Magnetism — Free Physics Review Games.

Every magnet has a north pole and a south pole.

Like magnetic poles repel each other; opposite poles attract.

A magnetic field is the area around a magnet where magnetic forces can be detected.

Iron (and its alloys) is the most commonly used ferromagnetic material.

An electromagnet is a temporary magnet made by passing electric current through a coil, often wrapped around an iron core.

A moving electric charge (current) creates a magnetic field around the conductor, as discovered by Oersted.

Electromagnetic induction produces voltage (and current) when a conductor experiences a changing magnetic field, discovered by Faraday.

A compass needle is a small magnet that freely rotates to align with Earth's magnetic field, pointing toward magnetic north.

Faraday's Law states that the induced EMF equals the negative rate of change of magnetic flux through a circuit.

Increasing current, adding more wire coils, or inserting a ferromagnetic core all increase an electromagnet's strength.

Lenz's Law states that induced current flows in a direction that opposes the change in flux, consistent with energy conservation.

An electric motor uses the force on current-carrying conductors in a magnetic field (Lorentz force) to produce rotational motion.

Electricity and magnetism are unified as electromagnetism: moving charges create magnetic fields, and changing magnetic fields create electric fields.

A generator converts mechanical energy into electrical energy through electromagnetic induction, while a motor does the reverse.

Magnetic flux is the product of the magnetic field strength, the area it passes through, and the cosine of the angle between them.

The tesla is the SI unit for magnetic field strength (magnetic flux density), defined as one weber per square meter. The 'Weber' is actually the unit of magnetic flux itself, not the field strength, so it is a related but distinct quantity. Students should remember that tesla measures how strong a field is at a point, while related units like the weber describe flux through an area.

Moving charges in the wire generate a magnetic field that wraps around the conductor in concentric circles, as described by the right-hand rule. 'Straight lines parallel to the wire' would not produce any circulating field capable of exerting a force on nearby charges or magnets, which contradicts observed effects like a compass deflecting near a wire. This circular pattern is fundamental to understanding how current loops and solenoids build stronger fields.

Iron filings become temporarily magnetized and align themselves along the magnetic field lines, visually mapping the field's shape and direction. They cannot indicate 'the exact strength of the magnetic field in teslas' because filing patterns show orientation, not a numeric measurement. This classic demonstration helps students visualize invisible field structure around magnets.

A magnetic domain is a microscopic region within a ferromagnetic material where the magnetic moments of atoms point in the same direction, contributing to a net magnetic effect. 'The distance a magnetic field can travel' describes field range, not an internal structural feature of the material. Understanding domains explains why materials like iron can be magnetized and demagnetized.

Copper is diamagnetic because its atomic structure produces no unpaired electron spins that align with an external field, resulting in a weak repulsion. 'Iron' is strongly attracted to magnets because it is ferromagnetic, with domains that align readily and amplify applied fields. Recognizing categories like diamagnetic, paramagnetic, and ferromagnetic helps predict how a material will respond to a magnetic field.

Iron is used as the core because its high permeability allows it to become strongly magnetized by the coil's field, greatly increasing the overall magnetic strength. 'Rubber' has no significant magnetic permeability and would not enhance the field at all. This is why electromagnets in devices like cranes and motors almost always use ferromagnetic cores.

Field lines are most concentrated near the poles, and since field strength corresponds to line density, the field is strongest there. 'In the exact middle of the magnet' is incorrect because field lines spread out and are less dense away from the poles inside the body of the magnet. This concept explains why paperclips and iron filings cluster most heavily at the ends of a bar magnet.

When the magnetic domains within iron align in the same direction, their individual fields combine to produce a strong, persistent net magnetic field. 'It is heated to a very high temperature' actually does the opposite, since heat above the Curie point randomizes domain alignment and destroys magnetism. This domain-alignment concept is central to explaining both magnetization and demagnetization.

Strong physical shocks or heat disrupt the alignment of magnetic domains, causing them to become randomly oriented and reducing net magnetism. 'Storing it in a cool, dry place' actually helps preserve magnetization rather than destroy it. This reinforces the idea that magnetism depends on the orderly arrangement of domains, which can be disturbed by energy input.

By convention, magnetic field lines emerge from the north pole and curve around to enter the south pole outside the magnet, forming closed loops. 'From the south pole to the north pole' actually describes the field's path inside the magnet, not outside. This convention is essential for correctly interpreting field diagrams and determining forces on other magnets or charges.

