In 1820, Ørsted accidentally discovered that a magnetic field forms around an electric current. This marked the beginning of the deep relationship between electricity and magnetism.
Basic Knowledge
When current flows through a wire, a concentric magnetic field forms around it. This is called the magnetic effect of current.
The direction of the magnetic field can be remembered using the right-hand screw rule: “When a screw is advanced in the direction of the current, the direction in which the screw turns is the direction of the magnetic field.”
Another method is to grip the wire with your right hand: remember that the thumb points in the direction of the current, and the other fingers point in the direction of the magnetic field.
When a wire is wound into a coil, a strong magnetic field like that of a bar magnet forms inside the coil. This is called an electromagnet.
Ways to strengthen an electromagnet:
① Increase the current
② Increase the number of turns in the coil
③ Insert an iron core
Magnetic effect of current (the phenomenon in which a magnetic field forms around a current. Discovered by Ørsted)
Right-hand screw rule (a rule relating the directions of current and magnetic field)
Electromagnet (a device that becomes a magnet when current flows. Coil + iron core)
Solenoid (a long, tightly wound coil. The inside has a uniform magnetic field)
Iron core (iron placed in the center of an electromagnet. Strengthens the magnetic field)
Ørsted (a Danish scientist who discovered the magnetic effect of current in 1820)
Further Exploration (Background and Significance)
Ørsted’s discovery was a turning point in the history of science. Until then, electricity and magnetism had been considered separate phenomena. After this discovery, electromagnetism rapidly developed as a unified field of study. In 1864, Maxwell predicted the existence of electromagnetic waves, which became the theoretical foundation of modern radio communication (radio, television, and smartphones).
The advantage of an electromagnet over a permanent magnet is that its magnetic force can be turned ON/OFF with a switch. Applications include:
・Cranes: lift scrap iron and drop it at the destination by turning off the power
・Bells and buzzers: an electromagnet attracts an iron piece to produce sound
・Relays: use a small current to operate a switch for a large current
・MRI: create a strong magnetic field with a superconducting electromagnet
・Linear motor trains: levitate and propel trains with powerful electromagnets
The right-hand rule is easy to confuse when remembering left, right, and directions, so drawing a diagram is an effective way to learn it accurately.
- A concentric magnetic field forms around a current (Ørsted, 1820)
- Use the right-hand screw rule to determine the direction of the magnetic field
- Coil + current = electromagnet
- Strengthen an electromagnet: increase current, add turns, insert an iron core
- An electromagnet can be turned ON/OFF with a switch
- Applications: cranes, bells, relays, MRI, linear motors
- Maxwell theoretically predicted electromagnetic waves (1864)
Points to Note (Easily Confused)
① The right-hand screw rule describes the relationship between current and magnetic field. If confused, think while gripping with your right hand. ② An electromagnet is a magnet only while current is flowing. Its magnetic force disappears when the power is turned off, which is why it is used in cranes. ③ An iron core is not essential, but inserting one can strengthen the magnetic force several hundred times. ④ Magnetic field strength and magnetic flux density are strictly different concepts, but at the middle-school level they are treated together as “magnetic field strength.”
Practice
- What rule determines the direction of the magnetic field around a current?
- Give one way to strengthen an electromagnet.
- What advantage does an electromagnet have that a permanent magnet does not?