IB+DP+Topic+5+Electricity+and+Magnetism

5.1 Electric fields

// ** Nature of science: ** // Modelling: Electrical theory demonstrates the scientific thought involved in the development of a microscopic model (behaviour of charge carriers) from macroscopic observation. The historical development and refinement of these scientific ideas when the microscopic properties were unknown and unobservable is testament to the deep thinking shown by the scientists of the time. (1.10)

// ** Understandings: ** // • Charge • Electric field • Coulomb’s law • Electric current • Direct current (dc) • Potential difference

// ** Applications and skills: ** // • Identifying two forms of charge and the direction of the forces between them • Solving problems involving electric fields and Coulomb’s law • Calculating work done in an electric field in both joules and electronvolts • Identifying sign and nature of charge carriers in a metal • Identifying drift speed of charge carriers • Solving problems using the drift speed equation • Solving problems involving current, potential difference and charge

• Early scientists identified positive charges as the charge carriers in metals; however, the discovery of the electron led to the introduction of “conventional” current direction. Was this a suitable solution to a major shift in thinking? What role do paradigm shifts play in the progression of scientific knowledge?
 * // Theory of knowledge: //**

// ** Guidance: ** // • Students will be expected to apply Coulomb’s law for a range of permittivity values

• I = ∆q / ∆t • F = k q 1 q 2 / r 2 • k = 1 / 4πε 0 • V= W/q • E= F/q • I = nAvq
 * // Data booklet reference: //**

//** Aims **// **Aim 2**: electrical theory lies at the heart of much modern science and engineering **Aim 3**: advances in electrical theory have brought immense change to all societies **Aim 6**: experiments could include (but are not limited to): demonstrations showing the effect of an electric field (eg. using semolina) simulations involving the placement of one or more point charges and determining the resultant field **Aim 7**: use of computer simulations would enable students to measure microscopic interactions that are typically very difficult in a school laboratory situation

Electricity and magnetism is produced by actions which go on in the surrounding medium as well as in the excited bodies and endeavouring to explain the action between distant bodies without assuming the existence of forces capable of action directly. It has to do with the space in the neighbourhood of the electric and magnetic bodies.
 * Theory of the Electromagnetic field by James Clerk Maxwell (1831 - 1879):**

There are two forms of electric charge either positive or negative. Positive charge is a property of protons and negative charge is a property of electrons. The amount of electric charge is quantised (an integral multiple of a basic unit: 1.6 x 10 -19 C).
 * Electric charges**

The law of conservation of charge.
The total charge cannot be created or destroyed in any process.

Conductors vs insulators Conductors: Materials that have many free electrons. Insulators: Materials that do not have many free electrons.

**Electric field strength.** (Students should understand the concept of a test charge.)

The electric field at various positions around a positive charge. Electric field is a vector quantity. The directiton of the electric field is the same as the direction of the force experienced by a __//**positive**//__ charge at the given point.

Coulomb’s law. (Charges in the force law are point charges.) The force between two point electric charges is given by Coulomb's law and can be either attractive or repulsive. =** coulomb's law **= where k is 1 / 4πε 0, ε 0 [epsilon]: permittivity of free space, ε 0 = 8.85 x 10 -12 C 2 N -1 m −2

PRACTICE 1: Two charges feel an attractive force of 36N. What force will they feel if a) distance between is tripled? 36N x (1/3 x 3) = 4 N b) distance is doubled and one charge is tripled? 36N x (3 / 2 x 2) = 27 N c) distance and both charges are tripled? 36N X ( 3 x 3 / 3 x 3) = 36 N PRACTICE 2: Two protons are distance d away from each other. a) Calculate the electric force between them. F E = k q 1 q 2 / d 2 = ( 8.99 x 10 9 ) (1.6 x 10 -19 ) 2 / d 2 b) Calculate the gravitational force between them. F G = G m 1 m 2 / d 2 = ( 6.67 x 10 -11 ) (1.7 x 10 -27 ) 2 / d 2 c) Which force is greater between electric force and gravitational force. F E / F E = ( 8.99 x 10 9 ) (1.6 x 10 -19 ) 2 / ( 6.67 x 10 -11 ) (1.7 x 10 -27 ) 2 The magnitude of the electric force is 10 36 times greater than the magnitude of gravitational force.