A compass needle is a small magnet that naturally aligns itself with the surrounding magnetic field, including Earth's field, pointing roughly toward magnetic north. It does not 'measure the exact strength of Earth's magnetic field' since it only indicates direction, not magnitude. This makes the compass a simple but reliable tool for orientation based on field direction rather than field strength.

Only moving charges, such as electrons forming a current, generate a magnetic field around a conductor, as described by Ampere's law. 'They must be stationary' is incorrect because stationary charges produce only electric fields, not magnetic ones. This distinction between electric and magnetic field sources is a foundational idea in electromagnetism.

Since the magnetic field around a wire is directly proportional to the current, given by \(B = \frac{\mu_0 I}{2\pi r}\), increasing the current increases the field strength. 'The magnetic field strength stays exactly the same' contradicts this direct proportionality shown in the formula. This relationship explains why increasing current is a common way to strengthen an electromagnet's field.

Using $F = BIL$ for a wire perpendicular to the field, \(F = (0.5)(4)(2) = 4\ \text{N}\), which correctly combines field strength, current, and length. The choice '\(8\ \text{N}\)' would result from mistakenly doubling the correct product rather than applying the formula directly. This formula is essential for calculating forces in motors and other current-carrying devices placed in magnetic fields.

The force on a moving charge is given by $F = qvB\sin\theta$, and since the velocity is perpendicular to the field, \(\sin\theta = 1\), giving \(F = (2)(3)(0.4) = 2.4\ \text{N}\). The value '\(1.2\ \text{N}\)' would result from an arithmetic error such as forgetting to multiply by the charge correctly. This formula underlies the circular motion of charged particles in devices like mass spectrometers and cyclotrons.

The magnetic field inside a solenoid follows \(B = \mu_0 n I\), where \(n\) is the number of turns per unit length, so increasing turns directly increases the field strength. 'It has no effect on field strength' is incorrect since the formula shows field strength scales linearly with turn density. This principle explains why tightly wound coils with many loops produce stronger electromagnets than loosely wound ones.

Magnetic flux is calculated as \(\Phi = BA\cos\theta\), and since the field is perpendicular to the loop, \(\theta = 0\) and \(\cos\theta = 1\), giving \(\Phi = (0.5)(0.2) = 0.1\ \text{Wb}\). The value '\(2.5\ \text{Wb}\)' would come from dividing rather than multiplying the given quantities. Correctly applying this formula is essential for solving Faraday's Law problems involving induced EMF.

Using Faraday's Law, \(\varepsilon = N \frac{\Delta\Phi}{\Delta t} = 50 \times \frac{0.06}{2} = 1.5\ \text{V}\), which correctly accounts for both the number of turns and the rate of flux change. The choice '\(3\ \text{V}\)' would result from forgetting to divide by the time interval of \(2\) seconds. This calculation shows why both the rate of change and coil turns matter in generating EMF.

Lenz's Law states that induced currents oppose the change in flux that created them, so the coil generates a field that repels the approaching north pole, resisting the motion. 'A direction that creates a magnetic field aiding the magnet's approach' would violate conservation of energy by accelerating the magnet without an external energy input. This opposition principle is why moving a magnet into a coil always requires doing work against the induced field.

Transformer voltage relates to turns ratio by \(\frac{V_s}{V_p} = \frac{N_s}{N_p}\), so \(V_s = 20 \times \frac{400}{100} = 80\ \text{V}\). The choice '\(5\ \text{V}\)' would incorrectly invert the turns ratio, which is the mistake made when confusing step-up and step-down configurations. This ratio is fundamental to how transformers change voltage levels in power distribution systems.

The magnetic force is given by $F = qvB\sin\theta$, and since velocity \(v = 0\) for a stationary charge, the force is zero regardless of field strength. 'Stationary charges have no electric field' is false, since even at rest a charge still produces an electric field. This velocity-dependence is a key distinction between magnetic and electric forces that students must apply when analyzing charged particle motion.

The split-ring commutator reverses the current direction in the coil every half rotation, which reverses the torque direction at the right moment to keep the coil spinning continuously in one direction. 'A permanent increase in magnetic field strength over time' does not occur in a standard DC motor design and would not solve the oscillation problem. This commutator mechanism is essential to converting electrical energy into continuous rotational motion.

Since the magnetic field of an electromagnet is proportional to current through \(B = \mu_0 n I\), a higher current produces a stronger field, which in turn increases the force experienced by the nearby bar magnet. 'Increasing current has no effect on magnetic interactions' directly contradicts this proportional relationship. This shows why electromagnet strength can be precisely controlled by adjusting current, unlike a fixed permanent magnet.

