Unit 3- Electricity  Ordinary matter is made up of atoms which have protons and electrons.  The magnitude of the charge on one proton or electron is called the elementary charge "e".  Charge is quantized as a multiple of the electron or proton charge:  proton charge: e=1.602 x Coulombs electron charge: e= x Coulombs

 One coulomb (C) of charge represents an excess or deficit of 6.24 x electrons.  Charge (Q) on an object is equal to the number of elementary charges on the object (N) multiplied by the elementary charge (e).  Q = Ne

Example 1:  How much charge does the capacitor hold if it has 4.5 billion elementary charges?

Solution:  Q= ? N = 4.5 x 10 9 elementary charges e = x Coulombs  Q = Ne Q = 4.5 x 10 9 x x Q = x

Example 2:  If an object has 4 coulombs of charge stored, how many elementary charges are present?

Solution:  N = ? Q = 4 Coulombs e = x Coulombs  N = Q/e  N = 4 / x [Note: you can remove the - from the charge] N = 2.5 x electrons

Example 3  How many electrons must be removed from an charged pair of wool socks with a -1µC charge to give it a charge of +2.3 µC?

Solution:  Qi= -1µC (Excess electrons) Qf= +2.3µC (Deficit electrons) N = ? e = x Coulombs  Qt = Qf – Qi; Qt = +2.3µC - (-1µC) Qt = +2.3µC + 1µC Qt = +3.3µC  N = Q/e  N = 3.3 x / x N = 2.05 x electrons

Fundamental Law of Electric Charge: 1. Opposite electric charges attract each other. 2. Similar electric charges repel each other. 3. Charged object attract some neutral object.  Clip at ysics30/elect/lessoniii_2_1.html ysics30/elect/lessoniii_2_1.html

Current Electricity  An electric charge in motion  Consists of a swarm of moving electrons  When electrons move from one location to another, they produce an electrical current  Electricity flows through wires as water flows through pipes

Units for current  I= Q/t  (I= current(A), Q= charge(c), t=time(s)  Measured in Amperes (A)  1 ampere of current flows when 1 coulomb of charge moves by a point in 1 second (about 6.3 billion billion electrons per second)  1A= 1C/1S

Types of currents  Direct current (DC): involves the continous flow of electrons in the SAME direction  Alternating current (AC): involves periodic reversal in the direction of electron flow

Reading current  Current is read using an ammeter, it must be hooked directly into the current to make measurements

Example  Find the amount of current flowing through an electric meter if it takes 900 C of charge to toast 2 slices of bread in 1.5 mins?

Solution  I=Q/t  I= 900C/ 90s  I=10A

Assignment:  current and potential difference questions