Dropping a phone onto a pad and watching the battery icon light up feels like magic, yet the process is rooted in classic physics. Wireless charging relies on time-varying electromagnetic fields predicted by Faraday, Ampère, and Maxwell nearly two centuries ago. By engineering coils, capacitors, and magnetic materials, modern devices turn those field equations into a convenient flow of energy. This article traces the physics from basic induction to resonant power transfer, explains why efficiency matters, and explores future directions such as true over-air charging.
Electromagnetic Induction: The Foundation
Michael Faraday’s 1831 discovery is still the bedrock: a changing magnetic flux through a loop of wire induces an electromotive force (EMF). Mathematically, E=−dΦBdt,mathcal{E}= -frac{dPhi_B}{dt},E=−dtdΦB,
where ΦB=∫B ⋅ dAPhi_B = int mathbf{B}!cdot!dmathbf{A}ΦB=∫B⋅dA is the magnetic flux. A coil driven by an alternating current produces a sinusoidally varying magnetic field; a nearby coil intercepts part of that flux and develops an AC voltage. Rectifiers then convert the AC into DC to charge batteries.
Inductive Coupling in Consumer Chargers
Most phone pads use tight inductive coupling at frequencies around 110–205 kHz. Two flat spiral coils—one in the pad (transmitter), one in the phone (receiver)—are separated by 2–5 mm. Their mutual inductance MMM dictates how much transmitter current I1I_1I1 creates receiver voltage V2V_2V2: V2=jωMI1.V_2=jomega M I_1.V2=jωMI1.
Getting useful power requires maximizing MMM while keeping coil resistance low. Engineers achieve this by:
- Litz wire – bundles of varnish-coated strands reduce skin-effect losses.
- Ferrite shields – high-μ slabs behind each coil confine flux, raising coupling and blocking eddy currents in metallic phone parts.
- Alignment magnets – small permanent magnets guide the phone to the pad’s sweet spot, increasing mutual inductance by up to 30 %.
Typical end-to-end efficiency is 60–72 %, respectable for short gaps but still lower than a plug.
Resonant Inductive Coupling: More Distance, Less Loss
To stretch power transfer beyond a few millimeters, designers add capacitors so both coils become tuned RLC circuits. At resonance ω0=1LC,omega_0=frac{1}{sqrt{LC}},ω0=LC1,
each coil’s reactive impedance cancels internally, and energy oscillates between electric and magnetic fields rather than dissipating as heat. The transmitter coil propels most of its magnetic field into space, and the receiver—tuned to the same frequency—captures it efficiently up to several centimeters away.
| Parameter | Tight Induction | Resonant Induction |
|---|---|---|
| Frequency | 110–205 kHz | 6.78 MHz (AirFuel) or 13.56 MHz (NFC) |
| Coil Q-factor | 20–30 | 100+ |
| Typical gap | ≤ 5 mm | 10–40 mm |
| Efficiency | 60–72 % | 80–90 % (good alignment) |
A high quality factor QQQ means energy sloshes back and forth many times before resistance damps it, giving the receiver multiple chances to grab power. However, higher QQQ also narrows bandwidth, so frequency tracking circuits adjust the drive slightly as load or temperature shifts.
Why Eddy Currents Matter
If a conductive object such as a coin or credit-card antenna wanders into the field, Faraday’s law induces loops of current that heat the metal—dangerous at tens of kilohertz where skin depth is shallow. Standards therefore mandate:
- Foreign-object detection (FOD) – the pad measures reflected impedance; an unexpected resistive rise triggers shutdown.
- Temperature monitors – NTC thermistors near the transmitter coil throttle power if the coil exceeds ≈ 50 °C.
These controls are essential because 15 W phone chargers can generate > 10 W of stray heat inside a misaligned metal-back phone.