Electric field strength
The electric force per unit charge experience by a samll, positive charge //**q**//: The direction of the electric field is the same as the direction of the force experienced by a positive charge at the given point. The unit of electric field is NC -1.

E = k q 1 / r 2 = q 1 / 4πε 0 r 2 = F/q [|Electric field questions] from physics-ref.blogspot.com Electric field patterns for different charge configurations. Electric field strength due to one or more point charges. (These fields are due to the following charge configurations: a point charge, a charged sphere, two point charges, and oppositely charged parallel plates. The latter includes the edge effect. Students should understand what is meant by radial field.)
 * Electric field strength formula**

e-fields plates
PRACTICE 2 : What is the magnitude of the force between a //2 nC// charge and a //-3 nC// charge separated by 3mm ? F = k q1 q2/ r^2 = (8.99 x 10^9) (2 x 10^-9) ( -3 x 10^9) / ( 3 x 10^-3)^2 = -6 x 10^-3 N = 6 mN (attractive force) EXAMPLE 1: (a) Define electric field strength. (b) Two flat parallel metal plates, each of length 12.0 cm, are separated by a distance of 1.5 cm. Space between the plates is a vacuum. The potential difference between plates is 210 V. The electric field may be assumed to be uniform in the region between the plates and zero outside this region. Calculate magnitude of the electric field strength between the plates. (c) An electron initially travels parallel to plates along a line mid-way between the plates, as shown in Fig.1. Speed of the electron is 5.0 × 107 m s–1. For the electron between the plates, (i) determine magnitude and direction of its acceleration, (ii) calculate time for the electron to travel a horizontal distance equal to the length of the plates. (d) Use answers in (c) to determine whether the electron will hit one of the plates or emerge from between the plates.

SOLUTION 1: (a) Electric field strength is defined as the force per unit positive charge (on a small test charge) (b) Electric field strength, E = (210 / { 1.5 × 10 -2 } ) = 1.4 ×10 4 N C -1

(c) (i) Electric force = Eq, Resultant force = ma , Eq = ma Acceleration, a = Eq / m = (1.4×10 4 × 1.6×10 -19 ) / (9.1×10 -31 ) = 2.5×10 15 (<= 2.46×10 15 ) m s -2 The acceleration is towards the positive plate / upwards (and normal to the plate) (ii) s = ut + ½ at 2 giving s = ut + 0 since acceleration is vertical – it has no horizontal component Time, t = s / u = (12×10 -2 / 5.0×10 7 ) = 2.4×10 -9 s (d) Compare the distance the electron would travel vertically in the amount of time calculated above. The vertical displacement after the acceleration for 2.4×10-9 s is Initial vertical velocity, u = 0. Using s = ut + ½ at 2 Displacement = 0 + [ ½ × 2.46 × 10 15 × (2.4×10 -9 ) 2 ] = 7.1 × 10 -3 m (= 0.71cm) Since the vertical displacement is less than half the separation ( 1.5 cm / 2 = 0.75 cm) of the plates. (Note that we are referring to half the separation since the electron is initially incident midway between the plates, so it only needs to move a distance of 0.75cm vertically to reach one of the plates).
 * EITHER

\ The electron will pass between the plates. (since 0.71cm < 0.75cm) || OR Compare the time the electron takes to travel halfway across (vertically) the plates to the time it takes to travel (horizontally) a distance equal to the length of the plate. If the electron has already travelled the horizontal distance of the plates before moving a distance of 0.75cm vertically, then it will not hit the plate. Displavement: 0.75 × 10 -2 = ½ × 2.46 × 10 15 × t 2 Vertical distance to travel, s = 0.75 × 10 -2 m. Consider the vertical motion: s = ut + ½ at 2 = 0 + ½ × 2.46 × 10 15 × t 2 where t is the time to travel the 0.75m distance vertically.

Time to travel ‘half-way across’ the plates, t = 2.47×10 -9 s Since the time to travel a distance equal to the length of the plates is less than the time travel ‘half-way across’ the plates, the electron will pass between the plates before hitting one of the plates.

