F | newton (N) |
| Mass | m | kilogram (kg) |
| Momentum | p | kg m s⁻¹ |
| Kinetic Energy | KE | joule (J) |
| Work | W | joule (J) |
| Power | P | watt (W) |
1.2 Vectors vs Scalars
- Scalar: magnitude only (e.g., speed, mass).
- Vector: magnitude + direction (e.g., velocity, force). Use arrows; add component diagrams when required.
1.3 Kinematics (distance‑time & velocity‑time graphs)
Interpret slopes (speed) and areas (displacement) on graphs. Example: a car travels 30 m in 5 s → average speed = 6 m s⁻¹.
1.4 Dynamics – Newton’s Laws
- 1st law – an object remains at rest or in uniform motion unless acted on by a net force.
- 2nd law – F = ma.
- 3rd law – for every action there is an equal and opposite reaction.
1.5 Work, Energy & Power
- Work: W = F s cos θ (J).
- Kinetic energy: KE = ½ mv².
- Power: P = W/t = F v.
1.6 Momentum
Momentum p = mv. Conservation of momentum applies to collisions (elastic & inelastic).
2. Thermal Physics (Core)
2.1 Particle Model of Matter
- Solids – particles vibrate in fixed positions.
- Liquids – particles close together but can move past each other.
- Gases – particles far apart and move freely.
2.2 Specific Heat Capacity
Energy required to raise 1 kg of a substance by 1 K: Q = mcΔT.
2.3 Phase Changes
- Latent heat of fusion (solid↔liquid) and vaporisation (liquid↔gas).
- Energy absorbed or released at constant temperature: Q = mL.
2.4 Heat Transfer
- Conduction – transfer through a material (rate ∝ area, temperature gradient, thermal conductivity).
- Convection – bulk movement of fluid.
- Radiation – electromagnetic waves; depends on temperature (Stefan‑Boltzmann law).
2.5 Example Experiment (Conduction)
Place a metal rod, a wooden rod and a plastic rod between a hot plate and a cold plate. Measure temperature rise after 2 min – metal conducts most efficiently.
3. Waves (Core)
3.1 Wave Terminology
| Term | Symbol | Definition |
|---|
| Wavelength | λ | Distance between successive crests (or troughs) |
| Frequency | f | Number of cycles per second (Hz) |
| Period | T | Time for one cycle (s) – T = 1/f |
| Speed | v | v = fλ |
3.2 Types of Waves
- Transverse: displacement ⟂ direction of travel (e.g., light, water surface).
- Longitudinal: displacement ∥ direction of travel (e.g., sound).
3.3 Reflection & Refraction
- Law of reflection – angle of incidence = angle of reflection.
- Snell’s law – n₁ sin θ₁ = n₂ sin θ₂.
3.4 Diffraction
When a wave passes an obstacle comparable to its wavelength it spreads out. Demonstrated with a ripple tank.
4. Light & Electromagnetic Spectrum (Core)
4.1 Ray Optics
- Reflection – mirrors.
- Refraction – lenses, prisms (focus, image formation).
- Key equations: 1/f = 1/v + 1/u (lens formula), M = h′/h = v/u (magnification).
4.2 Electromagnetic (EM) Spectrum
| Region | Wavelength (m) | Typical Use |
|---|
| Radio | 10⁻¹ – 10³ | Broadcasting, radar |
| Microwave | 10⁻³ – 10⁻¹ | Cooking, satellite communication |
| Infrared | 10⁻⁶ – 10⁻³ | Thermal imaging, remote controls |
| Visible | 4 × 10⁻⁷ – 7 × 10⁻⁷ | Human vision |
| Ultraviolet | 10⁻⁸ – 4 × 10⁻⁷ | Sterilisation, black‑light |
| X‑ray | 10⁻¹¹ – 10⁻⁸ | Medical imaging |
| Gamma | <10⁻¹¹ | Radioactive decay |
4.3 Real‑World Example
Fiber‑optic cables use total internal reflection of light (λ ≈ 1.3 µm) to transmit data with very low loss.
