4.1: Charge and Current

Mean Drift Velocity in simple terms

How fast electrons are moving

Speed of most charge carriers (like electrons)

Move slowly (but in large numbers)

Why electrons don’t move in a straight line but randomly

The free electrons repeatedly collide with the positive metal ions while they drift along the wire towards the positive terminal

Mean Drift Velocity

The average velocity of a charged particle along a conductor

Equation for mean drift velocity

I=Anev

I in I=Anev

Current (A)

A in I=Anev

Cross sectional area (m²)

e in I=Anev

Elementary charge (C)

v in I=Anev

Mean drift velocity (ms^-1)

n in I=Anev

Number density (m^-3)

Number density meaning

The number of charge carriers per cubic metre of a material

Number Density

The number of charge carriers per cubic metre of material

Higher number density

Means the greater the number of charge carriers per m³

The greater the number of charge carriers per m³

Means the better the electrical conductor

Conductors

Have a high number density (~10²⁸m^-3)

Insulators

Have a very low number density (near 0)

Semi conductors

Have a number density between conductors and insulators (~10¹⁷m^-3)

Kirchhoff 1st law

Statement of conservation of charge

Kirchhoff’s first law states

The sum of currents entering a junction must be equal to the sum of the currents leaving the junction

9A in the junction

9A out

2A and 5A go into the junction

7A out

6A and 4A into the junction

10A

Electrolytes

Liquids that are able to conduct an electrical current

Type of liquids electrolytes are

Commonly ionic solutions

Electric current in electrolytes

Flow of ions

Movement of cations in electrolytes

Move to the cathode

Cation

Positive ions

Cathode

Negative electrode

Movement of anions in electrolytes

Move to the anode

Anions

Negative ions

Anode

Positive electrode

Conventional current direction

Flows from +ve terminal to the -ve terminal

Conventional current charge

+ve charge

Electron flow direction

Flows from the -ve terminal to the +ve terminal

Electron flow charge

-ve charge

How electric current is measured

An ammeter

Ammeter use

Used to measure the electrical current at any point in a circuit

Placement of ammeter

In series

Ideal resistance of an ammeter

Low (0 if possible) (High resistance decreases the current)

Electric current in metals

A system of fixed metal ions, surrounded by electrons that are free

Electrons that are free

Free electrons

How to make electrons move

Make one end of the wire positive and the other negative

Why do electrons move

Electrons in metal wire are attracted towards the positive end of the wire so will move through the wire as an electric current

Charge on most objects

Comes from a loss or gain of electrons

Charge on an object is quantised

The charged value of an object is ALWAYS a multiple of e

Equation to find number of elementary charges

±n= Q / e

Equation to find charge from number of elementary charges

Q= ±ne

A charge carrier

Any particle that has an electric charge

Charge of proton

1.6×10^-19

Charge of electron

-1.6×10^-19

Charge of Copper²⁺ ion

3.2×10^-19

Charge of Sodium⁺ ion

1.6×10^-19

Charge of Chloride^- ion

-1.6×10^-19

The value e

Elementary charge \ fundamental charge

Elementary charge

e= 1.6×10^-19 C

Magnitude of charge on proton

+1e

Magnitude of charge in electron

-1e

Current

Rate of flow of charge

Equation to calculate electric current

I= ΔQ / Δt

Step 1 on how to calculate the average current

State what you know and convert units

Step 2 on how to calculate the average current

State equation, substitution, and calculate

Produces an electric current

Flow of charged particles

Electric charge

Physical property

Protons, electrons

Examples of charged particles

Neutrons

Example of particles that aren’t charged

Two types of charges

Positive and negative

Like charges

Repel

Opposite charges

Attract

Equation for measuring charge

ΔQ= IΔt

Derived unit of electric charge

Coulomb, C

One coulomb

Amount of charge which flows past a point when a current of 1 Amp flows for 1 second