23.1 Electric Current and Magnetism In 1819, Hans Christian Oersted, a Danish physicist and chemist, and a professor, placed a compass needle near a wire through which he could make electric current flow. When the switch was closed, the compass needle moved just as if the wire were a magnet.

An apparatus can be built that shows the magnetic field around a straight wire. The compass needles all form a circle when the current is switched on in the wire.

23.1 Electric Current and Magnetism Two wires carrying electric current exert force on each other, just like two magnets. The forces can be attractive or repulsive depending on the direction of current in both wires.

The direction of the force can be deduced from the right-hand rule. If you bend the fingers of your right hand as shown, your thumb, index, and middle finger indicate the directions of the force, current and magnetic field.

23.1 Electric Current and Magnetism The magnetic field around a single wire is too small to be of much use. There are two techniques to make strong magnetic fields from current flowing in wires: Many wires are bundled together, allowing the same current to create many times the magnetic field of a single wire. Bundled wires are made into coils which concentrate the magnetic field in their center.

When wires are bundled, the total magnetic field is the sum of the fields created by the current in each individual wire. By wrapping the same wire around into a coil, current can be “reused” as many times as there are turns in the coil

23.1 Electric Current and Magnetism The most common form of electromagnetic device is a coil with many turns called a solenoid. A coil takes advantage of these two techniques (bundling wires and making bundled wires into coils) for increasing field strength.

Coils are used in electromagnets, speakers, electric motors, electric guitars, and almost every kind of electric appliance that has moving parts.

23.1 The true nature of magnetism The magnetic field of a coil is identical to the field of a disk-shaped permanent magnet.

23.1 Magnetic force on a moving charge The magnetic force on a wire is really due to force acting on moving charges in the wire. A charge moving in a magnetic field feels a force perpendicular to both the magnetic field and to the direction of motion of the charge.

23.1 Magnetic force on a moving charge A magnetic field that has a strength of 1 tesla (1 T) creates a force of 1 newton (1 N) on a charge of 1 coulomb (1 C) moving at 1 meter per second. This relationship is how the unit of magnetic field is defined.

23.1 Magnetic field near a wire The field of a straight wire is proportional to the current in the wire and inversely proportional to the radius from the wire. Current (amps) B = 2x10-7 I r Magnetic field (T) Radius (m)

23.1 Magnetic fields in a coil The magnetic field at the center of a coil comes from the whole circumference of the coil. No. of turns of wire Magnetic field (T) B = 2p x10-7 NI r Current (amps) Radius of coil (m)

23.2 Electromagnets and the Electric Motor Electromagnets are magnets that are created when electric current flows in a coil of wire. A simple electromagnet is a coil of wire wrapped around a rod of iron or steel. Because iron is magnetic, it concentrates and amplifies the magnetic field created by the current in the coil.

23.2 Electromagnets and the Electric Motor The right-hand rule: When your fingers curl in the direction of current, your thumb points toward the magnet’s north pole.