AQA GCSE Physics (Separate Science) Unit 7: Magnetism and Electromagnetism Study Notes

AQA GCSE Physics (Separate Science) Unit 7: Magnetism and Electromagnetism

Poles of a Magnet

• Definition: A magnet has two ends known as the north pole and the south pole.

• Magnetic Forces: The magnetic forces are strongest at the poles.

• Interaction of Magnets:

  • When two magnets are brought close together:

  • Like Poles: Repel each other (e.g., N-N or S-S).

  • Opposite Poles: Attract each other (e.g., N-S).

• Non-contact Force: Magnets do not need to be in contact for the magnetic forces to be observed.

• Magnetic Materials: Only iron, cobalt, and nickel (and alloys containing these metals) are inherently magnetic.

• Types of Magnets:

  • Permanent Magnet: Has its own magnetic field that cannot be turned off (e.g., bar magnet, horseshoe magnet).

  • Induced Magnet: Becomes magnetic only when placed within a magnetic field. Induced magnets attract other magnetic materials but lose magnetism when removed from the field (e.g., iron filings).

Plotting Magnetic Field Lines

• Definition of Magnetic Field: The region surrounding a magnet where the magnetic force acts on other magnets or magnetic materials.

• Observation Method: Observed using a compass at various points around a bar magnet.

  • To draw field lines:

  1. Place the bar magnet in the center of paper.

  2. Position a magnetic compass around the magnet and note the direction of the needle, marking dots along the compass direction.

  3. Include arrows to indicate the direction of north.

  4. Repeat for various positions and connect the dots to complete the field lines.

• Magnetic Field Strength: Strongest at the poles; closer lines indicate stronger fields.

• Direction of Magnetic Field Lines:

  • Always emerge from the north pole and enter the south pole.

• Compass Use: The compass needle indicates the direction of the Earth's magnetic field, aiding navigation.

Electromagnetism

• Induced Magnetism: Occurs when a current flows through a conducting wire creating a circular magnetic field.

• **Characteristics of Induced Magnets: **

  • Switching off the current disables the magnetism.

  • Strength Increasing Factors:

  • Increasing current will strengthen the magnetic field.

  • Coiling the wire increases the strength of the magnetic field.

• Solenoid: A coiled wire that produces a strong and uniform magnetic field when current flows through it.

  • Methods to Increase Strength:

  • Add an iron core.

  • Increase number of wire coils.

  • Increase current.

• Electromagnet: A solenoid with an iron core that can be turned on and off. Found in devices such as electric motors, loudspeakers, electric bells, and remote-controlled locks.

The Motor Effect and Fleming's Left-Hand Rule

• Motor Effect Definition: The phenomenon whereby a current-carrying wire in a magnetic field experiences a force acting at a right angle to both the wire and the magnetic field.

• Electric Motors: Coiling wire allows the motor effect to enable the rotation of the wire. Electric motors work by getting a coil to rotate based on the force from the current, which changes direction every half rotation using a split ring commutator.

• Split Ring Commutator Functionality: It facilitates consistent rotation by switching the current direction in the motor for every half turn.

• Force Calculation Formula:

  • $F = B imes I imes l$

    • where:

      • $F$ = Force (N)

      • $B$ = Magnetic flux density (T)

      • $I$ = Current (A)

      • $l$ = Length (m)

  • Worked Example: If a current of 8A flows through a wire of 75cm (0.75m) length in a 0.5T magnetic field:

  • Calculation: $F = 0.5 imes 8 imes 0.75 = 3N$

• Fleming’s Left-Hand Rule: A method to determine the direction of force from the motor effect:

  • Left Hand Position:

  • Thumb = Direction of Force

  • Index Finger = Direction of Magnetic Field

  • Middle Finger = Direction of Current

Applications of Electromagnetic Principles

Headphones and Loudspeakers

• Loudspeaker Mechanism: Utilizes the motor effect to produce sound through the movement of a cone:

  • Alternating current through a coil produces a varying electromagnetic field.

  • This interacts with a permanent magnet, causing the cone to move, producing sound waves by vibrating air particles.

Transformers

• Induced Potential: Created when a potential difference arises across a conductor due to changes in magnetic fields, enabling current flow if connected in a closed circuit.

  • Induction Methods Include:

  • Moving a magnet through a coil.

  • Moving a conductor through magnetic field lines.

  • Inserting/removing a coil in a magnet's field.

• Transformers' Purpose: Change voltage using induced potential in alternating currents:

  • Types of Transformers:

  • Step-up Transformers: Decrease in primary coil turns, increase voltage.

  • Step-down Transformers: Increase in primary coil turns, decrease voltage.

  • Efficiency: Transformers function almost 100% efficiently with the formula:

  • $P<em>{in} = P</em>{out}$

  • $V<em>p imes I</em>p = V<em>s imes I</em>s$

• Worked Example: For Shannon’s hair straighteners (110V AC to 230V with 6A current):

  • $110 imes I_p = 230 imes 6$

  • Solve: $I_p = rac{1380}{110} = 12.55A$

Microphones

• Microphone Functionality: Operates like a loudspeaker, but uses the generator effect:

  • Sound waves vibrate a diaphragm, causing the coil to move and creating an induced potential.

  • The resulting current changes to match sound wave vibrations, transferring sound signals accordingly.