SONNY P. DE LEONMT 1

Electricity and Magnetism

Electric Current and Magnetism

He's not as famous as Darwin or Newton, but if you've ever used a modern gadget, chances are you have this 19th-century Danish physicist to thank.

Oersted’s Experiment

successfully connected electricity and magnetism

discovered that in addition to the electric field, a charge also has a magnetic field around it when it moves.

when current is present, the needle deflects perpendicular to the wire

when the current is reversed, the needle deflects in the opposite direction

Sources of Magnetic Field (B)

Magnetic fields exist around current-carrying wires, permanent magnets, and moving charged particles.

Oersted’s Principle

“A charge moving through a straight conductor produces a circular magnetic field around the conductor”

The Right Hand Rule

is used to determine the direction of the magnetic field:— If you hold a straight conductor in your right hand with your

right thumb pointing in the direction of the conventional current, your curled fingers will point in the direction of the magnetic field lines.

Note: If you use the electron flow model instead, you must use your left hand

Representing Currents and Magnetic Fields a cross-section of the wire is often shown

the current can either go into the page (x) or out of the page (.)

this model is based on an arrow

the magnetic fields get farther apart as you move away from the wire to indicate that it is getting weaker

Implications

led to new technologies like motors and generators

demonstrated using electricity to produce magnetism

can produce magnetic fields with properties that can be controlled:— can turn the magnetism on and off— can change the direction of the magnetic field— can change the strength of the magnetic field

Right-hand Rule It is used to determine the direction of magnetic field.

(1) Current Carrying Wire loop

The magnitude or strength of magnetic field B at a given point is

B = μ0 I / 2πr

B = μ0 I / 2 π r (current carrying wire)

Where μ0 is the permeability of free space, I is the current, and r is the distance from the center of the wire to the point of interest.

The SI unit for magnetic field is tesla ( T), named after Nicola Tesla, who invented the modern radio among other things.

The permeability of free space μ0 is 4 π x 10-7 tesla

Magnetic Field Through a Wire loop

(2) Wire loop

A current-carrying wire is more useful as magnet when it is wound in loop or many turns.

The magnitude of magnetic field at the center of a wire loop is B = μ0 N I / 2r

Right -hand rule (Coil)

Grasp the coil so that your curled fingers is the direction of the current and the thumb is the direction of the magnetic field.

A current carrying wire is more useful as a magnet when it is wound in a loop or many turns. The result is that the magnetic field at the center of the loop adds up, intensifying the field there.

Solenoid

When many turns are made such that the length of the coil is greater than the radius of each turn, the coil is solenoid.

The magnitude of magnetic field at the center of the solenoid at a given point is, B = μ0 N I / L, where L stands for the length of the solenoid.

Right - hand rule

Grasp the solenoid so that your curled fingers is the direction of the current and the thumb is the direction of the magnetic field.

Solenoid are described in terms of the ratio n, the number of turns per unit length.

The magnetic field at the center of the solenoid can be computed as B = μ0 N I / L

SolenoidThe magnetism of a solenoid may be turned on or off and the

magnetic field may be increased through the following ways:

Increasing the current

Increasing the number of turns or windings

Making the winding close and tight

Inserting soft iron core inside the solenoid.

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.

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:

1. Many wires are bundled together, allowing the same current to create many times the magnetic field of a single wire.

2. Bundled wires are made into coils which concentrate the magnetic field in their center.

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.

The true nature of magnetism

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

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.

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.

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.

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.

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

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 = 2x10-7 NI r

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.

Electromagnets and the Electric Motor

Key Question:

How does a motor work?

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.

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.

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.

The Principle of the Electric Motor

Commutation The process of reversing the current in the

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

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.

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.

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.

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

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.

Induction and the Electric Generator

Key Question:

How does a generator produce electricity?

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.

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.

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.

Magnetic flux

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

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

Generators A generator is a device that uses induction to

convert mechanical energy into electrical energy.

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.

Transformers A relationship between voltages and turns for

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

The induced emf in the primary and secondary coil is given by

Ve P = -Np Δ(BA) / Δt ( primary coil)

Ve S = -Ns Δ(BA) / Δt ( secondary coil)

Transformer efficiency is the ratio of the power output to the power input.

E = power output / power input

= Ve S is / Ve P i p

Step-up transformer – transformer that produces a larger output voltage.

Step-down transformer - gives a lower output voltage.

Solve:1. A friend being back from Europe a device that she

claims to be the world’s greatest coffee maker.Unfortunately, it was designed to to operate from 240 V line to obtain the 960 W of power that it needs.

a. What can she do to operate it at 120-V?

b. What current will the coffee maker draw from 120-V line?

2. An ac generator that delivers 20 A at 6000 V is connected to a step-up transformer . What is the output current at 120 000 V if the trnsformer efficiency is 100%?

3. A step-up transformer has 400 secondary turns and only 100 primary turns. A 120V alternating voltage is connected to primary coil.What is the output voltage?

Application: Trains that Float by Magnetic Levitation