A Level Physics Revision: All of Electromagnetism (in 38 minutes)

Electromagnetism Overview

Introduction to Electromagnetism

• The session focuses on revising electromagnetism, a crucial and fascinating topic in A-level physics, divided into two main sections: magnetic fields and their relationship with electric fields through Faraday's law.

Understanding Magnetic Fields

• The Earth behaves like a giant bar magnet, with distinct north and south poles. This analogy helps visualize the magnetic field around the planet.

• Key rule: Magnetic field lines always travel from the north pole to the south pole, curving back towards the south.

• If a free north pole existed (theoretical), it would follow these magnetic field lines back to the south pole. Finding such a monopole could lead to significant recognition in physics.

Patterns of Magnetic Fields

• Uniform magnetic fields are indicated by equally spaced parallel lines between two opposing poles; this uniformity is essential for understanding interactions between magnets.

• When two opposing poles are present, their field pattern resembles that of uniform fields directed toward each other.

Magnetic Fields Around Current-Carrying Wires

Characteristics of Magnetic Fields from Wires

• A current-carrying wire generates concentric circular magnetic fields around it.

• Notation: A circle with a cross indicates current flowing into the page, while a dot signifies current coming out of the page.

Directionality of Magnetic Fields

• The direction of the magnetic field depends on current flow; clockwise if current flows into the screen and anti-clockwise if it comes out.

Right Hand Grip Rule

• To determine magnetic field direction using the right hand grip rule: grip the wire with your right hand; thumb points in conventional current direction while fingers curl in magnetic field direction.

Applying Concepts Through Problem Solving

Example Problem Using Right Hand Grip Rule

• An example problem involves determining magnetic field directions at specific points (A and B). At point A, it emerges from the screen; at point B, it goes into the screen based on right-hand grip application.

Understanding Solenoids

• Inside solenoids, assume uniformity in magnetic fields. Identifying north and south poles relies on analyzing current directions within coils.

Understanding Magnetic Forces on Current-Carrying Wires

The Basics of Magnetic Fields and Currents

• The current flows downwards, with the north pole in one direction and the south pole in another. If the current reverses, so do the poles.

• A wire of length L is placed within an external magnetic field (magnetic flux density B). Without current, there’s no effect on the wire.

• When a current I runs through the wire (e.g., connected to a power supply), it experiences a magnetic force.

Calculating Magnetic Force

• The force (F) on the wire is calculated using F = BIL sin(θ), where:

• F = force,

• B = magnetic flux density (measured in Tesla),

• I = current (in amps),

• L = length of the wire (in meters),

• θ = angle between the wire and magnetic field.

• Maximum force occurs when θ = 90°, making sin(θ) equal to one. Conversely, if θ is 0° or 360°, there will be no force as the wire is parallel to the field.

Direction of Magnetic Force

• To determine direction, Fleming's left-hand rule is used:

• Thumb indicates motion/force,

• Index finger shows direction of magnetic field,

• Second finger represents direction of current.

• Applying this rule helps visualize how forces act on wires based on their orientation relative to magnetic fields.

Charge Particles in a Magnetic Field

• When a positive charge enters a magnetic field directed into the screen, its movement can be analyzed using Fleming's left-hand rule.

• As this charge moves right while experiencing an upward force due to magnetism, it curves instead of moving straight—this curvature results from centripetal force acting at right angles to its motion.

Relating Centripetal Force and Magnetic Force

• The relationship between centripetal force and magnetic force can be expressed as mv²/r = BIL. This allows for rearranging equations to find radius of curvature for charged particles in a magnetic field.

Understanding Magnetic Forces and Flux

Deriving the Radius in Circular Motion

• The equation MV^2 / R = QVB is introduced, where M is mass, V is velocity, R is radius, Q is charge, and B is magnetic field strength.

• Rearranging gives the formula for radius: R = MV / (QB) . This derivation should be memorized for exams.

Key Concepts of Magnetic Flux

• Magnetic flux ( Phi ) is defined as the product of magnetic flux density ( B ) perpendicular to an area and the area itself.

• The component of magnetic flux density that contributes to flux can be expressed as Bcos(theta) , where theta is the angle between field lines and the normal to the surface.

