physics

Qi (standard)

Qi (standard) is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Qi (standard) rather than just read about it. In short: Qi ( CHEE) is an open standard for inductive charging developed by the Wireless Power Consortium. It allows compatible devices, such as smartphones, to receive power when placed on a Qi charger, which can be effective over distances up to 4 cm (1.6 in).

Key takeaways

Qi (standard) belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Qi (standard) to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Qi (standard) from memory before moving on to harder problems.

Reference excerpt

Qi ( CHEE) is an open standard for inductive charging developed by the Wireless Power Consortium. It allows compatible devices, such as smartphones, to receive power when placed on a Qi charger, which can be effective over distances up to 4 cm (1.6 in). Devices that implement the optional Magnetic Power Profile, based on Apple's MagSafe wireless charging technology, using magnets for better device attachment and alignment to a charger may be labelled Qi2. Qi version 1.0 was released in 2010; by 2017, it had been incorporated into more than 200 models of smartphones, tablets, and other devices. In December 2023, 351 manufacturers were working with the standard, including Apple, Asus, Google, Huawei, LG Electronics, Samsung, Xiaomi, and Sony. The Qi specification version 2.2, released in April 2025, supports charging speeds of up to 25 watts and aims to improve compatibility across devices from various manufacturers. The current version 2.2.1 released in July 2025 includes Qi2 25 W branding for the 25-watt charging mode.

Naming The name of the Qi standard comes from the Chinese word qì (simplified Chinese: 气; traditional Chinese: 氣), meaning "life force" or "vital energy".

Design

Devices that operate using the Qi standard rely on electromagnetic induction between planar coils. A Qi system consists of two types of devices – the Base Station, which is connected to a power source and provides inductive power, and Mobile Devices, which consume inductive power. The Base Station contains a power transmitter that comprises a transmitting coil that generates an oscillating magnetic field; the Mobile Device contains a power receiver holding a receiving coil. The magnetic field induces an alternating current in the receiving coil by Faraday's law of induction. Close spacing of the two coils ensures the inductive power transfer is efficient. Base Stations typically have a flat surface – referred to as the Interface Surface – on top of which a user can place one or more Mobile Devices. There are two methods for aligning the transmitting coil (part of the Base Station) and receiving coil (part of the Mobile Device) in order for a power transfer to happen. In the first concept – called guided positioning – a user must place the Mobile Device on a certain location of the Base Station's surface. For this purpose, the Mobile Device provides an alignment aid that is appropriate to its size, shape, and function. The second concept – referred to as free positioning – does not require the user to place the Mobile Device in direct alignment with the transmitting coil. There are several ways to achieve free positioning. In one example a bundle of transmitting coils is used to generate a magnetic field at the location of the receiving coil only. Another example uses mechanical means to move a single transmitting coil underneath the receiving coil. A third option is to use a technique called Multiple Cooperative Flux Generators. The power transmitter includes a power conversion unit and a communications and control unit. Figure 1-1 shows the transmitting coil (array) generating the magnetic field as part of the power conversion unit. The control and communications unit regulates the transferred power to the level that the power receiver requests. A Base Station may contain numerous transmitters, allowing for multiple Mobile Devices to be placed on the same Base Station to charge each of its batteries. A power receiver comprises a power pick-up unit, and a communications and control unit. A power pick-up unit typically contains a single receiving coil only. The mobile device it charges typically contains a single power receiver. The communications and control unit regulates the transferred power to the level that is appropriate for the device.

Transmitters A reference Qi low-power transmitter has a coil of 20 turns (in two layers) in a flat coil, wound on a form with a 19 mm inner diameter and a 40 mm outer diameter, with a below-coil shield of soft iron at least 4 mm larger in diameter, which gives an inductance of 24 ± 1 microhenries. This coil is placed in a series resonant circuit that is driven by an H-bridge switching arrangement from the DC source. Power control is automatic and voltage is controllable in steps of 50 millivolts or less. In the device a proportional–integral–derivative controller is used to modulate the delivered power according to the primary cell voltage. Other Qi charge transmitters start their connections at 140 kHz, but can adjust to find a frequency to better match the distance between coils. Different Qi reference designs have different coil arrangements, including oval coil and multi-coil systems as well as more complex resonance networks with multiple inductors and capacitors. These designs allow operation at frequencies from 105 to 205 kHz and with maximum resonant circuit voltages as high as 200 volts.

Receivers The Qi power receiver hardware reference design starts with a rectangular coil of wire 44 mm × 30 mm outside size, with 14 turns of wire, and with an above-coil magnetic shield. Power output to the portable device is via a full-wave bridge; the power is typically filtered with a capacitor before delivery to the charge controller. Other Qi power receivers use alternate resonance modulators, including switching a resistor or pair of resistors across the receiver resonator capacitor, both before and after the bridge rectifier.

Features and specifications

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Qi (standard)

Start with the simplest possible case. Write down what Qi (standard) claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Qi (standard) before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Qi (standard) ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Qi (standard)

In research: Qi (standard) appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Qi (standard) in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Qi (standard) is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer-related introductions in 2009, Open standards, Wireless energy transfer, so understanding it makes those chapters shorter.
In everyday life: Look for Qi (standard) outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Qi (standard) in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Qi (standard) means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Qi (standard) out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Qi (standard) in simple terms?

Why does Qi (standard) matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Qi (standard)?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Qi (standard).