5. Sound (Core)
5.1 Generation & Propagation
- Produced by vibrating objects; travel as longitudinal pressure waves.
- Speed depends on medium (air ≈ 340 m s⁻¹, water ≈ 1500 m s⁻¹, steel ≈ 5000 m s⁻¹).
5.2 Pitch, Loudness & Quality
- Pitch ∝ frequency (Hz).
- Loudness ∝ amplitude (pressure variation) – measured in decibels (dB).
- Timbre determined by waveform (harmonics).
5.3 Application – Ultrasound
High‑frequency sound (> 20 kHz) used in medical imaging and non‑destructive testing.
6. Electricity & Magnetism (Core + Supplement)
6.1 Electric Charge & Fields
- Charge (q) measured in coulombs (C); two like charges repel, opposite charges attract.
- Electric field E defined as force per unit positive charge: E = F/q (N C⁻¹).
6.2 Potential Difference, Current & Resistance
- Potential difference (voltage) V = work done per unit charge: V = W/q (V).
- Current I = charge flow per second: I = Δq/Δt (A).
- Resistance R = V/I (Ω). Ohm’s law: V = IR.
6.3 Electrical Power & Energy
Power: P = VI = I²R = V²/R (W). Energy transferred: E = Pt (J).
6.4 Simple Circuits
- Series – same current, total resistance Rₜₒₜ = ΣR.
- Parallel – same voltage, 1/Rₜₒₜ = Σ1/R.
6.5 Magnetic Fields
- A magnetic field **B** is a region where a moving charge or a magnetic material experiences a force.
- Field lines exit the north pole of a magnet and enter the south pole; line density = field strength.
6.5.1 Visualising B‑fields
- Iron‑filings experiment: Sprinkle fine filings over a paper placed on a bar magnet – filings align with field lines.
- Compass‑needle method: Record needle direction at several points; join arrows to sketch field lines.
6.6 Forces Between Magnetic Fields
- Like poles (N‑N or S‑S) repel; unlike poles (N‑S) attract – the result of overlapping magnetic fields exerting forces on each other.
- Permanent magnets retain magnetisation; induced (temporary) magnets acquire a field only while in an external field.
6.7 Magnetic Field of a Current‑Carrying Conductor
Biot–Savart law (qualitative): a straight conductor carrying current I produces concentric circular field lines around the wire.
6.7.1 Fleming’s Left‑Hand Rule (Motors)
Arrange the fingers of the left hand as follows:
- First finger (index): direction of magnetic field **B** (from N to S).
- Second finger (middle): direction of conventional current **I** (positive to negative).
- Thumb: direction of the force **F** on the conductor.
6.7.2 Magnetic Force on a Straight Conductor
\[
\mathbf{F}=BIL\sin\theta
\]
- B – magnetic flux density (T).
- I – current (A).
- L – length of wire within the field (m).
- θ – angle between **I** and **B**.
6.8 Solenoids and Electromagnets
A long solenoid (coil of N turns per unit length) produces an almost uniform field inside:
\[
B=\mu_{0}nI
\]
- μ₀ = 4π × 10⁻⁷ T·m A⁻¹ (permeability of free space).
- n = number of turns per metre.
- I = current (A).
Inserting a ferromagnetic core (iron) multiplies the field by the relative permeability μᵣ (μᵣ ≫ 1).
6.8.1 Example – Electromagnet
Coil: 200 turns, I = 2 A, core: soft iron (μᵣ ≈ 5000). The field inside the core is roughly 5000 times larger than the air‑core field.
6.9 Everyday Applications of Magnetic‑Field Interaction
- Compass: tiny permanent magnet aligns with Earth’s magnetic field.