The electron will pass between the plates. (since 2.4ns < 2.47ns) || E = F/q Energy = Fd= Eq d = qV [since V = W/q, (W = Energy)] => V = Ed,
 * Potential difference**
 * E = V/d** [ unit: v / m ]

Electric current
//I// is the current //n//(the number density) is the number of electrons per unit volume in a conductor, A is the cross sectional area of the conductor, v is the drift velocity of the charges, //q// is the charge that each electron carries ||
 * Direct current (dc):** An electric current flowing in only one direction
 * Drift velocity**
 * =//I =// //nAvq//= || //where//

5.2 – Heating effect of electric currents Essential idea: One of the earliest uses for electricity was to produce light and heat. This technology continues to have a major impact on the lives of people around the world.

// ** Nature of science: ** // Peer review: Although Ohm and Barlow published their findings on the nature of electric current around the same time, little credence was given to Ohm. Barlow’s incorrect law was not initially criticized or investigated further. This is a reflection of the nature of academia of the time, with physics in Germany being largely non-mathematical and Barlow held in high respect in England. It indicates the need for the publication and peer review of research findings in recognized scientific journals. (4.4)

// ** Understandings: ** // • Circuit diagrams • Kirchhoff’s circuit laws • Heating effect of current and its consequences • Resistance expressed as RI = V • Ohm’s law • Resistivity • Power dissipation

// ** Applications and skills: ** // • Drawing and interpreting circuit diagrams • Identifying ohmic and non-ohmic conductors through a consideration of the V/I characteristic graph • Solving problems involving potential difference, current, charge, Kirchhoff’s circuit laws, power, resistance and resistivity • Investigating combinations of resistors in parallel and series circuits • Describing ideal and non-ideal ammeters and voltmeters • Describing practical uses of potential divider circuits, including the advantages of a potential divider over a series resistor in controlling a simple circuit • Investigating one or more of the factors that affect resistance experimentally

• A set of universal symbols is needed so that physicists in different cultures can readily communicate ideas in science and engineering
 * // International-mindedness: //**

• Sense perception in early electrical investigations was key to classifying the effect of various power sources; however, this is fraught with possible irreversible consequences for the scientists involved. Can we still ethically and safely use sense perception in science research?
 * // Theory of knowledge: //**

//** Utilization: **// • Although there are nearly limitless ways that we use electrical circuits, heating and lighting are two of the most widespread • Sensitive devices can employ detectors capable of measuring small variations in potential difference and/or current, requiring carefully planned circuits andhigh precision components

// ** Guidance: ** // • The filament lamp should be described as a non-ohmic device; a metal wire at a constant temperature is an ohmic device • The use of non-ideal voltmeters is confined to voltmeters with a constant but finite resistance • The use of non-ideal ammeters is confined to ammeters with a constant but non-zero resistance • Application of Kirchhoff’s circuit laws will be limited to circuits with a maximum number of two source-carrying loops

//** Data book reference: **// • Kirchhoff’s circuit laws: • ΣV = 0 (loop) • ΣI = 0 (junction) • R = V/I • P = VI = I 2 R = V 2 / R • R total = R 1 + R 2 + ...  • 1/ R total = 1/ R 1 + 1/ R 2 + ...  • ρ = RA / L  • Refer to electrical symbols on page 4 of the physics data booklet

**Aim 2:** electrical theory and its approach to macro and micro effectscharacterizes much of the physical approach taken in the analysis of theuniverse **Aim 3:** electrical techniques, both practical and theoretical, provide arelatively simple opportunity for students to develop a feeling for thearguments of physics **Aim 6:** experiments could include (but are not limited to)- use of a hot-wireammeter as an historically important device **Aim 7:** there are many software and online options for constructing simple and complex circuits quickly to investigate the effect of using different components within a circuit
 * // Aims //**

Kirchhoff's circuit laws
• ΣV = 0 (loop) • ΣI = 0 (junction) [|Kirchoff's first and second laws] Images from www.s-cool.co.uk Example 1) Using Kirchhoff's Circuit Law, find the current flowing in the 40Ω resistor, R 3

SOLUTION: Question taken from [|electronics-tutorials] =Homework) Quizizz revision : Kirchoff's circuit laws quiz from mrwaynesclass.com= [|Kirchoff's law] Tippens Physics quiz = EXIT Quizizz revision : [|Quiz on Circuits and Kirchhoff's Laws] from www.antonine-education.co.uk= Example 2) Use Kirchoff's Laws to find the internal resistance of the cell.

