Dr Pusey

www.puseyscience.com

Syllabus points There are two types of charge that exert forces on each other Electric current is carried by discrete charge carriers; charge is

conserved at all points in an electrical circuit This includes applying the relationship

Energy is conserved in the energy transfers and transformations that occur in an electrical circuit

Note: The electrostatics in your Physics program should not contain anything to do with Coulombs law, or electric fields. It has been stripped down to fit in the Waves section.

= q

I t

Learning Goals Define:

Net charge Electric field Electrostatic induction Conductor Insulator Semiconductor Electric current Drift velocity Conventional current

Identify that electrons are mobile Identify that like charges repel and unlike charges attract Identify that a neutral object has the same number of positive and negative charges Recognise that charge cannot be created or destroyed, but it can be transferred Recognise that energy flow is only possible in a circuit if the charges are mobile Identify that current flows due to an electric field established by a potential difference

Be able to use and rearrange the equation for electric current 𝐼 =𝑞

𝑡 Recognise the direction of an electric current is the direction of the transfer of positive charges Distinguish between conventional current and electron current Give the SI unit for: electric current, time

Intro

Have you ever noticed…

◊ your hair standing up on end on dry days.

◊ you get shocked when touching a door knob, or when going down a slide.

◊ you sometimes hear a crackling noise when taking off a shirt.

◊ you get an electric shock when you touch a car after it has been travelling for a while.

All of these effects are caused by electric charges

Recap of the atom An atom consists of a nucleus containing

protons (positively charged) and neutrons (no charge).

These are not removable by usual everyday methods

Electrons are present in the region of space outside the nucleus. They are negatively charged and weakly bound to the atom.

Electrons are often removed from and added to an atom by normal everyday occurrences.

Charges

Positive charge +

Negative charge -

In atoms only the electrons (negative charges) are moving around

Neutral = no overall charge, means charges are balanced, same number of protons and electrons

Opposite charges attract, like charges repel

In a closed system, electric charge is always conserved

From: http://blog.ssis.edu.vn/103059/2013/09/17/does-the-color-of-hair-affect-how-much-static-electricity-it-can-carry/

Units SI unit for charge is the Coulomb, represented by C

Can also use elementary charge, represented by e

Elementary charge = smallest charge in nature, amount of charge on one proton, or electron

Charge on proton = +e Charge on electron = -e

1 e = 1.602 x 10-19 C

Conductors and Insulators Conductor = electrons are free to move,

charge can flow easily

Insulators = electrons can’t move around, will not allow charge to flow through them

Semiconductor = in between a conductor and insulator, can conduct electricity but not very well (Silicon!)

Electrostatic effects can cause issues in industry. Technicians often wear static-straps to ensure they don’t accidently zap a sensitive electrical component when working.

Electric force Electric forces (of attraction or repulsion) are non-

contact, objects don’t need to be touching to experience the force

Vector quantity – has a direction!

Strength of electric force depends on distance between charges

Greater the unbalanced charges the greater the force

Coulomb’s law This is not actually in your program anymore, but

whatever.

Only applies to charges at rest, ‘point’ charges (much smaller than the distance between them)

𝑭 =𝑘𝑞1𝑞2𝑟2

Electric Force (N)

Constant = 9x109

Amount of charge (C)

Distance between charges (m)

Example 1

Two electrons are placed a distance of 6 nm away from one another. What is the force between them? What direction is the force acting in?

Example 1

Two electrons are placed a distance of 6 nm away from one another. What is the force between them? What direction is the force acting in?

Electric field diagrams Also not in the program anymore, woohoo!

Electric field = the region of space around a fixed charge in which a force is exerted on another charge

Used to explain the force one charge exerts on another, the ‘field’ is not an actual ‘thing’, you can’t see or feel it

Similar to magnetic field diagrams

Current Electric current = flow of electric charge

To have a current need

an electric field – to supply energy to move the charges

charges which are free to move

Electric current is net amount of charge passing a certain point in a given time interval

𝐼 =𝑞

𝑡Electric Current (A)

Amount of charge (C)

Time (s)

Current - units Current is measured with an ammeter, it

measures how much charge is passing a certain point per second

Symbol for electric current is capital i – I

Unit for electric current is amperes (A) Cool kids just say “Amps”

1 A = 1 C/s (6.2*1018 electrons) passing a point per second

Creating a current Consider an electric field

Electric field exerts a force on charges

Charges move due to force, creating a current

Example 1

A current of 0.5 A flows for 10 minutes in an electrical conductor, calculate the number of Coloumbs of charge that pass a given point.

Example 1

A current of 0.5 A flows for 10 minutes in an electrical conductor, calculate the number of Coloumbs of charge that pass a given point.

Example 2

If 1.2x104 C of charge flows through a conductor in 1.0 hour, what is the electric current?

Example 2

If 1.2x104 C of charge flows through a conductor in 1.0 hour, what is the electric current?

Example 3

If 5.0x1018 electrons per second pass through a current-measuring device, what is the current, in amperes?

Example 3

If 5.0x1018 electrons per second pass through a current-measuring device, what is the current, in amperes?

Conventional current Conventional electric current =

flows from positive terminal to negative terminal

Conventional electric current = flow of positive charges (even though in this case PROTONS ARE NOT MOVING)

Left over convention from olden times

Electrons flow from negative to positive terminal

Conventional current Conventional current is the direction that positive

charge would flow

Conventional current = from positive to negative

Depending on the situation, positive charges, negative charges, or both may move

In metal wires, current is carried by electrons, negative charges move

In ionic solutions, such as salt water, both positive and negative charges move

In nerve cells, both positive and negative charges move

Drift velocity Good conductors = large number of free

charges In metals, free charges = free electrons Electrons move through a metal conductor

like the motion of atoms in a gas Electric field causes electrons to drift (in a

direction opposite to electric field) Drift velocity = average velocity of the free

charges, usually very small Free electrons collide with atoms in the

conductor, transferring some of their energy (come back to this idea in resistance ppt)

From: http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@8.8:154

Dc vs AC Direct current (DC) = net charge flows in one direction,

power get from batteries

Alternating current (AC) = charge flow alternates, moves back and forth, power we get from wall sockets

Look more into this later in the topic

Thinking Questions Can a wire carry a current and still be neutral – that is,

have a total charge of zero? Explain.

Why are two conducting paths from a voltage source (ie battery) to an electrical device needed to operate the device?

Why isn’t a bird sitting on a high-voltage power line electrocuted? Contrast this with a situation in which a large bird hits two wires simultaneously with its wings.

Resources pHet – DC Circuits

http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Electric current boosts memory -http://www.sciencedaily.com/releases/2014/08/140828142742.htm

Warning over electrical stimulation of brain -http://www.bbc.com/news/health-27343047