Current and magnetism (AQA GCSE Physics): Revision Notes
Current and magnetism
Electric current creates magnetic fields around conductors. Understanding how these fields work is essential for explaining motors, generators and electromagnets.
How electric current creates magnetic fields
When electric current flows through any conductor, it produces a magnetic field around it. The shape, direction and strength of this magnetic field depend on several important factors.
Think of it like this: electricity and magnetism are closely linked. Wherever you have moving electric charges (current), you automatically get magnetism too.
Magnetic field around a straight wire
Shape of the magnetic field
When current flows through a long straight wire, the magnetic field forms concentric circles around the wire. These are circular field lines that get further apart as you move away from the wire.
Imagine looking down at a wire from above - the magnetic field lines would look like ripples spreading out in circles around the wire.
Direction of the magnetic field
The direction of the magnetic field depends on which way the current is flowing. You can work this out using the right-hand rule:
Right-Hand Rule for Magnetic Field Direction:
- Point your right thumb in the direction of the conventional current (from positive to negative)
- Your fingers curl in the direction of the magnetic field lines
Remember: Don't confuse current direction with electron flow. Use conventional current (positive to negative) for this rule.
Strength of the magnetic field
The strength of the magnetic field around a straight wire depends on two main factors:
- Current size: The magnetic field strength is directly proportional to the current. Double the current, double the field strength.
- Distance from wire: The magnetic field strength is inversely proportional to the distance from the wire. Move twice as far away, and the field becomes half as strong.
The solenoid
What is a solenoid?
A solenoid is a long coil of conducting wire covered in insulating material. When current flows through a solenoid, it creates a much stronger and more useful magnetic field than a straight wire.
Magnetic field properties
The solenoid produces a strong, uniform magnetic field inside the coil. The field pattern is very similar to that of a bar magnet - it has a north pole at one end and a south pole at the other.
The magnetic field inside a solenoid is remarkably uniform and predictable, making solenoids extremely useful in practical applications like electromagnets and electric motors.
Making the field stronger
You can increase the magnetic field strength inside a solenoid by:
- Increasing the current flowing through the wire
- Adding more turns to the coil
- Inserting an iron bar inside the coil (this creates an electromagnet)
Practical Application: Electromagnet Design
To create a powerful electromagnet:
- Wind 200 turns of copper wire around an iron core
- Connect to a 6V power supply
- The iron core becomes strongly magnetised and can lift metal objects
- Switch off the current - the magnetism disappears immediately
Iron cores are particularly effective because iron is easily magnetised and makes the magnetic field much stronger.
Understanding proportionality
Directly proportional means that as one factor increases, the other increases at the same rate. For example, if you double the current in a wire, you double the magnetic field strength.
Mathematically: (magnetic field strength is proportional to current)
Inversely proportional means that as one factor increases, the other decreases. For example, if you double your distance from a wire, the magnetic field strength halves.
Mathematically: (magnetic field strength is inversely proportional to distance)
Key Points to Remember:
- Electric current always creates magnetic fields around conductors
- Straight wires produce circular magnetic field lines that get weaker with distance
- Use the right-hand rule to find magnetic field direction (thumb = current, fingers = field)
- Solenoids create much stronger, more uniform magnetic fields than straight wires
- Field strength increases with more current and decreases with distance from the wire