ELECTROMAGNETS

WHAT IS AN ELECTROMAGNET? An electromagnet is a magnet that runs on electricity. Unlike a permanent magnet, the strength of an electromagnet can easily be changed by changing the amount of electric current that flows through it. The poles of an electromagnet can even be reversed by reversing the flow of electricity.

Electrical Quantity

Symbol Used

DescriptionUnit Used to Express Quantity

Device Used to Measure Quantity

Electric Current I

The movement of charged particles in a specific direction

Ampere (A)

A= C s

ammeter

Voltage V

The electric pressure that

causes current to flow

Volt (v)V= J C

voltmeter

Resistance R

The opposition a material offers to

currentOhm () ohmmete

r

CHANGING THE AMOUNT OF ELECTRIC CURRENT 1. INCREASING VOLTAGE

2. DECREASING RESISTANCE

FACTORS THAT AFFECT RESISTANCE

There are three external factors that influence the resistance in a conductor. Thickness (cross sectional area of the wire), length, and temperature all have some effect on the amount of resistance created in a conductor. The fourth factor is the conductivity of the material we are using. Some metals are just more electrically conductive than others. This however, is considered an internal factor rather than an external one.

CROSS SECTIONAL AREA

The cross-sectional area of a conductor (thickness) is similar to the cross section of a hallway. If the hall is very wide, it will allow a high current through it, while a narrow hall would be difficult to get through due to it's Cross Sectional Area (8k)restriction to a high rate of flow. The animation at the left demonstrates the comparison between a wire with a small cross sectional area (A) and a larger one (A). Notice that the electrons seem to be moving at the same speed in each one but there are many more electrons in the larger wire. This results in a larger current which leads us to say that the resistance is less in a wire with a larger cross sectional area.

LENGTH OF THE CONDUCTOR

Longer wire greater resistance

Shorter wire lesser resistance

TEMPERATURE Lower Temperature - Lesser resistance

Higher Temperature- higher resistance

23.1 Electric Current and Magnetism

Key Question:Can electric current create a magnet?

*Students read Section 23.1 AFTER Investigation 23.1

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.

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.

If a wire carrying an electric current is formed into a series of loops, the magnetic field can be concentrated within the loops. The magnetic field can be strengthened even more by wrapping the wire around a core. The atoms of certain materials, such as iron, nickel and cobalt, each behave like tiny magnets. Normally, the atoms in something like a lump of iron point in random directions and the individual magnetic fields tend to cancel each other out.

However, the magnetic field produced by the wire wrapped around the core can force some of the atoms within the core to point in one direction. All of their little magnetic fields add together, creating a stronger magnetic field.As the current flowing around the core increases, the number of aligned atoms increases and the stronger the magnetic field becomes.

At least, up to a point. Sooner or later, all of the atoms that can be aligned will be aligned. At this point, the magnet is said to be saturated and increasing the electric current flowing around the core no longer affects the magnetization of the core itself.

FACTORS AFFECTING THE STRENGTH OF ELECTROMAGNETS 1. Number of turns in the coil

-as the number of turns in the coil increases, magnetic field also increases

2. Size of the soft iron core- as the size of the soft-iron core

increases, magnetic field also increases

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.

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 Electric Current and Magnetism The electrons moving

around the nucleus carry electric charge.

Moving charge makes electric current so the electrons around the nucleus create currents within an atom.

These currents create the magnetic fields that determine the magnetic properties of atoms.

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 force on a moving charge A charge moving perpendicular to a

magnetic field moves in a circular orbit.

A charge moving at an angle to a magnetic field moves in a spiral.

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.

Magnetic field (T) Radius (m)

Current (amps)

B = 2x10-7 I r

23.1 Magnetic fields in a coil The magnetic field at the center of a coil

comes from the whole circumference of the coil.

Magneticfield (T)

Radiusof coil (m)

Current (amps)

No. of turns of

wireB = 2 p x10-7 NI r

23.1 Calculate magnetic field

A current of 2 amps flows in a coil made from 400 turns of very thin wire.

The radius of the coil is 1 cm.

Calculate the strength of magnetic field (in tesla) at the center of the coil.

23.2 Electromagnets and the Electric Motor

Key Question:How does a motor

work?

*Students read Section 23.2 AFTER Investigation 23.2

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.

23.2 The principle of the electric motor

An electric motor uses electromagnets to convert electrical energy into mechanical energy.

The disk is called the rotor because it can rotate.

The disk will keep spinning as long as the external magnet is reversed every time the next magnet in the disk passes by.

One or more stationary magnets reverse their poles to push and pull on a rotating assembly of magnets.

23.2 The principle of the electric motor

23.2 Commutation The process of reversing the current in the

electromagnet is called commutation and the switch that makes it happen is called a commutator.

23.2 Electric Motors

Electric motors are very common.

All types of electric motors have three key components:

1. A rotating element (rotor) with magnets.2. A stationary magnet that surrounds the

rotor.3. A commutator that switches the

electromagnets from north to south at the right place to keep the rotor spinning.

23.2 Electric Motors

If you take apart an electric motor that runs on batteries, the same three mechanisms are there; the difference is in the arrangement of the electromagnets and permanent magnets.

23.2 Electric motors

The rotating part of the motor, including the electromagnets, is called the armature.

This diagram shows a small battery-powered electric motor and what it looks like inside with one end of the motor case removed.

23.2 Electric motors

The permanent magnets are on the outside, and they stay fixed in place.

The wires from each of the three coils are attached to three metal plates (commutator) at the end of the armature.

commutator

23.2 Electric Motors As the rotor spins, the three plates come

into contact with the positive and negative brushes.

Electric current flows through the brushes into the coils.

23.3 Induction and the Electric Generator

Key Question:How does a generator

produce electricity?

*Students read Section 23.3 AFTER Investigation 23.3

23.3 Induction and the Electric Generator

If you move a magnet near a coil of wire, a current will be produced.

This process is called electromagnetic induction, because a moving magnet induces electric current to flow.

Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

23.3 Induction

Current is only produced if the magnet is moving because a changing magnetic field is what creates current.

If the magnetic field does not change, such as when the magnet is stationary, the current is zero.

23.3 Induction If the magnetic field is increasing, the

induced current is in one direction.

If the field is decreasing, the induced current is in the opposite direction.

23.3 Magnetic flux

A moving magnet induces current in a coil only if the magnetic field of the magnet passes through the coil.

23.3 Faraday's Law

Faraday’s law says the current in a coil is proportional to the rate at which the magnetic field passing through the coil (the flux) changes.

23.3 Faraday's Law

23.3 Generators A generator is a device that uses induction

to convert mechanical energy into electrical energy.

23.3 Transformers Transformers are

extremely useful because they efficiently change voltage and current, while providing the same total power.

The transformer uses electromagnetic induction, similar to a generator.

23.3 Transformers A relationship between voltages and turns for

a transformer results because the two coils have a different number of turns.

Application: Trains that Float by Magnetic Levitation