Electromagnetism
The discovery of a magnetic effect around an electric current was made accidentally by Hans Christian Oersted in 1820. He noticed that a wire carrying a current was able to deflect a nearby compass needle.
Magnetic Effect of a Current
We can use iron filings or a small plotting compass to investigate the pattern of the magnetic field produced by a current flowing through different conductors such as a straight wire, a flat coil or a solenoid.
Magnetic Field Around a Long Straight Wire
The wire is placed in a vertical position and the card is placed horizontally (Figure 21.1). When the magnetic field lines are joined on the card, they form concentric circles around the wire, the circles nearer the wire being closer to one another. This shows that the magnetic field is stronger in the region around the wire. The further away from the wire, the weaker is the magnetic field. Hence we find that the circles are further apart.
When the magnitude of the current flowing through the wire is increased, more magnetic field lines will be formed, indicating a stronger field.
The plotting compass will show us the direction of the magnetic field.
When the current is reversed, the direction of the magnetic field will also be reversed.
Current going into and out of the page
Figure 21.2 shows the magnetic field as it would appear when you look down on the page. Imagine the current direction as an arrow. When the arrow moves away from you into the page, you see the tail of the arrow - the cross. When the arrow moves towards you out of the page, you see the tip of the arrow - the dot.
