Magnetism

Units

ampere (A): unit of electric current, measuring the rate of flow of electric charge in a circuit; 1 ampere means 1 coulomb of charge passes a point each second

volt (V): unit of potential difference, equal to the energy transferred per unit charge; 1 volt = 1 joule per coulomb

watt (W): unit of power, measuring the rate of energy transfer; 1 watt = 1 joule per second

Magnetism

Properties of Magnets

  • Magnets have two poles: north and south.

  • Like poles repel, unlike poles attract.

  • The force between magnets acts at a distance through a magnetic field.

  • Magnets attract magnetic materials such as iron, steel, nickel, and cobalt.

  • The magnetic force is strongest at the poles of a magnet.

Magnetically Hard and Soft Materials

  • Magnetically hard materials:

    • Difficult to magnetise but retain magnetism for a long time.

    • Have high coercivity (resist demagnetisation).

    • Used to make permanent magnets (e.g. steel).

  • Magnetically soft materials:

    • Easy to magnetise but lose magnetism quickly when the field is removed.

    • Have low coercivity.

    • Used in electromagnets and transformer cores (e.g. iron).

Magnetic Field Lines

  • A magnetic field is a region where a compass will align/ is a region where a magnetic material will experience a force

  • Magnetic field lines represent the direction and strength of a magnetic field.

  • They point from the north pole to the south pole outside the magnet.

  • Inside the magnet, they go from south to north, forming closed loops.

  • The closer the lines are, the stronger the magnetic field.

  • Field lines must never cross or touch, as this would imply two directions at once.

  • The stronger the field, the closer togther the lines are

Induced Magnetism

  • When a magnetic material is placed in a magnetic field, it can become magnetised.

  • This happens because domains (regions of aligned atoms) align with the field.

  • In soft materials, this effect is temporary.

  • In hard materials, some magnetism may remain after removal of the field.

Magnetic Field Patterns (Practical)

  • Iron filings can be sprinkled around a magnet to reveal field patterns.

  • A plotting compass can be used to trace field lines and direction.

  • Around a bar magnet: curved lines from north to south.

  • Between two magnets:

    • Opposite poles: field lines connect, showing attraction and a strong field.

    • Same poles: field lines push apart, showing repulsion and a weaker region between them.

  • The density of filings indicates field strength.

Uniform Magnetic Field

  • Produced by placing two flat, parallel magnets with opposite poles facing each other.

  • Field lines are straight, parallel, and evenly spaced.

  • This shows the magnetic field has constant strength and direction throughout the region.

  • Uniform fields are useful in experiments where a constant force is needed.

Different magnet examples

Single bar magnet:

2 bar magnets - North poles facing towards each other:

2 magnets - opposite poles facing:

Electromagnetism

Magnetic Field Around a Current

  • A current flowing through a conductor produces a magnetic field around it.

  • The field consists of concentric circular lines centered on the wire.

  • The direction of the field can be determined using the right-hand grip rule (thumb = current, fingers = field direction).

  • Increasing current increases the strength of the magnetic field.

  • Summary:

Electromagnets

  • Made by wrapping insulated wire into a coil (solenoid) around a soft iron core.

    • Makes the magnetic field stronger

    • Uses iron because it can be turned on and off whereas steel would stay magnetic

  • When current flows, the coil produces a magnetic field similar to a bar magnet.

  • The soft iron core becomes magnetised, greatly increasing field strength.

  • Strength can be increased by:

    • Increasing current

    • Increasing number of turns

    • Using a better core material (soft iron)

  • Can be switched on and off, unlike permanent magnets.

  • Electric bell:

  • Relay:

    • A relay is a magnetic switch and they are used for turning on high current or dangerous appliances so that the user stays away from the danger

  • Circuit breaker:

Magnetic Field Patterns

  • Straight wire: circular field lines around the wire, closer near the wire indicating stronger field.

  • Flat circular coil: field lines loop through the coil, resembling a weak bar magnet.

  • Solenoid:

    • Inside: strong, uniform, parallel field lines.

    • Outside: weaker, curved lines similar to a bar magnet.

Force on a Charged Particle

  • A charged particle moving through a magnetic field experiences a force.

  • The force acts perpendicular to both the direction of motion and the magnetic field.

  • If the particle moves parallel to the field, no force acts.

  • This can cause circular or curved motion of charged particles.

  • Catapult field:

    • What is the combined magnetic field for a wire and a uniform field?

Force on a Current-Carrying Wire

  • A wire carrying current in a magnetic field experiences a force due to interaction between magnetic fields.

  • This is called the motor effect.

  • The force is perpendicular to both current and magnetic field direction.

  • Applications:

    • d.c. motors: convert electrical energy into kinetic energy (rotation).

