Phys: Magnetism and Electromagnetism

What is a magnetic field, and what key rules govern magnetic field lines around a bar magnet?

A magnetic field is a region where magnets or magnetic materials (iron, steel, nickel, cobalt) experience a non-contact force; magnetic field lines always point from north to south, show the force direction on a north pole, and are closest together at the poles where the field is strongest.

How do magnetic poles interact, and how does the force between a magnet and an unmagnetised magnetic material behave?

Like poles repel and unlike poles attract; the force between a magnet and an unmagnetised magnetic material is always attractive regardless of which pole is placed near it.

How can a plotting compass be used to map the magnetic field pattern around a bar magnet?

Place a small plotting compass near a bar magnet, mark the direction its needle points, move the compass so its tail aligns with the previous needle mark, repeat across multiple locations, and join the points to trace the field lines.

What is the fundamental difference between permanent magnets and induced magnets?

Permanent magnets produce their own persistent magnetic field; induced magnets are magnetic materials that temporarily become magnetized only when placed inside a magnetic field and quickly lose most or all of their magnetism when the external field is removed.

What shape is the magnetic field around a straight current-carrying wire, and how does field strength vary?

The field consists of concentric circles perpendicular to the wire with the wire at the centre; the field is strongest closest to the wire and increases in strength when a larger current flows.

How is the right-hand thumb rule used to determine the direction of a magnetic field around a straight wire?

Point the right thumb in the direction of conventional electric current flow and curl the fingers; the direction the fingers point indicates the direction of the magnetic field.

What is the shape and nature of the magnetic field inside and outside a current-carrying solenoid?

Inside the solenoid, the magnetic field is strong and uniform (constant strength and parallel direction); outside, the field shape is identical to that of a bar magnet.

What is an electromagnet, and how can its magnetic field strength be increased?

An electromagnet is a solenoid wrapped around an iron core whose magnetic field can be turned on and off with an electric current; its strength is increased by increasing current, adding more wire turns, or inserting an iron core (which becomes an induced magnet).

How are electromagnets used in scrap cranes and relay switches?

In cranes, electromagnets are turned on to attract/pick up magnetic metals and turned off to drop them; in relays, a low-current primary circuit energizes an electromagnet to attract an iron contact, closing contacts to switch on a secondary high-current circuit.

What is the motor effect, and what conditions determine the magnitude of the force produced?

When a current-carrying wire is placed in a magnetic field, the magnetic field around the wire interacts with the magnetic field of the magnets, causing a force to be exerted on the wire; maximum force occurs when the wire is at $90^\circ$ to the magnetic field lines (zero force if parallel), and the magnitude increases if the magnetic field strength or the current increases.

What equation calculates the force on a current-carrying conductor at right angles to a magnetic field, and what do the variables represent?

$F = B I l$, where $F$ is force in newtons ($\text{N}$), $B$ is magnetic flux density in tesla ($\text{T}$), $I$ is current in amperes ($\text{A}$), and $l$ is the length of conductor inside the magnetic field in metres ($\text{m}$).

How does Fleming's left-hand rule determine the direction of the force on a conductor?

Using the left hand, point the **F**irst finger in the direction of the magnetic **F**ield (north to south), the se**C**ond finger in the direction of the **C**urrent (positive to negative), and the **Th**umb will point in the direction of **M**otion / force; reversing either the current or field reverses the force direction.

How does a basic electric motor work, and what role does the split-ring commutator play?

A current-carrying coil placed in a magnetic field experiences equal and opposite forces on its two parallel sides, causing it to rotate on a spindle; the split-ring commutator swaps the electrical contacts every half turn to swap current direction and keep the coil rotating continuously in the same direction.

How can the rotation direction of a dc electric motor be reversed?

The direction of motor rotation can be reversed by either swapping the polarity of the dc supply (reversing current direction) or swapping the magnetic poles (reversing field direction).

How do loudspeakers and headphones convert alternating electrical signals into sound waves?

An alternating current (ac) passes through a coil of wire attached to the base of a paper cone surrounding a permanent magnet pole; as current direction alternates, the magnetic force rapidly reverses direction, causing the coil and paper cone to vibrate, creating pressure variations in the surrounding air at the same frequency as the ac signal.

