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 10-19 Coulombs electron charge: e=-1.602 x 10-19

Coulombs

One coulomb (C) of charge represents an excess or deficit of 6.24 x 1018 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 109 elementary chargese = -1.602 x 10-19 Coulombs

Q = NeQ = 4.5 x 109 x -1.602 x 10-19

Q = -7.209 x 10-10

Example 2:

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

Solution:

N = ?Q = 4 Coulombse = -1.602 x 10-19 Coulombs

N = Q/e N = 4 / 1.602 x 10-19 [Note: you can

remove the - from the charge]N = 2.5 x 1019 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 = -1.602 x 10-19 Coulombs

Qt = Qf – Qi; Qt = +2.3µC - (-1µC)Qt = +2.3µC + 1µCQt = +3.3µC

N = Q/e N = 3.3 x 10-4 / -1.602 x 10-19

N = 2.05 x 1015 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

http://www.saskschools.ca/curr_content/physics30/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