Current makes a magnet

The idea

When electric current flows through a conductor such as a wire, it produces a magnetic field around it; the field disappears when the current stops. This is the magnetic effect of electric current.

A surprise in 1820

Hans Christian Oersted was giving a demonstration. Whenever he closed or opened an electrical circuit, a compass needle lying near it deflected.

What do you think made the needle move?

• The electric current in the wire
• The table shaking
• The light in the room

Close and open the circuit

Switch the current ON, then OFF. Watch the compass needle each time.

The needle follows the current

When the current flowed, the needle deflected. When it stopped, the needle went back. So the current itself makes the magnetic field.

When electric current flows through a conductor such as a wire, it produces a magnetic field around it. The field disappears when the current stops. This is the magnetic effect of electric current.

• Current | Compass needle
• ON | Deflects
• OFF | Returns to its original direction

Notes

When electric current flows through a conductor it produces a magnetic field around it. The field disappears when the current stops. This is the magnetic effect of electric current.

Check yourself

A current flows through a wire. The compass needle near it …

The current is switched OFF. The compass needle …

What is the effect called in which a current produces a magnetic field?

What does a wire need for it to have a magnetic effect?

• does not move at all. A deflection is exactly what shows the magnetic effect.
• moves only after the current stops. The needle deflects while the current flows.
• deflects — correct. Yes!
• returns to its original direction — correct. Yes!
• stays deflected. The magnetic field disappears when the current stops, so the needle returns.
• points the opposite way. It goes back to its original direction.
• The heating effect of electric current. That is the effect in which a wire gets hot.
• The magnetic effect of electric current — correct. Yes!
• The effect of cold. There is no such effect in this chapter.
• To be made of iron. A conductor like a wire with current flowing has the effect.
• To be hot. Heat is not needed. The current itself produces the field.
• Electric current flowing through it — correct. Yes!