Magnetism
56 flashcards covering Magnetism for the MCAT Chem / Phys / Psych / Soc section.
Magnetism is a fundamental force in physics that causes certain materials, like iron or electromagnets, to attract or repel each other. It arises from the motion of electric charges, such as electrons spinning in atoms, and is closely linked to electricity. This phenomenon powers everyday devices, from compasses to MRI machines, and forms a key part of electromagnetism, which is essential for understanding the physical world.
On the MCAT, magnetism appears in the Chemical and Physical Foundations section, typically through multiple-choice questions on magnetic fields, forces on moving charges, and electromagnetic induction. Common traps include confusing magnetic poles with electric charges or misapplying formulas like the Biot-Savart law. Focus on mastering concepts like the right-hand rule, Faraday's law, and calculating magnetic forces, as these often require quick problem-solving and integration with other topics.
Practice drawing magnetic field lines for better visualization.
Terms (56)
- 01
Magnetic field
A region around a magnet or a moving electric charge where magnetic forces can be detected, characterized by field lines that indicate the direction a north pole would move.
- 02
Magnetic force on a moving charge
The force experienced by a charged particle moving through a magnetic field, given by F = q(v × B), where it is perpendicular to both the velocity and the field, causing circular or helical motion.
- 03
Lorentz force law
The equation that combines electric and magnetic forces on a charged particle, F = q(E + v × B), where E is the electric field, v is velocity, and B is the magnetic field.
- 04
Right-hand rule for magnetic force
A method to determine the direction of the magnetic force on a moving charge, by pointing fingers in the direction of velocity, curling them toward the magnetic field, with the thumb indicating the force direction for a positive charge.
- 05
Magnetic field due to a straight current
The magnetic field produced by a long straight wire carrying current, calculated as B = (μ₀ I) / (2 π r), where I is current, r is distance from the wire, and μ₀ is the permeability of free space.
- 06
Biot-Savart law
A law that calculates the magnetic field at a point due to a current element, similar to Coulomb's law for electricity, integrating dB = (μ₀ / 4π) (I dl × r̂) / r² over the current path.
- 07
Ampere's law
A law relating the magnetic field around a closed loop to the electric current passing through the loop, stated as ∮ B · dl = μ₀ Ienclosed, useful for symmetric current distributions.
- 08
Magnetic flux
The measure of the amount of magnetic field passing through a given area, calculated as Φ = B A cosθ, where B is the magnetic field strength, A is the area, and θ is the angle between them.
- 09
Faraday's law of induction
A law stating that the induced electromotive force in a closed loop is equal to the negative rate of change of magnetic flux through the loop, ε = -dΦ/dt, fundamental to electromagnetic induction.
- 10
Lenz's law
A principle that the direction of an induced current opposes the change in magnetic flux that produced it, conserving energy by creating a magnetic field that counters the original change.
- 11
Electromagnetic induction
The process by which a changing magnetic field induces an electric current in a conductor, as seen in generators and transformers.
- 12
Solenoid
A coil of wire that creates a uniform magnetic field inside when current passes through it, with field strength B = μ₀ n I, where n is turns per unit length and I is current.
- 13
Torque on a current loop
The rotational force on a current-carrying loop in a magnetic field, given by τ = μ × B, where μ is the magnetic moment, causing the loop to align with the field.
- 14
Magnetic dipole moment
A vector quantity for a current loop or magnet, equal to the product of current, area, and a direction normal to the loop, used to describe interactions with external fields.
- 15
Ferromagnetism
A property of materials like iron that can be permanently magnetized due to aligned magnetic domains, resulting in strong attraction to magnets and retention of magnetism.
- 16
Paramagnetism
A weak attraction to magnetic fields exhibited by materials with unpaired electrons, such as aluminum, where the atoms align with the field but lose magnetism when removed.
- 17
Diamagnetism
A property of materials that creates a weak opposing magnetic field when exposed to an external field, causing repulsion, as seen in substances like copper with all paired electrons.
- 18
Hysteresis in ferromagnets
The lag in the magnetization of a ferromagnetic material as the external magnetic field is changed, forming a loop on a graph, which represents energy loss in applications like transformers.
- 19
Curie temperature
The temperature above which a ferromagnetic material loses its permanent magnetism and becomes paramagnetic, due to thermal agitation disrupting magnetic domains.
- 20
Force between parallel currents
The attractive or repulsive force between two parallel wires carrying currents, proportional to the product of currents and inversely to the distance, defining the ampere unit.
- 21
Cyclotron motion
The circular path followed by a charged particle in a uniform magnetic field, with radius r = mv / (qB), used in devices like cyclotrons for particle acceleration.
- 22
Hall effect
The production of a voltage difference across a conductor carrying current in a magnetic field, perpendicular to both, which allows measurement of charge carrier properties.
- 23
Self-inductance
The property of a coil that opposes changes in current through it by inducing an EMF, given by ε = -L di/dt, where L is inductance, affecting circuits with changing currents.
- 24
Mutual inductance
The induction of EMF in one circuit due to changing current in a nearby circuit, quantified by M, the mutual inductance coefficient, as in transformers.