From AC Fields to DC Batteries
The receiver module rectifies the induced AC with synchronous MOSFET bridges, then steps the DC to desired battery voltage via buck converters. Pulse-width modulation adjusts the transmitter’s power so the receiving phone sees exactly the wattage requested via a back-channel (modulating load impedance lets the phone talk to the pad). Early Qi versions topped out at 5 W; Qi 2 now negotiates 15–30 W, and future revisions eye 60 W for slim laptops.
Efficiency Checklist for Engineers
- Coil geometry – maximize overlap area; circular spirals waste corners, while square coils fit better in rectangular phones.
- Wire selection – choose strand diameters near skin depth δ ≈ √(ρ/π f μ) for minimum resistance.
- Capacitor ESR – low equivalent series resistance preserves QQQ.
- Adaptive frequency control – lock transmitter to receiver resonance to stay on-peak despite magnetic detuning.
- Thermal vias and copper pours – conduct coil heat into the phone’s chassis instead of the battery.
Standards and Safety Regulations
- Qi (Wireless Power Consortium) – ubiquitous in phones; caps frequency near 200 kHz.
- AirFuel Alliance – promotes resonant systems at 6.78 MHz and RF charging at 915 MHz.
- IEC 62311 – limits human exposure to magnetic fields; consumer chargers must keep specific absorption rate below 2 W kg⁻¹.
- UL 2738 – outlines FOD, temperature shutdown, and insulation standards for household devices.
Beyond Smartphones: Emerging Applications
Electric vehicles (EVs)
Ground pads at 85 kHz send up to 11 kW through 10 cm air gaps, charging an EV parked over the coil. Precision parking assists, plus ferrite flux guides, maintain coupling.
Medical implants
Pacemakers and drug pumps use resonant coils near 13.56 MHz to harvest energy through skin without infection-prone wires.
Kitchen countertops
Induction cooktops already heat pans; research prototypes add a second modulation channel to power small appliances anywhere on the surface.
Internet-of-Things sensors
Sub-watt resonant networks trickle-charge battery-free sensors scattered around a room, using frequency-diverse packets so each device tunes to its optimal sub-band.
Wireless Charging versus Radio-Frequency Harvesting
Inductive systems move energy mainly via magnetic near-fields, dropping off roughly as 1/r31/r^31/r3. RF harvesters tap far-field electromagnetic waves where intensity scales as 1/r21/r^21/r2 but legal power limits are far lower (FCC Part 15 allows only milliwatts). Result: RF trickle charging suits ultra-low-power gadgets but not phones.
A Glimpse into True Over-Air Charging
Start-ups are experimenting with steerable phased-array antennas that sweep 915 MHz beams to phones across a room. Beam-forming keeps power density below safety thresholds by spreading energy except at the focal point. Real-world demos reach ~1 W at 3 m—enough for idle maintenance, not full charge. Key physics challenges:
- Free-space path loss ∝(4πr/λ)2propto (4pi r/lambda)^2∝(4πr/λ)2 rises steeply with distance.
- Multipath fading in rooms causes destructive interference.
- Regulatory ceilings on EIRP limit total transmitted power.
Breakthroughs will hinge on higher-gain antennas, AI-controlled beam steering, and high-efficiency rectennas.
Practical Tips for Everyday Users
- Center the phone – misalignment halves coupling and wastes energy as heat.
- Remove metal cases or cards – avoid eddy-current heating and FOD shutdowns.
- Keep firmware updated – pads and phones negotiate power profiles; updates improve efficiency.
- Ventilation helps – cooler coils mean lower resistance and higher efficiency, so don’t cover a charging pad with books.
Conclusion: Field Equations in Your Pocket
Every time a device charges wirelessly, Maxwell’s equations play out in miniature: oscillating currents launch magnetic fields; Faraday’s law converts flux back into voltage; resonance stores and shuttles energy with minimal loss. By tweaking coil shapes, materials, and control algorithms, engineers push efficiency ever closer to wired perfection. Though true across-room charging remains a frontier, the physics is sound—and steadily closing the gap between convenience and capacity.
From your bedside pad to future EV driveways, wireless power shows how classical physics still powers the technologies of tomorrow.
