Magnetism is a fundamental force of nature produced by moving electric charges. It is responsible for the forces of attraction or repulsion between objects.
A fridge magnet sticking to your refrigerator and a compass needle pointing North.
Every magnet has two ends called poles: a North pole (N) and a South pole (S).
Bar Magnet
Another example
This is one of the most fundamental rules of magnetism:
Observe the behavior of two magnets as you change their polarity.
A magnetic field is the area of magnetic force around a magnet. It is an invisible field that can exert a force on other magnetic materials.
We visualize magnetic fields using magnetic field lines. These lines:
Watch how iron filings (particles) trace out the invisible field lines around a bar magnet.
The Earth has its own magnetic field, generated by molten iron in its core. This field protects us from harmful solar radiation, creating a protective "shield" called the magnetosphere. Interact with the 3D model below.
Interactive 3D Model
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Magnetism is fundamentally caused by moving electric charges. In magnetic materials, groups of atoms form magnetic domains that act like tiny magnets.
Domains are randomly oriented.
Domains are aligned, creating a strong field.
These are materials that can be strongly magnetized. They have a high concentration of aligned magnetic domains. Examples include:
The link between electricity and magnetism. When electric charge flows (a current), it creates a magnetic field. This is the basis of electromagnets.
In 1820, Hans Christian Ørsted discovered that a compass needle deflects when placed near a current-carrying wire, proving that electricity and magnetism are related.
A current-carrying wire produces a circular magnetic field around it. The strength decreases with distance from the wire.
To determine the direction of the magnetic field around a wire, imagine grasping the wire with your right hand:
For a long, straight current-carrying wire, the magnetic field strength ($B$) is given by:
$$B = \frac{\mu_0 I}{2\pi r}$$
A long wire carries a current of $5 \, \text{A}$. What is the magnetic field strength at a distance of $0.1 \, \text{m}$ from the wire?
Hint: Use the formula $B = \frac{\mu_0 I}{2\pi r}$.
Using the formula:
$$B = \frac{(4\pi \times 10^{-7} \, \text{T} \cdot \text{m/A}) \times 5 \, \text{A}}{2\pi \times 0.1 \, \text{m}}$$
$$B = \frac{20\pi \times 10^{-7}}{0.2\pi} = 1.0 \times 10^{-5} \, \text{T}$$
A magnetic field exerts a force on a moving electric charge. This force is known as the Lorentz force.
The magnitude of the force ($F$) on a charge ($q$) moving with velocity ($v$) in a magnetic field ($B$) is:
$$F = |q|vB \sin\theta$$
To determine the direction of the magnetic force on a positive charge:
A positive charge entering a uniform magnetic field will experience a force causing it to move in a circular path. Click to reset.
A proton (a positively charged particle) is moving from left to right. It enters a uniform magnetic field that is directed out of the page. What is the initial direction of the magnetic force on the proton?
Hint: Apply the Right-Hand Rule.
Using the Right-Hand Rule:
The initial magnetic force on the proton is upwards, perpendicular to both its velocity and the magnetic field.
A changing magnetic field through a coil of wire induces an electric current. This is the principle behind Faraday's Law of Induction and the operation of electric generators.
Move the magnet in and out of the coil to see the induced current light up the bulb.
These four equations are the foundation of classical electromagnetism, uniting electricity, magnetism, and optics. They describe how electric and magnetic fields are generated by charges, currents, and by each other.
1. Gauss's Law for Electricity:
$$\nabla \cdot \mathbf{E} = \frac{\rho}{\varepsilon_0}$$
This equation states that electric fields originate from electric charges. The divergence of the electric field ($\nabla \cdot \mathbf{E}$) is proportional to the electric charge density ($\rho$).
2. Gauss's Law for Magnetism:
$$\nabla \cdot \mathbf{B} = 0$$
This states that there are no magnetic monopoles. The magnetic field lines always form closed loops, and the net magnetic flux through any closed surface is zero.
3. Faraday's Law of Induction:
$$\nabla \times \mathbf{E} = - \frac{\partial \mathbf{B}}{\partial t}$$
A changing magnetic field ($\frac{\partial \mathbf{B}}{\partial t}$) creates a circulating electric field ($\nabla \times \mathbf{E}$). This is the principle of induced current.
4. Ampere-Maxwell's Law:
$$\nabla \times \mathbf{B} = \mu_0 \left( \mathbf{J} + \varepsilon_0 \frac{\partial \mathbf{E}}{\partial t} \right)$$
A magnetic field ($\nabla \times \mathbf{B}$) can be generated by an electric current density ($\mathbf{J}$) or by a changing electric field ($\frac{\partial \mathbf{E}}{\partial t}$). The latter term was Maxwell's crucial addition.