The induced EMF depends on the rate of change of flux, which is greatest when the loop's plane is parallel to the field because the flux is changing most rapidly at that orientation, per \(\varepsilon = -N\frac{d\Phi}{dt}\). 'When the plane of the loop is perpendicular to the magnetic field' actually corresponds to maximum flux but zero rate of change, giving zero EMF. This relationship between flux and its rate of change is the core mechanism behind AC generator output.

Faraday's Law states that EMF is induced only when flux changes over time, \(\varepsilon = -N\frac{d\Phi}{dt}\), so a constant flux produces zero EMF and therefore no induced current. 'A constant current is induced' is incorrect because induction fundamentally requires change, not just presence, of magnetic flux. This principle explains why a stationary magnet near a stationary coil produces no current, even though flux is present.

As the metal plate moves through the field, the changing flux within the conducting material induces loops of current, called eddy currents, that circulate within the metal itself according to Faraday's Law. 'Eddy currents only form in insulators, not conductors' is backwards, since insulators lack the free electrons needed to sustain such currents. This effect is used practically in magnetic braking systems, where eddy currents oppose motion and dissipate energy as heat.

Pointing the fingers in the direction of velocity (right) and curling them toward the field (into the page) shows the thumb points upward, giving the force direction via \(\vec{F} = q\vec{v} \times \vec{B}\). 'Downward, toward the bottom of the page' would be the correct direction only for a negative charge moving the same way, not a positive one. Mastering this cross-product rule is essential for predicting particle trajectories in magnetic fields.

Each current-carrying wire produces a magnetic field that exerts a force on the other wire, and when currents flow in the same direction, this force is attractive due to the orientation of the resulting fields. 'They repel each other' actually describes the case when the currents flow in opposite directions. This wire-pair interaction is the basis for the historical definition of the ampere as a unit of current.

The iron core has high magnetic permeability, which concentrates the magnetic flux produced by the primary coil and efficiently channels it through the secondary coil, enabling effective energy transfer via mutual induction. 'It converts AC current into DC current' is false since transformers work specifically because of the changing nature of AC current and do not perform rectification. This core function is why real transformers achieve much higher efficiency than air-core coils.

Since transformers rely on Faraday's Law, requiring a changing magnetic flux to induce EMF, a steady DC current produces a constant flux with no rate of change, resulting in no induced voltage in the secondary coil. 'DC current is too weak to create any magnetic field' is incorrect because DC current does create a magnetic field, just a non-changing one. This limitation explains why AC power is used for long-distance transmission where voltage transformation is essential.

The magnetic force $F = qvB$ always acts perpendicular to the velocity of a charged particle, which makes it act exactly as a centripetal force that continuously redirects the particle into a circular path. 'The particle's own inertia' actually resists changes in motion and would cause the particle to travel in a straight line without an external centripetal force. This principle underlies devices such as cyclotrons and mass spectrometers that manipulate charged particle trajectories using magnetic fields.

The changing current in coil A creates a changing magnetic flux that links coil B, and by Faraday's Law this changing flux induces an EMF and current in coil B, a phenomenon called mutual induction. 'Nothing happens because coil B has no power source' ignores that induction does not require an internal power source, only a changing external flux. This mutual induction principle is the operating basis of transformers and wireless charging systems.

Earth's magnetic field at the surface is only about \(25\) to \(65\ \mu\text{T}\), which is far weaker than laboratory electromagnets that can easily reach field strengths on the order of \(0.1\) to several teslas. 'Earth has no measurable magnetic field near its surface' is clearly false since Earth's field is what allows compasses to function for navigation. This comparison helps students appreciate the vast range of magnetic field strengths encountered in nature versus engineered devices.

Since induced EMF depends on the rate of change of flux, \(\varepsilon = -N\frac{d\Phi}{dt}\), increasing rotational speed increases how quickly the flux changes, directly boosting the induced EMF. 'Slowing down the rotation to reduce flux change' would actually decrease the EMF, since a slower rate of change produces a smaller induced voltage. This relationship is why generators are often designed to spin at controlled but relatively high speeds to maximize power output.

A stronger magnet generally produces a greater flux at any position, so as it moves through the coil at the same speed, the rate of change of flux, \(\frac{d\Phi}{dt}\), is larger, resulting in a greater induced EMF and current by Faraday's Law. The claim being 'always false regardless of magnet strength' ignores the direct dependence of flux, and therefore EMF, on field strength. This reasoning highlights that both field strength and speed of motion jointly determine the magnitude of induced current.

