Module 4.1

CHARGE

Flow of charged particles produces a massive electric current

Electric Charge

  • Physical property

  • Some particles are charged (protons, electrons)

  • Some particles aren’t charged (neutrons) → called neutron

  • Positive charge and negative charge

  • Like charges repel

  • Opposite charges attract

Measuring charge

ΔQ= IΔt

ΔQ= change in charge (C)

I= Current (A)

Δt= Change in time (s)

DERIVED UNIT OF ELECTRIC CHARGE: Coulomb, C

One coulomb is the amount of charge which flows past a point when a current of 1 Amp flows for one second

DEFINING ELECTRIC CURRENT

Definition

Current is the rate of flow of charge

Calculate electric current with

I= ΔQ/Δt

CHARGE CARRIER

is any particle that has an electric charge

Elementary/ fundamental charge

The value e

e= 1.6x10^-19C

Magnitude of charge on an electron: -1e

Magnitude of charge on a proton: +1e

NET CHARGE

Charge on most objects come from either a gain or loss of electrons

The charge on an object is quantised

This means the charged value of an object is ALWAYS a multiple of e

Q= +-ne

MODELLING ELECTRIC CURRENTS IN METALS

To help model electric current in metals@

  • Model a system of fixed metal ions

  • Surrounded by free electrons

Ions don’t move but vibrate→ electrons move

How to make electrons move

  • Make one end of the wire positive and the other negative:

The electrons in the metal wire will be attracted towards the positive end so will move through the wire as an electrical current

MEASURING ELECTRIC CURRENT

An ammeter is used to measure electrical current at any point in a circuit

Ideal value of the resistance of an ammeter

  • Low resistance (0 if possible)

Because high resistance decreases the current

CONVENTIONAL CURRENT AND ELECTRON FLOW

Conventional Current

The direction of a +ve charge flows and goes from the +ve to the -ve terminal

Electron flow

Direction a -ve charge flows and goes from the -ve to the +ve terminal

ELECTRIC CURRENT IN ELECTROLYTES

  • Several liquids are able to conduct an electrical current

  • These are called electrolytes

  • The liquids are commonly ionic solutions

In these cases electric current is a flow of ions not electrons

Movement of ions in electrolytes

When electrodes are placed in electrolytes:

Cations go to the cathode

Anions go to the anode

KIRCHHOFF’S FIRST LAW

  • Is a statement of conservation of charge

  • States that the sum of two currents entering a junction must be equal to the sum of currents leaving the junction

MOVEMENT OF CHARGE CARRIERS

  • Most charge carriers (electrons included) move slowly

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

These repeated random collisions between the positive ions and free electrons means the electrons don’t move in a straight line but randomly

I=Anev

I= Current (A)

A= cross sectional area (m^2)

n= number density (m^-3)

e= elementary charge (C)

v= mean drift velocity (ms^-1)

n in I=Anev

Number density= the number of charge carriers per cubic metre of material

Mean drift velocity

The average velocity of a charged particle along a conductor

NUMBER DENSITY AND CONDUCTORS

The higher the number density, the greater the number of charge carriers per m^3, so better the electrical conductors

Conductors

Have a high number density (~10^28m^-2)

Insulators

Have a very low number density (near 0)

Semiconductors

Have a number density between the two (~10^17m^-3)