Post on 15-Aug-2015
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