Current and magnetism (AQA GCSE Physics Combined Science): Revision Notes
Current and magnetism
What happens when current flows
When electricity flows through a wire, something amazing happens - it creates a magnetic field around the wire. This magnetic field has a specific shape, direction, and strength that depends on several important factors.
The fundamental law of electromagnetism: Any electric current will always create a magnetic field. This is one of the most important connections between electricity and magnetism.
Magnetic field around a straight wire
Shape of the field
When current flows through a long straight wire, the magnetic field forms concentric circles around the wire. Think of these circles like ripples spreading out from the wire in all directions.
Imagine dropping a stone into still water - the ripples form perfect circles spreading outward. The magnetic field lines around a current-carrying wire behave similarly, forming circular patterns around the wire.
Direction of the field
To work out which way the magnetic field points, use the right-hand rule:
- Point your right thumb in the direction of the conventional current (from + to -)
- Your fingers will curl in the direction of the magnetic field lines
Don't confuse conventional current direction with electron flow direction. Electrons actually flow from negative to positive, but we always use conventional current (+ to -) for the right-hand rule. This historical convention is used consistently in electromagnetic calculations.
Strength of the magnetic field
The strength of the magnetic field depends on two key factors:
Current size:
- Directly proportional to the current
- If you double the current, the magnetic field strength also doubles
- Bigger current = stronger magnetic field
Distance from the wire:
- Inversely proportional to the distance
- If you double the distance from the wire, the field strength halves
- Closer to the wire = stronger magnetic field
- Further from the wire = weaker magnetic field
Worked Example: Magnetic Field Relationships
If the current in a wire is 2A and produces a certain magnetic field strength at 1cm from the wire:
Step 1: Doubling the current (2A → 4A)
Result: Magnetic field strength doubles
Step 2: Doubling the distance (1cm → 2cm) with original current
Result: Magnetic field strength halves
Step 3: Both changes together (4A current at 2cm)
Result: Field strength = 2 × 0.5 = same as original
The solenoid
What is a solenoid?
A solenoid is a long coil of conducting wire wrapped around in a spiral and covered with insulating material. When current flows through a solenoid, it creates a much stronger and more useful magnetic field.
Magnetic field in a solenoid
The solenoid creates a strong, uniform magnetic field inside the coil. This field is similar in shape to the field around a bar magnet, with clear north and south poles.
Unlike the circular field lines around a straight wire, the magnetic field inside a solenoid runs in parallel straight lines from one end to the other, creating a uniform field strength throughout the interior.
Making it stronger
You can make the magnetic field inside a solenoid even stronger by:
- Increasing the current flowing through the wire
- Adding more turns to the coil
- Inserting an iron core inside the coil (this creates an electromagnet)
Uses of solenoids
Solenoids are used in many devices to create strong, controlled magnetic fields. They're especially useful because you can turn the magnetism on and off by controlling the electric current.
Key relationships to remember
Understanding Proportionality:
Directly proportional means when one thing increases, the other increases at the same rate. For example: current doubles → magnetic field strength doubles.
Inversely proportional means when one thing increases, the other decreases. For example: distance doubles → magnetic field strength halves.
Key Points to Remember:
- Electric current always creates a magnetic field around the conductor
- Magnetic field lines around a straight wire form concentric circles
- Use the right-hand rule to find the direction of the magnetic field
- Stronger current means stronger magnetic field
- Closer to the wire means stronger magnetic field
- Solenoids create strong, uniform magnetic fields that can be controlled by adjusting the current