SOLUTION: Question taken from [|www.s-cool.co.uk] 10 V = (0.3 x 4) + (0.3 x 3) + (0.3 x r) 10 = 1.2 + 0.9 + 0.3r 7.9 = 0.3r,
 * Energy in = Energy out, and V = IR, so** r = 26.3 Ω

// **Resistance** // The electric resistance **//R//** of a conductor is defined as the potential difference **//V//** across its ends divided by the current **//I//** passing through it. It is the __ opposition __ to the electric current. The rate of the flow of electron currents can be reduced by adding more resistance to the circuit. Resistance is measured in ohm, the unit (Ω) ohm(s) is the volt per ampere. A resistor limits the flow of electricity. The __ bigger __ the resistance, the __ smaller __ the electric current.

//**Resistivity**// The resistivity of a material is defined in terms of its resistance (R), length (l) and cross-sectional area (A).

The Greek alphabet //r // is the resistivity of the particular material the resistor is made from. The unit for resistivity is the ohm-meter (Ωm)
 * ⍴ = R A / L**

Answer to the questions below. media type="custom" key="29317377" Electricity from the Physics Classroom Watch resistance.flv
 * Resistance in a wire ** from Phet
 * 1) What variables affect the resistance in the wire?
 * 2) What must happen for the wire’s resistance to be at its greatest?
 * 3) What must happen for the wire’s resistance to be at its least?

// **Explain what Ohm's law is** and find **the properties of resistivity** using the website below; //
 * What are three variables affecting electrical resistance ** from tutorvista

**//Heating effect of current//**
Collisions of electrons with lattice atoms: The electric field accelerate the electrons and the electrons keep proviing energy to the atoms of the wire. Theatoms in the wire vibrate about their equilibrium positions with increased kinetic energy. This results in an increate in the temperature of the wire.

= **Investigation lab on Ohm's Law** =

The volt is defined as the __ energy __ transfer per __ c oulomb __ of charge as charges move between two points in a circuit (work done per unit charge). ( 1 V = 1 J C -1 )
 * //Voltage// **
 * V = I R **
 * V = ΔW / ΔQ **

Work is done when an electric charge **//q//** is moved from one point to another when there is a potential difference **//V//** between these points. Therefore, //W = qV//

** //Power// **

 * Power = work done / time taken = **//q////V/ t =// //I////V//
 * //P = I V = R I//** 2 **//= V//** 2 **/// R//**

** //Current// **
Electric current (Electricity) is the rate of the __ flow __ of invisible particles called __ electrons __. They go round a track of wire. Current is measured in __ ampere __, the unit of __ amp __ ( __ A __ ). 1 ampere of electric current is the rate of electron motion equal to 1 coulomb per second: Q = Charge in motion (coulombs) t = Time (seconds) || Coulomb: a unit of electrical charge equal to the amount of charge transferred by a current of 1 ampere in 1 second. The __ brighter __ the bulb, the __ greater __ the current flowing.
 * **I = Q/t** || Where, || I = Electric current (amperes)


 * Do you remember this!!! **
 * **Measuring**
 * Voltage and Current** || * We always place the ammeter in__ series __ with the component we are testing.
 * We always measure the Voltage in __ parallel __ with the component we are testing. ||
 * **Voltage and Current** || * In a SERIES circuit the __ voltage __ is divided but the __ current __ remains the same. This is why we measure current in series.
 * In PARALLEL circuits the __ current __ is divided but the __ voltage __ remains the same. This is why we measure voltage in parallel. ||

**// Resistors in circuits //**
Some resistors can be made to vary their resistance by tapping them at various places. These are called variable resistors and potentiometers.

