Cambridge IGCSE Physics 0625 UNIT 4 Electricity and Magnetism Revision #igcsephysics

Properties of Magnets

Magnets possess two distinct poles: the North Pole and the South Pole, with magnetic forces peaking at these ends. When magnets are positioned near each other, they exhibit attractions between unlike poles and repulsions between like poles. Magnetic materials, which contain iron, nickel, or cobalt, are inherently attracted to magnets and can also be magnetized; for example, steel primarily consists of iron.

Types of Magnetic Materials

• Hard Magnetic Materials: Materials like steel that are difficult to magnetize but maintain their magnetism. These are typically used to create permanent magnets.

• Soft Magnetic Materials: Materials such as iron that are easily magnetized but lose their magnetism quickly. These are commonly employed in electromagnets and transformers due to their ability to have their magnetism easily controlled.

• Non-Magnetic Materials: Materials that are not attracted to magnets and cannot be magnetized, including metals that lack iron, nickel, or cobalt as well as non-metals.

Identifying Magnetic Properties

To determine a material’s magnetism:

• Magnets exhibit repulsion.

• Magnetic Materials reveal attraction without repulsion.

• Non-Magnetic Materials show no response to a magnet.

Example

Consider three unknown metal bars tested with a known magnet:

• Bar AB shows attraction at both ends, indicating it is a magnetic material.

• Bar CD repels at end C, identifying it as a magnet with North at C and South at D.

• Bar F shows no interaction, marking it as a non-magnetic material.

Magnetizing Magnetic Materials

Magnetization involves inducing magnetism within magnetic materials, accomplished through several methods:

1. Induced Magnetism: Involves placing a magnetic material near a strong magnet, resulting in weak and temporary magnetism (e.g., a steel bar retains magnetism while an iron bar does not after removal).

2. Magnetizing by Stroking: A magnet is stroked along a steel bar to impart magnetism in one direction, creating designated North and South poles.

3. Using Direct Current: A coil with a direct current can magnetize a steel nail by generating a magnetic field around it, establishing North and South poles.

Demagnetizing the Magnets

Demagnetization refers to the process of eliminating magnetism, which can be achieved through:

1. Heating: Applying heat to demagnetize a magnet.

2. Hitting: Physically striking the magnet to disrupt its magnetic structure.

3. Alternating Current: Using alternating current to generate a changing magnetic field that demagnetizes the magnet when it is pulled away from the coil.

Magnetic Fields

Magnetic fields are the regions in space influenced by a magnet where magnetic materials experience forces.

• Field Lines: These imaginary lines show the magnetic field's direction (from North to South) and strength (closer lines indicate stronger fields, while farther apart lines indicate weaker fields).

Earth’s Magnetic Field

The Earth itself contains a magnetic field due to its iron and nickel composition, acting like a giant magnet with its magnetic North and South poles.

• A compass needle aligns with Earth’s magnetic field, pointing towards the magnetic North.

Plotting Magnetic Field Lines

Magnetic field lines can be visualized by:

• Using Iron Fillings: Sprinkle iron fillings on paper over a magnet to see field lines forming.

• Using a Compass: Move a compass around a magnet and mark the needle's orientation to trace the magnetic field lines.

Electromagnetic Induction

Electromagnetic induction is the process where an EMF is generated in a conductor due to interaction with a changing magnetic field. This phenomenon occurs when conductors, such as wires, are moved through a magnetic field, inducing current and voltage.

Inducing Current with Wires

• If wires move perpendicular through the magnetic field, maximum current is induced. Conversely, when moving parallel, no current is induced. The Flemings Right-Hand Rule helps determine the direction of the induced current.

Electromagnetic Induction in Solenoids

Solenoids experience induced current based on the movement of magnets:

• A North Pole entering a solenoid induces current, indicated by galvanometer deflection. The speed of the magnet affects the induced current's magnitude; faster movements induce greater currents.

Lenz's Law

Induced currents produce magnetic fields opposing the change causing them, aligning with Lenz's law, which highlights the conservation of energy principle.

The AC Generator

AC generators convert kinetic energy to electrical energy, operating via electromagnetic induction principles. The mechanism involves a coil rotating within a magnetic field, generating alternating EMF and current. Using the Fleming Right-Hand Rule denotes the direction of induced current.

Graphing EMF vs. Time

The induced EMF peaks when the coil is horizontal due to maximum interaction with the magnetic field and falls to zero when the coil is vertical. As rotation speed increases, so does the induced EMF.

Magnetic Fields Around Conductors

Current flowing through a wire generates a surrounding magnetic field, which can also be analyzed using plotting compasses. The right-hand grip rule instructs on field direction, showing reversing the current alters the magnetic field direction.

Electromagnets

When current flows through a solenoid with an added soft iron core, the core amplifies the solenoid’s magnetic field. Consequently, electromagnets can be controlled and used in various applications, such as relays and electric bells.

Electric Relay

Operated by an electromagnet, electric relays allow smaller currents to actuate larger power circuits, efficient for several electrical applications.

Electric Bell

Electric bells leverage electromagnetism to produce sounds through a mechanically operated hammer.

Force on Current-Carrying Conductors

When current moves through a wire in a magnetic field, it experiences a force. The direction can be discerned with Fleming’s Left-Hand Rule. The force magnitude varies based on current direction relative to the magnetic field.

Loudspeakers and Headphones

These devices convert electrical signals into sound waves via the oscillating magnetic field around coils, leading to vibrations in cones.

Moving Charged Particles

Charged particles interact with magnetic fields, experiencing forces causing them to move in circular paths unless exiting the field, wherein they revert to linear motion.

The DC Motor

DC motors transform electrical energy into kinetic energy, driven by the rotating forces produced between current-carrying coils and magnetic fields. Using a split ring commutator, the current direction is reversed to maintain consistent rotational motion.

Mutual Induction

When an electromagnet energizes a second solenoid, inducing brief EMF pulses, its behavior mirrors a magnet moving swiftly toward another solenoid. Continuous current prevents induced EMF due to a stable magnetic field.

Transformers

Transformers adjust AC voltage levels through mutual induction and consist of primary coils and secondary coils around iron cores. DC fail to induce EMF due to unchanging magnetic fields.

Types of Transformers

• Step-Up Transformers: Increase voltage from primary to secondary.

• Step-Down Transformers: Decrease voltage from primary to secondary.

National Grid

Essential for electricity distribution, national grids utilize transformers to increase voltage for reduced energy loss in transmission. By keeping currents low and utilizing step-up transformers, high voltage transmission minimizes energy loss across long distances.

High Voltage Calculations

Illustrated calculations prove less power loss during high voltage transmission compared to low voltage scenarios, emphasizing efficient energy conservation methods.