Magnetic effect of a current
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Magnetic Field Around a Straight Wire When current flows through a straight wire, a magnetic field is produced around it. FIG 4.5.9: Field around a current-carrying wire Cross-section view: wire shown as dot (current coming out of page) or cross (current going into page).
Magnetic field lines are concentric circles around the wire. Arrows on circles show field direction. Lines closer together near wire (stronger field). Right-Hand Grip Rule (for straight wire) Point thumb in direction of current .
Fingers curl in direction of magnetic field . Properties of the Field Field lines are concentric circles around the wire Field is stronger closer to the wire (lines closer together) Field strength increases with larger current Reversing current reverses field direction Magnetic Field Around a Solenoid A solenoid is a coil of wire.
When current flows through it, the field pattern resembles that of a bar magnet . FIG 4.5.11: Field around a solenoid Coil of wire with current flowing. Field lines emerge from one end (N pole) and enter the other end (S pole).
Inside the solenoid, field lines are parallel and evenly spaced (uniform field). Outside, pattern like a bar magnet. Right-Hand Grip Rule (for solenoid) Curl fingers in direction of current . Thumb points to N pole of the solenoid.
Factors Affecting Solenoid Field Strength Increasing current → stronger field Increasing number of turns → stronger field Adding a soft iron core → much stronger field (electromagnet) Reversing current → reverses polarity (N and S swap) Experiment: Investigating Magnetic Fields Method 1: Plotting Compasses Place plotting compass near current-carrying wire/solenoid Mark position of compass needle Move compass and repeat to trace field pattern Add arrows showing N pole direction Method 2: Iron Filings Place card around wire/solenoid Sprinkle iron filings on card Tap gently – filings align with field Note: cannot show direction, only pattern Applications: Relays A relay is an electromagnetic switch that allows a small current to control a larger current in a separate circuit.
FIG 4.5.14: Relay circuit Two circuits shown: Control circuit (low current) with switch and electromagnet coil wound on iron core. Main circuit (high current) with contacts that close when electromagnet attracts iron armature.
When control circuit is on, electromagnet pulls armature, closing contacts in main circuit. How a Relay Works Small current flows through electromagnet coil Electromagnet becomes magnetised Iron armature is attracted to electromagnet Armature movement closes contacts in main circuit Large current can now flow in main circuit Uses of Relays Car starter motor: Small current from ignition switch controls large starter current Industrial machines: Safe control of high-power equipment Home automation: Low-voltage switches controlling mains appliances Applications: Loudspeakers A loudspeaker converts electrical signals into sound using the magnetic effect of current.
FIG 4.5.15: Loudspeaker Cross-section: permanent magnet (ring shape) creates radial field. Wire coil attached to paper cone sits in the gap of the magnet. When a.c. signal passes through coil, it moves back and forth in the magnetic field, vibrating the cone to produce sound.
How a Loudspeaker Works A.c. signal from amplifier passes through voice coil Coil experiences force in permanent magnet's field Direction of force alternates with a.c. signal Coil (and attached cone) vibrates back and forth Cone vibrations create sound waves in air
Interactive revision notes, videos and practice questions load below.