Magnetic field of a current; electromagnet and the electric bell

Oersted's experiment: a current deflects a compass

1. 1
   Stretch a copper wire just above a magnetic compass, parallel to its needle. Join the ends of the wire to a battery through a switch.
2. 2
   Close the switch. Observe: the needle turns away from its north-south line.
3. 3
   Increase the current. Observe: the needle turns further.
4. 4
   Reverse the battery connections. Observe: the needle turns the opposite way.
5. 5
   Switch off the current. Observe: the needle returns to north-south.
6. 6
   Conclude.

   A current produces a magnetic field around the wire. H. C. Oersted of Denmark first saw this in 1820. Iron filings on a card around a vertical current-carrying wire form circles round it.

Solenoid and electromagnet

Wind many turns of insulated copper wire closely on a paper tube. This long coil is a solenoid. When a current flows, the solenoid behaves like a bar magnet: one end acts as a north pole and the other as a south pole. If the current is reversed, the poles swap. When the current stops, the magnetism goes. Put a soft iron core, such as iron nails, inside the coil. The field becomes much stronger, because the tiny magnets in the iron line up with it. A solenoid with a soft iron core is an electromagnet. Its strength rises with more current, more turns and a soft iron core. Electromagnets are used in motors, generators, electric bells, MRI machines, cranes and very fast trains.

Activity: make an electromagnet

You need a thick drawing sheet, insulated copper wire, a 9 V battery, a switch, an iron scale and some iron nails.

1. 1
   Roll a thick drawing sheet into a tube about 15 cm long and 1 cm across.
2. 2
   Wind 100 to 150 turns of insulated copper wire around the tube. Leave the inside of the tube empty.
3. 3
   Join the ends of the wire, through a switch, to a 9 V battery.
4. 4
   Hold an iron scale near the tube with the switch off. Observe: no pull.
5. 5
   Switch on. Observe: the iron scale is pulled towards the tube. The coil has become a magnet.
6. 6
   Fill the tube with iron nails and switch on again. Observe: the pull is much stronger.
7. 7
   Switch off. Observe: the pull stops.

   An electromagnet is a temporary magnet: it works only while current flows.

Bar magnet vs electromagnet

How an electric bell works

1. 1
   Pressing the push button completes the circuit, so current flows through the coils of a U-shaped (horseshoe) electromagnet.
2. 2
   The soft iron core becomes a magnet and pulls the soft iron armature towards it.
3. 3
   The hammer fixed to the armature strikes the gong, and the bell rings.
4. 4
   As the armature moves, it leaves the contact screw, so the circuit breaks. The electromagnet stops being a magnet.
5. 5
   The spring pulls the armature back. It touches the contact screw again, and the circuit is complete once more.
6. 6★
   The cycle repeats many times a second, so the hammer keeps striking the gong while the button is pressed.

   To draw it: horseshoe electromagnet with coils, armature on a spring, hammer and gong, contact screw, push button and battery. Label every part.

★ key step