What is the generator effect (electromagnetic induction)?

The induction of a potential difference (and current if part of a complete circuit) in a conductor that is moving relative to a magnetic field, or experiencing a change in magnetic field.

How can potential difference and current be induced in a conductor using a magnet and coil?

By moving a magnet inside a coil of wire or moving a conductor through a magnetic field ("cutting" magnetic field lines); moving the magnet/conductor in the opposite direction or reversing magnetic polarity reverses the induced potential difference/current.

How does continuous relative movement (moving backwards/forwards or rotating a magnet inside a coil) produce alternating current (ac)?

Moving a magnet backwards and forwards continuously swaps the direction of the induced potential difference/current; similarly, rotating a magnet inside a coil changes the magnetic field direction through the coil every half turn, causing the current to reverse direction every half turn to produce ac.

How does Lenz's law apply to the magnetic field created by an induced current?

The magnetic field created by an induced current always acts against the change that made it, meaning the induced current always opposes the change that caused it.

How can the size of an induced potential difference (or induced current) be increased?

By increasing the speed of movement (cutting more magnetic field lines in a given time) or by increasing the strength of the magnetic field (providing more field lines to be cut).

How do alternators use slip rings and brushes to generate alternating current (ac)?

As a coil rotates inside a magnetic field (or a magnet inside a coil), a current is induced that changes direction every half turn; alternators use slip rings and brushes so the electrical contacts do not swap every half turn, outputting an alternating potential difference.

How do dynamos use a split-ring commutator to generate direct current (dc)?

Dynamos rotate a coil inside a magnetic field, but use a split-ring commutator that swaps the circuit connection every half turn to keep the induced current flowing in the same direction, producing direct current.

How do oscilloscope traces distinguish between ac and dc generated potential difference, and how does increasing coil speed affect the trace?

On an oscilloscope, ac shows a wave that goes up and down crossing the horizontal axis, whereas dc stays above the axis (pd remains positive); increasing rotation frequency increases overall peak pd (height) and creates more wave peaks in a given time.

How do moving-coil microphones convert sound waves into electrical signals using the generator effect?

Sound waves hit a flexible diaphragm attached to a coil of wire wrapped around a magnet, causing the coil to move back and forth in the magnetic field; this induces a current whose variations match the pressure variations of the sound wave.

Why do transformers only work with alternating current (ac) rather than direct current (dc)?

An alternating pd applied across the primary coil creates a continuously changing magnetic field in the iron core, which is necessary to induce an alternating pd in the secondary coil; a direct current creates a constant magnetic field, which cannot induce a pd.

What are the main components of a basic transformer, and why is an iron core used?

A transformer consists of two coils of insulated wire (primary and secondary) wrapped around a shared soft iron core; iron is used because it magnetises and demagnetises easily to efficiently transfer the changing magnetic field.

What is the structural and functional difference between a step-up transformer and a step-down transformer?

A step-up transformer increases potential difference ($V_s > V_p$) and has more turns on the secondary coil than the primary coil ($n_s > n_p$); a step-down transformer decreases potential difference ($V_s < V_p$) and has more turns on the primary coil than the secondary coil ($n_p > n_s$).

What equation links primary potential difference ($V_p$), secondary potential difference ($V_s$), primary turn count ($n_p$), and secondary turn count ($n_s$)?

$\frac{V_p}{V_s} = \frac{n_p}{n_s}$ or $\frac{V_s}{V_p} = \frac{n_s}{n_p}$, where $V_p$ and $V_s$ are potential differences in volts ($\text{V}$), and $n_p$ and $n_s$ are the respective number of turns on each coil.

What equation relates potential difference and current across primary and secondary coils for a 100% efficient transformer?

$V_s I_s = V_p I_p$, where secondary power output ($V_s I_s$) equals primary power input ($V_p I_p$), with potential difference ($V$) in volts ($\text{V}$) and current ($I$) in amperes ($\text{A}$).

Why are step-up and step-down transformers used in the National Grid to transmit power efficiently?

Step-up transformers increase potential difference to extremely high levels before transmission, which decreases current ($I$) for a given power level, drastically reducing thermal energy lost as heat in the transmission lines; step-down transformers then reduce the potential difference to safe levels for consumer use.