Thermistors are temperature dependent resistors, changing their resistance in response to their temperature. - ( As temperature increases resistance decreases. )

Light-dependent resistors (LDRs) change their resistance in response to light intensity.- ( As brightness increases resistance decreases. )

Com plete questions on ** Combination circuits ** from www.physicsclassroom.com

**//Circult symbols//**

 * > [[image:IB circuit symbols 1.png width="764" height="534"]] ||
 * > [[image:IB circuit symbols 2.png width="759" height="211"]] ||

//**Potential divider**//
The PD of a source is divided over two resistors in series in proportion to their resistances. <span style="background-image: url(">[|Potential divider] from BBC Bitesize



V 2 out = [ R 2 / (R 1 +R 2 ) ] V
//__ Application __// Potentiometer: A variable resistor with a third adjustable terminal. The potential at the third terminal can be adjusted to give any fraction of the potential across the ends of the resistor.

EXAMPLE 1) Explain the use of sensors in potential divider circuits such as light-dependent resistors (LDRs), negative temperature coefficient (NTC) thermistors and strain gauges. ELECTRICAL SENSORS: A component whose electrical properties (usually resistance) changes with its physical conditions. THERMISTOR: A resistor whose resistance falls when its temperature rises. LIGHT DEPENDENT RESISTOR: A resistor whose resistance falls as the intensity of light falling on it increases. STRAIN GAUGE: A long thin wire whose resistance increases when it is stretched. POTENTIAL DIVIDER: The PD of a source is divided over two resistors in series in proportion to their resistances. SENSOR CIRCUITS: One of the resistors in a potential divider is replaced by a sensor so that the PD across the resistors in the potential divider changes as a physical condition changes (eg temperature, light intensity or strain). [|If you connect a voltmeter in series in a circuit] from www.furryelephant.com 5.3 – Electric cells Essential idea: Electric cells allow us to store energy in a chemical form.
 * [[image:http://www.technologystudent.com/images7/resist1.gif]] || [[image:nothingnerdy/LDR.png width="382" height="254"]] || [[image:nothingnerdy/LDR_graph.png]] || [[image:http://www.qsl.net/4f5aww/images/vdiv3.gif width="465" height="265"]] || [[image:nothingnerdy/thermistor.png width="530" height="265"]] ||
 * Voltage out per resistance || LDR || LDE Graph || LDR Potential divider || Thermistor ||

// ** Nature of science: ** // Long-term risks: Scientists need to balance the research into electric cells that can store energy with greater energy density to provide longer device lifetimes with the long-term risks associated with the disposal of the chemicals involved when batteries are discarded. (4.8)

// ** Understandings: ** // • Cells • Internal resistance • Secondary cells • Terminal potential difference • Electromotive force (emf)

** // Applications and skills: // ** • Investigating practical electric cells (both primary and secondary) • Describing the discharge characteristic of a simple cell (variation of terminal potential difference with time) • Identifying the direction of current flow required to recharge a cell • Determining internal resistance experimentally • Solving problems involving emf, internal resistance and other electrical quantities

//** Guidance: **// • Students should recognize that the terminal potential difference of a typical practical electric cell loses its initial value quickly, has a stable and constant value for most of its lifetime, followed by a rapid decrease to zero as the cell discharges completely

//** International - mindedness: **// • Battery storage is important to society for use in areas such as portable devices, transportation options and back-up power supplies for medical facilities

//** Theory of knowledge: **// • Battery storage is seen as useful to society despite the potential environmental issues surrounding their disposal. Should scientists be held morally responsible for the long-term consequences of their inventions and discoveries?

//** Utilization: **// • The chemistry of electric cells (see Chemistry sub-topics 9.2 and C.6)

//** Aims: **//
 * • ** **Aim 6**: experiments could include (but are not limited to): investigation of simple electrolytic cells using various materials for the cathode, anode andnelectrolyte; software-based investigations of electrical cell design; comparison of the life expectancy of various batteries
 * • ** **Aim 8**: although cell technology can supply electricity without direct contribution from national grid systems (and the inherent carbon output issues), safe disposal of batteries and the chemicals they use can introduce land and water pollution problems
 * • ** **Aim 10:** improvements in cell technology has been through collaboration with chemists

