How does a changing magnetic field produce an electric current, and how is this used in generators?
Describe electromagnetic induction as the production of an electromotive force by a changing magnetic field through a conductor, and explain how generators and transformers use induction.
A Regents Physics answer on electromagnetic induction: how a changing magnetic field through a conductor induces an electromotive force and current, the factors that increase the induced EMF, and how generators and transformers work, with worked reasoning examples.
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What this topic is asking
Electromagnetic induction is the converse of the motor effect: where a current in a field produces a force, a changing magnetic field produces a current. The Physical Setting/Physics course asks you to describe induction as the production of an electromotive force (EMF) when the magnetic field through a conductor changes, to know the factors that increase the induced EMF, and to explain how generators and transformers rely on it. The Regents tests this conceptually, focusing on the condition for induction and the factors that affect its size.
What induction is
The discovery, due to Faraday, is that magnetism can create electricity, mirroring the way a current creates magnetism. The crucial word is change: the magnetic field through the conductor must be changing for an EMF to be induced. This can happen by moving a magnet relative to a coil, moving or rotating the coil in a field, or changing the current (and so the field) in a nearby coil.
The condition for induction: a changing field
This is the single most-tested idea: students often think a magnet sitting inside a coil should induce a current, but a static field induces nothing. Only relative motion or another change keeps the field varying and the current flowing. The direction of the induced current opposes the change that produced it (Lenz's law), which is why a generator resists being turned.
Factors that increase the induced EMF
These factors are a standard Regents short-answer question ("state two ways to increase the induced voltage"). Each ties back to the same principle: a faster or larger change in the field, applied across more turns, gives a bigger EMF.
Generators and transformers
A generator is the practical use of induction: a coil is rotated in a magnetic field (often turned by a turbine), so the field through the coil continually changes and an EMF is induced, converting mechanical energy into electrical energy. It is essentially a motor run in reverse. A transformer uses a changing current in a primary coil to create a changing magnetic field, which induces an EMF in a secondary coil wound on the same iron core; by choosing the ratio of turns, it steps the voltage up or down, which is how electricity is transmitted efficiently at high voltage and delivered safely at low voltage.
Reference Tables note
The Reference Tables do not include an equation for electromagnetic induction (no Faraday's-law or transformer formula on the Physical Setting/Physics tables), so this topic is assessed qualitatively. You recall the condition (a changing field), the factors that increase the induced EMF, and the operation of generators and transformers. This complements magnetism and the motor effect: a current makes a field (the motor), and a changing field makes a current (the generator).
Try this
Q1. State the condition required to induce a current in a coil using a magnet. [1 point]
- Cue. The magnetic field through the coil must be changing (for example, by moving the magnet relative to the coil).
Q2. State the energy conversion that takes place in an electrical generator. [1 point]
- Cue. Mechanical (kinetic) energy is converted into electrical energy.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (style)1 marksPart A (multiple choice). A bar magnet is held stationary inside a coil of wire connected to a sensitive meter. The meter reads zero current. To induce a current in the coil, you should (1) hold the magnet still in the coil (2) move the magnet into or out of the coil (3) increase the coil's resistance (4) cool the coil. Justify your choice.Show worked answer →
A 1-point Part A item on the condition for induction. The answer is (2).
A current is induced only when the magnetic field through the coil is changing. A stationary magnet gives a steady field and no induced current. Moving the magnet into or out of the coil changes the field through it, inducing an electromotive force and current. The trap is (1): a magnet at rest, even inside the coil, induces nothing.
Regents (style)2 marksPart B-2 (constructed response). State two ways to increase the size of the electromotive force induced when a magnet is moved near a coil, and explain why each works.Show worked answer →
A 2-point constructed-response conceptual item on induction.
Two ways (1 point each, any two): move the magnet faster (a faster change in the magnetic field induces a larger EMF); use a stronger magnet (a larger change in field strength); use more turns on the coil (each turn adds to the induced EMF).
Explanation: the induced EMF depends on how quickly the magnetic field through the coil changes and on the number of turns, so anything that speeds the change or multiplies it across more turns increases the EMF.
Markers reward two valid methods, each with a reason tied to a faster or larger change in the magnetic field, or more turns.
Related dot points
- Describe magnetic fields and the field produced by an electric current, apply to the force on a moving charge in a magnetic field, and explain the force on a current-carrying wire that underlies the electric motor.
A Regents Physics answer on magnetism and the motor effect: magnetic fields and field lines, the magnetic field of a current, the force on a moving charge using the Reference-Table equation, and the force on a current-carrying wire that drives electric motors, with worked examples.
- Define current as rate of flow of charge, , state Ohm's law , and apply the electrical power equations to calculate power and energy in a resistor.
A Regents Physics answer on current, Ohm's law and electrical power: current as rate of charge flow, the voltage-current-resistance relationship, and the power and energy equations from the Reference Tables, with worked examples.
- Define the electric field as force per unit charge, , describe the uniform field between parallel plates with , and define electric potential difference as work per unit charge, .
A Regents Physics answer on electric fields and potential difference: the field as force per unit charge, the uniform field between parallel plates, field-line diagrams, and potential difference as work per unit charge, using the Reference-Table equations, with worked examples.
- Describe charging by friction, conduction and induction, state that charge is conserved and quantised in multiples of the elementary charge, and apply Coulomb's law to calculate the force between point charges.
A Regents Physics answer on static electricity and Coulomb's law: how objects are charged by friction, conduction and induction, the conservation and quantisation of charge, and how to apply the Reference-Table equation for the force between point charges, with worked examples.
- Apply the rules for series and parallel circuits to current, voltage and total resistance, and analyze simple circuits to find the current through and voltage across each component.
A Regents Physics answer on series and parallel circuits: the rules for current, voltage and total resistance in each, how total resistance increases in series and decreases in parallel, and how to analyze a simple circuit, with worked examples.
Sources & how we know this
- Reference Tables for Physical Setting/Physics — NYSED (2006)
- Physical Setting/Physics Core Curriculum — NYSED (2010)