    • loudspeakers: convert electrical signals into vibrations (sound).

  • Speaker:

    • When the current flow one way round the speaker coil, the speaker moves left

    • When it flows the other way round the speaker moves right

    • Use Fleming’s left hand rule to work out which way it will move

    • Just focus on one point of the could to work out the direction

  • Motors:

    • Motors spin anticlockwise

    • Half a turn later - the could as turned over so now the current in the purple half is flowing left instead of right

    • This keeps the motor spinning anticlockwise

    • Parts of a motor:

      • Coil - acts as the current carrying wire in a magnetic field so there is a force

      • Magnets - provide the magnetic field

      • Brushes - transfer current into the col without wires getting tangled up

      • Split-ring commutator - keeps the current flowing back on the left hand side and forwards on the right hand side so the motor keeps spinning the same way

Left-Hand Rule

  • Used to determine direction of force on a current-carrying conductor.

  • First finger: direction of magnetic field (north to south).

  • Second finger: direction of current (positive to negative).

  • Thumb: direction of force or motion.

  • Overall with each abbreviation for what every finger represent it spells out FBI (Force, Field, Current)

  • If a current is parallel to the field, there will not be a force!

Factors Affecting Force

  • Increasing current increases the force.

  • Increasing magnetic field strength increases the force.

  • Increasing length of wire in the field increases the force.

  • Reversing current or field reverses the direction of the force.

Electromagnetic Induction

Reminder:

  • Left hand rule and catapult effect:

    • If you have a current carrying wire in a magnetic field you get movement due to the force

  • Magnetic field + current = movement (e.g. motor)

  • Current (+movement) = magnetic field (electromagnet)

  • SO movement + magnetic field = current (generator)

Induced Voltage

  • A voltage is induced when:

    • We move the wire into the magnetic field, it cuts the field lines

    • This generates induced voltage

    • As there will be a complete circuit, a current will flow

  • Other ways to get an induced voltage:

    • Move a magnet into a coil

    • Spin a magnet in a coil

    • Move a magnet out of a coil

    • Move a wire out of the magnetic field

    • Place a turned of electromagnet into a coil and then turn it on or off

  • Factors increasing induced voltage:

    • Faster relative motion

    • Stronger magnetic field

    • Greater number of turns in the coil

    • Larger area of the coil

Generating Electricity

  • Occurs when mechanical energy is converted into electrical energy.

  • Methods:

    • Rotating a magnet inside a stationary coil

    • Rotating a coil within a magnetic field

  • Produces alternating current (a.c.).

  • Increasing speed of rotation increases frequency and voltage.

Transformers

  • Consist of a primary coil, secondary coil, and an iron core.

  • The alternating current in the primary coil produces a changing magnetic field.

  • This induces a voltage in the secondary coil.

  • The size of the voltage depends on the number of turns in each coil.

  • Only work with a.c. because a changing magnetic field is required.

How do transformers work step by step

  • An alternating voltage is applies across the primary coil (the input)

  • The direction whihc the current flows though the primary coil therefore keeps reversing

  • This means that the current in the primary coil magnetises the iron core in one direction, then in the other, then back and so on

  • The magnetic field produced by the iron core there continually changes

  • As the field changes the magnetic field lines around the iron core move

  • These moving field lines cut through the wires in the secondary coil

  • This induces a voltage in the secondary coil

  • The direction in which the field lines move through keeps changing

  • The voltage is therefore induced in the secondary coil (the output) is an alternating voltage

Why transformers only work with a.c.

  • If a direct voltage is applied by the primary coil, then the iron core will be continually magnetised in the same direction

  • This means that the field produced by the iron core will not change

  • The magnetic field lines around the iron core will therefore be stationary and so will not cut the wires in the secondary coil

  • Meaning that no voltage will be induced in the secondary coil

Step-Up and Step-Down Transformers

  • Step-up transformer:

    • Increases voltage

    • Secondary coil has more turns than primary

  • Step-down transformer:

    • Decreases voltage

    • Secondary coil has fewer turns than primary

  • Calculations:

  • National grid use:

    • Step-up transformers increase voltage for transmission to reduce energy loss (less current → less heating).

    • Step-down transformers reduce voltage to safe levels for homes and appliances.

Transformer Equation
input (primary) voltage / output (secondary) voltage = primary turns / secondary turns

Power in Transformers
input power = output power (for 100% efficiency)

Vp × Ip = Vs × Is

  • This means that if the voltage is increase by the transformer, the current will be decreased by the same factor

    • i.e. if voltage doubles, current will halve

Efficiency Considerations

  • In real transformers, some energy is lost as heat and sound.

  • Losses are reduced by using soft iron cores and insulating materials.