In a superconductor, electrical resistance drops to exactly zero below a critical temperature, so once a current is induced, there is no resistive energy loss and the current can persist indefinitely without an external EMF source. 'Superconducting loops are immune to Faraday's Law' is false because the initial current was induced precisely because of Faraday's Law during the flux change. This property is why superconducting magnets can maintain extremely strong, stable magnetic fields used in devices like MRI machines.

The motional EMF is given by $\varepsilon = BLv$, so as the rod's velocity \(v\) increases, the EMF increases proportionally, and with constant resistance, the current \(I = \varepsilon / R\) increases as well. 'The induced current stays constant regardless of velocity' contradicts this direct proportionality between EMF and velocity in the motional EMF equation. This scenario is a classic application connecting mechanical motion directly to electromagnetic induction.

As the motor spins, it acts partly like a generator, producing a back-EMF that opposes the applied voltage according to Lenz's Law, which reduces the net voltage driving current through the coil and thus decreases the current. 'The magnetic field surrounding the motor weakens as speed increases' is not generally true for a motor with a fixed field source such as a permanent magnet. This back-EMF concept explains why a motor draws large starting current but much less current once running freely at speed.

Because the induced current experiences an opposing magnetic force as the loop is pulled out, per Lenz's Law, work must be done against this force, and by conservation of energy that mechanical work is converted into electrical energy dissipated as resistive heat in the loop. 'It equals zero because no external force is needed' ignores the opposing force created by the induced current, which is the whole basis of Lenz's Law. This energy balance illustrates that electromagnetic braking always requires an external energy input to maintain constant velocity motion.

Faraday's Law, in its most general form as one of Maxwell's equations, states that a time-varying magnetic field produces a circulating electric field regardless of whether a physical conductor is present, since the induced field is a property of space itself. 'Electric fields can only exist inside conductors' is false, since electric fields exist throughout space and conductors merely respond to them by allowing charge to flow. This concept is essential to understanding how electromagnetic waves, including light, propagate through a vacuum.

When the electric force \(qE\) exactly balances the magnetic force $qvB$, the net force on the particle is zero, so by Newton's first law it continues moving in a straight line at constant velocity, which is the operating principle of a velocity selector. 'The particle accelerates uniformly in a circle' would only occur if the forces were unbalanced, causing a net centripetal force. This balanced-force configuration is used practically in mass spectrometers to select particles of a specific speed before further analysis.

Because isolated magnetic monopoles have never been observed, every north pole is always paired with a south pole, forcing magnetic field lines to form continuous closed loops that pass through the magnet from south to north internally and north to south externally. 'Closed loops only occur when current is alternating' is incorrect since even a static permanent magnet's field lines form closed loops without any current at all. This principle is formally expressed as Gauss's Law for magnetism, one of Maxwell's four fundamental equations.

For an ideal transformer, power is conserved, so \(V_p I_p = V_s I_s\); since the turns ratio doubles the voltage to \(240\ \text{V}\), conservation of power requires the current to be halved to \(1\ \text{A}\) to keep power constant. The choice '\(4\ \text{A}\)' incorrectly assumes current scales the same way as voltage rather than inversely. This inverse relationship between voltage and current in transformers is essential for understanding power transmission efficiency.

As the ring falls through a non-uniform field, the changing flux through it induces eddy currents, and by Lenz's Law these currents create a magnetic force opposing the ring's motion, slowing its fall compared to free fall alone. 'The ring falls at exactly the same rate since gravity is unaffected by magnetism' ignores that while gravity itself is unaffected, the net force on the ring includes an additional opposing magnetic force that changes its acceleration. This magnetic braking effect is a well-known demonstration of Lenz's Law and is used practically in eddy current brakes.

As the coil rotates at constant angular velocity \(\omega\), the flux through it varies as \(\Phi = BA\cos(\omega t)\), and taking the derivative for EMF gives $\varepsilon = -N\frac{d\Phi}{dt} = NBA\omega\sin(\omega t)$, producing a naturally sinusoidal output. 'The rotational speed of the coil constantly speeds up and slows down' is false since the generator is specifically designed to rotate at constant angular velocity, with the sinusoidal EMF arising purely from geometry rather than changing speed. This mathematical relationship explains why standard household AC power follows a smooth sine wave pattern.