Mathematical Representation of Magnetic Flux

• The formula for magnetic flux can be written as:

• Phi = BAcos(theta)

• It’s crucial to note that maximum flux occurs when the field lines are perpendicular to the area.

Understanding Angles in Magnetic Flux Calculations

• A common mistake involves misidentifying angle theta; it must be measured from the normal line, not directly from the surface.

• When field lines are parallel to an area, no magnetic flux passes through; thus, if they are at a 90° angle relative to each other, then:

• Φ = BAcos(90°)=0.

Example Calculation of Magnetic Flux

• An example problem illustrates calculating magnetic flux with given values. If:

• Density ( B = 3 mT = 3 × 10^-3 T)

• Area dimensions (3 cm by 5 cm converted into meters).

Units and Base Units in Electromagnetism

• The unit for magnetic flux density ( B) is Tesla (T), while magnetic flux ( Φ) uses Weber (Wb).

• To derive base units for magnetic flux density:

• Using force equation rearranged gives:

• Base unit becomes kg·A⁻¹·m⁻².

Introduction to Magnetic Flux Linkage

• Definition provided for magnetic flux linkage:

• Expressed as:

• Number of coils ( n) multiplied by total magnetic flux ( Φ).

Understanding Faraday's Law and Electromagnetism

Introduction to Faraday's Law

• The discussion begins with an overview of magnetism, leading into Michael Faraday's significant discovery known as Faraday's law.

• Faraday reversed the concept of electric current creating a force on a conductor by demonstrating that moving a magnet changes the magnetic field around a conductor, inducing an electromotive force (emf).

• The induced emf is directly proportional to the rate of change of magnetic flux linkage, expressed mathematically as E = -Delta Phi/Delta t .

Key Concepts in Faraday's Law

• The formula can also be represented as E = -Delta (NBA cos theta)/Delta t , where Phi represents magnetic flux.

• Lenz's law states that the direction of induced current opposes the change causing it; for example, if a north pole approaches a wire, the wire generates its own north pole to repel it.

• This opposition is crucial for energy conservation; without it, energy would not be conserved during interactions between magnets and conductors.

Application: AC Generators

• An explanation follows regarding how an alternating current (AC) generator operates using coils within a magnetic field.

• As the coil rotates through different positions relative to the magnetic field, the angle ( theta ) changes, affecting magnetic flux and thus inducing varying emf over time.

• A graph illustrating flux linkage against time shows maximum flux when perpendicular to field lines and zero when parallel.

Understanding Induced EMF

• The changing angle results in alternating current generation due to continuous variation in direction as the coil moves through its cycle.

• The gradient of this graph indicates the magnitude of induced emf, emphasizing its importance in understanding electromagnetic principles.

Transformers Explained

• Transitioning to transformers, they consist of an iron core with primary and secondary coils connected to an AC power supply.

• AC current creates changing magnetic fields which induce emf in nearby coils due to fluctuating magnetic flux linkage.

• The soft iron core enhances efficiency by linking primary and secondary coils effectively.

This structured summary captures key insights from discussions on Faraday’s law and related concepts while providing timestamps for easy reference.

Understanding Electromagnetic Induction and Transformers

The Basics of Electromagnetic Induction

• The induced electromotive force (emf) in the secondary coil is explained by Faraday's law, which states that a changing magnetic field can induce an emf without a direct physical connection between the primary and secondary coils.

• The primary coil generates a magnetic field that changes over time, leading to variations in magnetic flux. This change induces an emf in the secondary coil due to the rate of change of magnetic flux.

Transformer Equations and Power Conservation

• The transformer equation relates the number of turns in the coils (NS for secondary, NP for primary) to their respective voltages (VS for secondary, VP for primary), expressed as NS/NP = VS/VP.

• Current relationships are also crucial; VS/VP equals IP/IS, where IP is current in the primary coil and IS is current in the secondary coil. This relationship highlights power conservation principles within transformers.

• Power conservation indicates that power input into the primary must equal power output from the secondary: PP = PS, or mathematically VP × IP = VS × IS. Rearranging gives insights into voltage and current relationships across transformer coils.