- 25
Time constant in RL circuit
The time τ = L/R for an RL circuit, where the current reaches about 63% of its final value after one time constant when the circuit is closed.
- 26
Electric motor
A device that converts electrical energy to mechanical work by using magnetic fields to rotate a current-carrying loop, based on the torque from the Lorentz force.
- 27
Electric generator
A device that converts mechanical energy to electrical energy by rotating a coil in a magnetic field, inducing an EMF through Faraday's law.
- 28
Gauss's law for magnetism
A law stating that the magnetic flux through any closed surface is zero, indicating that magnetic monopoles do not exist and field lines form closed loops.
- 29
Magnetic permeability
A measure of how much a material concentrates magnetic field lines, with μ = μ₀ μr, where μr is the relative permeability, higher in ferromagnetic materials.
- 30
Right-hand rule for field from current
A rule to find the direction of the magnetic field around a current-carrying wire, by grasping the wire with the right hand, thumb in current direction, fingers curl in field direction.
- 31
Velocity selector
A device using perpendicular electric and magnetic fields to allow only particles of a specific velocity to pass undeflected, where v = E/B.
- 32
Mass spectrometer
An instrument that uses magnetic fields to separate ions by mass, based on their deflection in a field, allowing analysis of molecular weights.
- 33
Induced EMF in a moving loop
The EMF generated when a conductor moves in a magnetic field, calculated as ε = B l v sinθ, for a straight conductor of length l moving at velocity v.
- 34
Common trap: Magnetic fields do no work
A key point that magnetic forces act perpendicular to velocity, so they change direction but not speed, meaning they do not perform work on charged particles.
- 35
Strategy for solving induction problems
First calculate the change in magnetic flux, then apply Faraday's law to find induced EMF, and use Lenz's law to determine current direction in the circuit.
- 36
Magnetic field at loop center
For a circular loop of radius r carrying current I, the field at the center is B = (μ₀ I) / (2 r), pointing perpendicular to the loop's plane.
- 37
Ferromagnetic domains
Regions within a ferromagnetic material where atomic magnetic moments are aligned, and the material's overall magnetism depends on how these domains are oriented.
- 38
Saturation magnetization
The maximum magnetization a ferromagnetic material can achieve when all domains are aligned with the external field, beyond which increasing the field has no effect.
- 39
Remanence
The residual magnetization left in a ferromagnetic material after the external magnetic field is removed, representing the material's ability to retain magnetism.
- 40
Coercivity
The measure of the external magnetic field strength required to reduce the magnetization of a ferromagnetic material to zero, indicating its resistance to demagnetization.
- 41
Earth's magnetic field
The planet's global magnetic field, generated by electric currents in the outer core, which protects from solar wind and defines magnetic north and south.
- 42
Example: Force on a proton in field
For a proton with velocity 5 × 10^6 m/s perpendicular to a 2 T magnetic field, the force is F = q v B = (1.6 × 10^{-19}) (5 × 10^6) (2) = 1.6 × 10^{-12} N, causing circular motion.
- 43
Difference between B and H fields
B is the magnetic flux density in a material, while H is the magnetic field strength, related by B = μ H, with H more useful in materials with permeability.
- 44
Displacement current
A term in Maxwell's equations representing the changing electric field that contributes to the magnetic field, essential for explaining electromagnetic waves.
- 45
Inductor in AC circuits
A component that opposes changes in current, leading to a phase difference in AC, where the reactance XL = 2πf L, affecting impedance.
- 46
Magnetic resonance in MRI
The absorption and emission of energy by atomic nuclei in a magnetic field when exposed to radiofrequency pulses, used in medical imaging to visualize tissues.
- 47
Common trap: Direction of induced current
Students often forget to apply Lenz's law, which ensures the induced current creates a field opposing the flux change, so always check for opposition.
- 48
Cyclotron frequency
The frequency of a charged particle's rotation in a magnetic field, f = q B / (2π m), independent of velocity, used in particle accelerators.
- 49
Permanent magnet
A material that retains magnetization without an external field, due to ferromagnetic properties, commonly used in speakers and motors.
- 50
Electromagnet
A temporary magnet created by winding a coil around a ferromagnetic core and passing current through it, with strength adjustable by current.
- 51
Example: EMF in falling loop
For a loop of area 0.1 m² falling at 2 m/s into a 1 T field, the induced EMF is ε = B l v = 1 × 0.1 × 2 = 0.2 V, assuming the loop's width is perpendicular.
- 52
Superconducting magnets
Magnets that use superconducting materials to produce strong, stable fields with no resistance, as in MRI machines, once cooled below critical temperature.
- 53
Relative permeability
A dimensionless ratio indicating how much a material enhances the magnetic field compared to vacuum, greater than 1 for paramagnetic and ferromagnetic materials.
- 54
Meissner effect
The expulsion of magnetic fields from a superconductor below its critical temperature, resulting in perfect diamagnetism and levitation in strong fields.
- 55
Poynting vector
A vector representing the directional energy flux of an electromagnetic field, with magnitude related to E × B, indicating power flow in waves.
- 56
Maxwell's equation for magnetic fields
One of Maxwell's equations states that the divergence of B is zero, ∇ · B = 0, meaning no magnetic monopoles exist.