Magnetism Section 2

Section 2: Magnetism from Electric Currents

Preview• Key Ideas • Bellringer • Electromagnetism • Electromagnetic Devices

Magnetism Section 2

Key Ideas

〉What happens to a compass near a wire that is carrying a current?

〉Why are electric motors useful?

Magnetism Section 2

Bellringer

The temporary magnetic field created by current flowing

through a wire is used in many small appliances.

1. List as many things as possible that contain a small electric motor.

2. What keeps the coil in an electric motor spinning?

3. A sewing machine needle can be magnetized by gently stroking it with a magnet in one direction. What is a possible explanation for this?

Magnetism Section 2

Electromagnetism

〉What happens to a compass near a wire that is carrying a current?

〉When the wire carries a strong, steady current, all of the compass needles move to align with the magnetic field created by the electric current.

• Hans Christian Oersted found that magnetism is produced by moving electric charges.

– Electric currents produce magnetic fields.

Magnetism Section 2

Electromagnetism, continued

• Use the right-hand rule to find the direction of the magnetic field produced by a current.

• right-hand rule:

– If you hold a wire in your right hand and point your thumb in the direction of the positive current, the direction that your fingers curl is the direction of the magnetic field.

Magnetism Section 2

The Right-Hand Rule

• The thumb points in the direction of the current.

• The fingertips point in the direction of the magnetic field.

• The magnetic field runs counterclockwise.

Magnetism Section 2

Magnetic Field of a Current-Carrying Wire

Magnetism Section 2

Electromagnetism, continued

• Solenoids and bar magnets have similar magnetic fields.

• solenoid: a coil of wire with an electric current in it

– In a solenoid, the magnetic field of each loop of wire adds to the strength of the magnetic field of any neighboring loops.

• The strength of a solenoid can be increased.

– More loops or more current can create a stronger magnetic field.

• electromagnet: a coil that has a soft iron core and that acts as a magnet when an electric current is in the coil

– The magnetic field of the rod adds to the coil’s field.

Magnetism Section 2

Visual Concept: Solenoid

Magnetism Section 2

Electromagnetism, continued

• Moving charges cause magnetism.

– Negatively charged electrons moving around the nuclei of atoms make magnetic fields.

– Atomic nuclei also have magnetic fields because protons move within the nucleus.

– Each electron has a property called electron spin, which also produces a tiny magnetic field.

• The magnetism of the uncanceled fields in certain materials combines to make the materials magnetic overall.

Magnetism Section 2

Electromagnetic Devices

〉Why are electric motors useful?

〉A motor can perform mechanical work when it is attached to an external device.

• electric motor: a device that converts electrical energy to mechanical energy

• Galvanometers detect current.

– galvanometer: an instrument that detects, measures, and determines the direction of a small electric current

Magnetism Section 2

Visual Concept: Galvanometer

Magnetism Section 2

Electromagnetic Devices, continued

• ammeter: measures current• voltmeter: measures voltage.

• Motors use a commutator to spin in one direction.– commutator: a device used to make the current

change direction every time the flat coil makes a half revolution.

– brushes: devices that connect the wires to the commutator

Magnetism Section 2

Electric Motor