IGCSE Physics 0625 – Simple Phenomena of Magnetism
4.1 Simple Phenomena of Magnetism
Objective
Explain that magnetic forces are due to interactions between magnetic fields.
1. What is a magnetic field?
A magnetic field is a region of space around a magnet or a current‑carrying conductor in which other magnetic objects experience a force. The direction of the field is shown by field lines that exit the north pole and enter the south pole of a magnet.
Suggested diagram: Field lines around a bar magnet, showing north (N) and south (S) poles.
2. How magnetic fields produce forces
When two magnetic fields overlap, the fields interact. The resulting force on a magnetic object depends on the direction and strength of the fields involved.
If the fields are parallel and in the same direction, they attract each other.
If the fields are parallel but opposite, they repel each other.
If the fields are at an angle, a component of the force acts to align the objects.
3. Magnetic force on a current‑carrying conductor
The force on a straight conductor of length L carrying a current I in a magnetic field of flux density B is given by
\$\mathbf{F}=BIL\sin\theta\$
where θ is the angle between the direction of the current and the magnetic field.
4. Interaction of magnetic fields in common phenomena
4.1 Compass needle
A compass needle is a small bar magnet. The Earth’s magnetic field exerts a torque on the needle, aligning it with the field lines (north‑south direction).
4.2 Two bar magnets
When the north pole of one magnet is brought near the south pole of another, the overlapping fields attract, pulling the magnets together. Like poles (N‑N or S‑S) cause repulsion because the fields oppose each other.
4.3 Electromagnet
When a coil of wire carries a current, it creates a magnetic field around the coil. Placing a ferromagnetic core (e.g., iron) inside the coil concentrates the field, producing a strong magnet. The force on a nearby magnetic object is due to the interaction between the coil’s field and the object’s field.
Suggested diagram: Cross‑section of a solenoid with a ferromagnetic core, showing magnetic field lines inside the coil.
5. Summary of key points
Magnetic fields exist around magnets and current‑carrying conductors.
Forces arise when magnetic fields overlap; the nature of the force depends on the relative directions of the fields.
The magnetic force on a conductor is given by \$F = BIL\sin\theta\$.
Every everyday magnetic phenomenon (compass, attraction/repulsion of magnets, operation of an electromagnet) is an example of magnetic‑field interaction.
6. Key equations
Quantity
Symbol
Equation
Units
Magnetic flux density
\$B\$
–
tesla (T)
Current
\$I\$
–
ampere (A)
Length of conductor in field
\$L\$
–
metre (m)
Magnetic force on conductor
\$F\$
\$F = BIL\sin\theta\$
newton (N)
7. Practice questions
A straight wire 0.20 m long carries a current of 3 A and is placed perpendicular to a uniform magnetic field of 0.05 T. Calculate the magnitude of the magnetic force on the wire.
Two identical bar magnets are placed with their north poles facing each other at a distance of 2 cm. Describe the direction of the magnetic force and explain why it occurs.
Explain how a compass works in terms of magnetic‑field interaction.
An electromagnet consists of a coil of 200 turns wrapped around an iron core. If a current of 2 A flows through the coil, what happens to the magnetic field inside the core compared with the field when the coil is empty? (No calculation required – give a qualitative answer.)