Unit 2 - Electricity

Electric Charge

Property of matter associated with electric force

Coulumb’s Law

Attractive force between two charged, static particles in a vacuum

Field Forces

Long range electric force that act through space without physical contact

Field Model

The field is the agent that exerts a force on object

As field decreases

Distance increases

Positively Charged fields

Points outwards

Negatively Charged Fields

Points inwards

If q is positive

Force is with F_e

If q is negative

Force is against F_e

Electric Dipole

System of two equal and opposite charges that are close, separated at distance d (u=qd)

Field Lines

Continuous lines that depict dipole field vectors

Rules for Field Lines

1. Number of lines proportional to magnitude of electric field

2. Points in direction of electrical field vector at every point

3. Arrows go from positive to negative

4. Field lines are closer together when stronger

5. Lines DO NOT cross

Electric Field of a Charge Particle is

Independent of position and its magnitude proportional to surface charge density

Conductors only have

1 field from positive to negative

Plates are assumed

To be infinite because the edges aren’t constant

Capacitor

Stores electrical energy by building up charge on each plate, creating a positive and negative plate

Work happens when

Force is in the same direction as motion

Conservative Force

Work of force is independent of the path

Potential Energy is

Relative to something (W_conservative= -U)

If Potential energy is negative

Energy is needed to bring it to the reference point

If potential energy is positive

No energy is needed to get to reference point

As 2 same charged points are brought together,

Energy is needed to keep them there

As 2 opposite charged points are brought together

Energy is needed to keep them apart

What’s the Total Charge on an Electric Dipole?

0C

Work Equation

F_ed = qEd

What way does a dipole moment point?

Negative to positive

Electric Potential

Property of space and a function of the position of the charge

Electric Potential at a point represents

Negative work by Fe

Equipotential Surface

All points have the same potential, doesn’t need electric work to move between points and always perpendicular to electric field

Electric Field Points in Direction of 

Decreasing Potential

A positive charges slow down when it moves through

a positive V (Ek to PE)

A positive charge speeds up when it moves through

a negative V (PE to Ek)

Change in KE causes

Opposite change in U

Capacitor

Device storing electrical PE by separating negative and positive charge 

Capacitance

Ability of object to store charge, defined by area and distance

Dielectrics

Insulator placed between capacitor to increase capacitance

Capacitor Energy equal to

Amount of work to separate the charge

Magnets

Material that makes magnetic field with north and south poles forming magnetic dipole

Permanent Magnet

Stays magnetized for a long time without ant help

The south pole of the Earth faces

North Pole

The north pole of the Earth faces

South pole

Ampere’s Law

Magnetic field of a straight conductor at a point if proportional to the current and inversely to distance from conductor 

When 2 wires have current in the same direction

They will be attracted to each other

When 2 wires have current in the opposite direction

They will be repulsed by each other

Charges will only feel Fb if

Both charges are moving

North in loops of currents is

Where field emerges 

South in loop of currents is

Where field re-enters

Solenoid

Long coiled wires to create helix with strong uniform magnetic field inside it

Cyclotron Motion

UCM from a charge entering a magnetic field with a velocity and Fb that acts as a_c

Cyclotron Frequency and Period doesn’t depend on 

Charge velocity

Aston’s Mass Spectrometer

Consists of ion source, velocity selector and mass selector

Ion Source

Hot filament ionizes molecules

Velocity Selector

Sorts molecules by velocity so that all particles that reach the mass selector has the same velocity

If molecule is too fast

Fb becomes stronger and goes up

If molecule is too slow

Fe becomes stronger and goes down

Mass Selector

Uses another magnetic field to create cyclotron motion to curve the and separate ions according to m:z ratio