Electromagnetism

Hans Christian Oersted (1777-1851)

• Discovered that moving electric charges (current) induces a magnetic field perpendicular to the flow of current.

• When current is turned off (or charges are stationary) NO MAGNETIC FIELD EXISTS, and no force exists.

Electromagnetism ~ Magnetic Induction

• Moving electric charges create a magnetic field.

• When the direction of the current is reversed, the direction of the field is reversed.

1st Hand Rule ~ Magnetic Field Around A Current-Carrying Wire

1st Right-Hand Rule (conventional current)

1st Left-Hand Rule (electron flow)

• Used to find the direction of a magnetic field around a wire

1. Grasp wire with your hand

2. Point thumb in the direction of the current

3. The fingers curl around the wire and point in the direction of the magnetic field

YouTube - The Right Hand Rule and the Magnetic Fie with a Current Loop

1st Hand Rule ~ Magnetic Field Around A Current-Carrying Wire

Magnetic Field Near A Coil (Solenoid)

Solenoid – long coil of wire consisting of many loops

• If a wire is looped several times (solenoid) the current flowing creates a field that is the same for each loop

• The field created by a current flowing through a solenoid resembles a bar magnet

Electromagnet – magnet created by current flowing in a solenoid

» YouTube - Magnetic field in a solenoid

Magnetic Field Near A Coil (Solenoid)

2nd Hand Rule ~ Magnetic Field Around A Solenoid

2nd Right Hand Rule (conventional current)2nd Left Hand Rule (electron flow)

• Used to find the direction of a field produced by an electromagnet (solenoid)

1. Grasp coil with hand2. Curl fingers around the coil in the

direction of the current3. Your thumb points toward the N-pole of

the electromagnet

2nd Hand Rule ~ Magnetic Field Around A Solenoid

2nd Hand Rule Continued…

You can increase the strength of an electromagnet by:

1. Placing an iron core inside the solenoid

2. Increasing the current

3. Increasing the amount of loops in the coil

Forces Caused By Magnetic Fields

Michael Faraday (1791-1867) discovered that the force on a current-carrying wire is at right angles to the magnetic field and the direction of the current.

F β

I

3rd Hand Rule ~ Forces on Wires In A Magnetic Field

3rd Right Hand Rule (conventional current)

3rd Left Hand Rule (electron flow)• Used to find the direction of the force on a

current-carrying wire placed in a magnetic field.

1. Point the fingers in the direction of the magnetic field.

2. Point the thumb in the direction of the current.

3. The force exerted comes out of the palm.

3rd Hand Rule ~ Forces on Wires In A Magnetic Field

Measuring the Force on a Wire in a Magnetic Field

Symbols: Force, Fcurrent, IMagnetic Field, β

• The magnitude of the force is proportional to three factors:

1. The strength of the field, β2. The current, I3. The length of wire, l

Force = β I l

Strength of a Magnetic Field (Flux Density)

• The strength of a magnetic field is the force on a section of straight wire one meter long, carrying one Ampere of current

β = F I l

Measured in: Teslas, T1 T = N/Am

1 Tesla = 1 x 104 Gauss = Webers/m2

Force On a Single Charged Particle

• A magnetic field can exert a force on individual charges that are not confined to a wire (as long as air has been removed to avoid molecular collisions)

The force on the charged particle depends on:1. Velocity of the particle, v

2. Strength of the field, β3. Angle b/t the velocity and the field (max @ 90°)

F = q v β

Force on a Single Charged Particle

• We measure force when the charge is moving ┴ (90°) to the field.

• To determine the direction of the force, use the 3rd Rule~ replace current with velocity.

• The path is circular because force is perpendicular to velocity

Force on a Single Charged Particle

Meters and Motors

(see torque on a current-carrying loop)

Galvanometer – device used to measure very small currents.

The interaction between the field around the solenoid and the permanent field b/t the magnets creates the movement.

Galvanometer

1. A small coiled wire placed in a strong magnetic field of a permanent magnet.

2. Each loop follows the 3rd rule.

3. On one side, the force pushes down and on the other side, the force pushes up, creating torque which causes it to rotate.

The coil of wire is attached to a small spring for measuring. The forces due to current are opposed by the spring, therefore, the amount of rotation of the coil is proportional to current.

Galvanometer

Ammeter

Ammeter – a device made from a galvanometer to measure larger currents.

1. Place a resistor (with resistance smaller than the galvanometer) in parallel with the meter. (resistor = shunt)

2. Most of the current flows through the resistor (shunt) b/c current is inversely proportional to R.

Ammeter

Voltmeter

Voltmeter – a device made from a galvanometer to measure voltage.

1. Place a resistor (multiplier) in series with the meter.

2. The galvanometer measures I through the multiplier. I =V/R where V is voltage over the voltmeter and R is the effective resistance of the multiplier & galvanometer

Voltmeter

The Electric Motor

Electric Motor – converts electrical energy to mechanical energy.

A simple loop of wire will not rotate more than 180° in a magnetic field.

In order to rotate 360°, the current has to continually change direction at the vertical position.

In an electric motor, current is continually changing direction every 180° of rotation.

The Electric Motor

To achieve 180° of change:1. A commutator and brushes are used 2. The brushes are attached to a battery (+ and -)3. The commutator is a split-ring attached to a

coil.4. When the wire rotates 180°, the commutator

sides hit the brushes~ each turn being a different terminal.

5. By changing connection with the brushes, the direction of the current changes.

The Electric Motor