- Motor: current in a coil within a magnetic field experiences a force (Fleming’s left‑hand rule) causing rotation.
- Generator: rotating a coil in a magnetic field induces an emf (Faraday’s law – covered in the supplement).
- Magnetic levitation (maglev) trains: repulsive forces between superconducting magnets and track.
- MRI scanner: strong solenoid field (several tesla) aligns nuclear spins for imaging.
7. Summary of Key Points
- Magnetic fields exist around permanent magnets, induced magnets and current‑carrying conductors.
- When magnetic fields overlap they exert forces on each other – the basis of attraction, repulsion and motor action.
- Force on a straight conductor: F = BIL sin θ.
- Field inside a long solenoid: B = μ₀nI; a ferromagnetic core increases the field dramatically.
- All magnetic phenomena can be explained by field interactions and are linked to the broader physics syllabus (motion, energy, thermal physics, waves, light, sound, electricity).
8. Key Equations
| Quantity | Symbol | Equation | Units |
|---|
| Force (magnetic) | F | F = B I L sin θ | N |
| Magnetic flux density | B | – | T |
| Magnetic field (solenoid) | B | B = μ₀ n I | T |
| Ohm’s law | V | V = IR | V |
| Power (electrical) | P | P = VI = I²R = V²/R | W |
| Kinetic energy | KE | KE = ½ mv² | J |
| Work (force) | W | W = F s cos θ | J |
| Wave speed | v | v = fλ | m s⁻¹ |
| Lens formula | | 1/f = 1/v + 1/u | m⁻¹ |
9. Practice Questions
- A straight wire 0.20 m long carries a current of 3 A and is placed perpendicular to a uniform magnetic field of 0.05 T. Calculate the magnitude of the magnetic force on the wire.
- Two identical bar magnets are placed with their north poles facing each other at a distance of 2 cm. Describe the direction of the magnetic force and explain why it occurs.
- Explain how a compass works in terms of magnetic‑field interaction.
- An electromagnet consists of a coil of 200 turns wrapped around an iron core. If a current of 2 A flows through the coil, what happens to the magnetic field inside the core compared with the field when the coil is empty? (No calculation required – give a qualitative answer.)
- A solenoid 0.30 m long has 600 turns and carries a current of 1.5 A. Calculate the magnetic flux density inside the solenoid (assume it is long enough for the field to be uniform). Use μ₀ = 4π × 10⁻⁷ T·m A⁻¹.
- A 0.10 m segment of a horizontal wire carrying 4 A is placed in a magnetic field of 0.20 T that points into the page. The current flows from left to right. Using Fleming’s left‑hand rule, state the direction of the force on the wire and calculate its magnitude.
- Calculate the kinetic energy of a 0.15 kg ball moving at 5 m s⁻¹. Then state how much work would be required to bring it to rest.
- Water at 20 °C is heated from 20 °C to 80 °C. If the mass of water is 0.5 kg and its specific heat capacity is 4180 J kg⁻¹ K⁻¹, calculate the energy required.
- A sound wave travels from air into water. If its speed in air is 340 m s⁻¹ and in water is 1500 m s⁻¹, what happens to its wavelength when the frequency remains constant?
- In a series circuit, three resistors of 4 Ω, 6 Ω and 10 Ω are connected to a 12 V battery. Find the current flowing through the circuit.
10. Suggested Laboratory Activities
- Magnetic field mapping: Use iron filings and a bar magnet to produce field‑line diagrams.
- Force on a current‑carrying wire: Suspend a wire between two pins, place it in a known magnetic field, and measure the deflection to verify F = BIL.
- Solenoid field measurement: Use a Hall‑probe to compare the field of an air‑core solenoid with that of the same solenoid containing an iron core.
- Thermal conduction experiment: Compare temperature rise in metal, wood and plastic rods placed between hot and cold plates.
- Wave speed in a ripple tank: Measure wavelength and frequency to confirm v = fλ.