//** Data booklet reference: **// • <span style="color: #ad13da; font-family: Symbol,sans-serif; font-size: 18.2px;">e = I (R + r)

<span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">EMF (E): The total electrical energy given by the battery to each coulomb of charge OR the terminal PD of the source when no current flows. Image from [|IOP EMF and Internal resistance] Electromotive force is the work done per unit charge in moving charge across the battery terminals.
 * //Electromotive force (emf)// ** **from** <span style="background-image: url(">**<span style="background-image: url(">[| Teaching Advanced Physics Episode 105] **

Electromotive force (voltage across a source of electrical energy) is potential difference (voltage across a component that uses electrical energy) across the battery terminals when the battery has no internal resistance. Despite its name, emf is not a force but a __ volta g e __ It is measured in volts.

Emf is the power provided by the battery per unit current. <span style="font-family: Symbol,sans-serif; font-size: 13px;">e ** //= emf = W / q = P / I// **

<span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 10pt;">INTERNAL RESISTANCE (**//r//**): The resistance of the battery. Some of the energy provided by the source is converted to heat here and is not available in the external circuit components. <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif; font-size: 13.3333px;">TERMINAL PD (**V**): The PD measured across the terminals of the source which is less than the EMF due to the internal resistance of the source. Thank you Mr. Armstrong for sharing this precious resource [|Difference between emf and potentional difference] from www.s-cool.co.uk PRACTICE 1) A 'potato cell' has emf 1.0 V and internal resistance 5000 <span style="font-family: Symbol,sans-serif;">W . How many of these cells in what arrangement would adequately light a 5.0 W, 6.0 V filament lamp? SOLUTION: 6 in series + several in parallel

Aim: To determine the emf and internal resistance of an electrical cell
electrical cell resistor variable resistor voltmeter ammeter electrical wire Design your circuit.
 * Introdudction/Background information : **
 * Hypothesis: **
 * Variables: **
 * || What is it? Name it. || Methods of management or measuremenet and control ||
 * Independent varialbe (unit) ||  ||   ||
 * Dependent variable (unit) ||  ||   ||
 * Controlled variable 1 ||  ||   ||
 * Controlled variable 2 ||  ||   ||
 * Controlled variable 3 ||  ||   ||
 * Equipment: **
 * Sefaey ** :
 * Diagram: **

<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">1. The cell and the resistor, labelled //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">r //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">, should be connected in series and used as a single circuit element. As a plan for this practical, draw a circuit that will connect the 100 Ω variable resistor across the cell (and resistor //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">r //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">) and measure the potential difference //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">V //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">across the variable resistor labelled //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">R //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">and the current //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">I //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">through it. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">2. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13.3333px;">Have your circuit layout checked before you begin. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">Have your circuit design checked (show Ms. LEE) and, when she is happy, set it up on the bench. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">3. Comment on how you will ensure the equipment is safely used and continues to function properly. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">4. Vary the resistance of the variable resistor and record values for //V// and //I//. Take readings for the whole range of the variable resistor.
 * Procedure: ** <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13.3333px;">Take more than the usual six readings so that you have more points on your graph. This will make it easier to recognise anomalies.


 * Results: Make your own tables **


 * Tips for your discussion **
 * 1) Recognise and make use of appropriate units in calculations.
 * 2) Use an appropriate number of significant figures.
 * 3) Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined by addition, subtraction, multiplication, division and raising to powers.
 * 4) Translate information between graphical, numerical and algebraic forms.
 * 5) Plot two variables from experimental or other data.
 * 6) Determine the slope and intercept of a linear graph and suggest<span style="font-family: Arial,sans-serif; font-size: 13.3333px;"> your understanding if the results present a linear relationship

1. The mathematical model for this circuitis E – Ir = V. 2. So V = –rI + E (A graph of V against I should give a straight line with gradient –r) 3. Measure the gradient of your graph and compare it with the manufacturer’s value of the resistor r. Ensure you take account of the powers of 10 (prefix) in the measurement of current. 4. Justify the number of significant figures you use in your answer. 5. Comment on the likely accuracy of your values for E and r. Think of change of resistance or resistance in the circuit you have not considered. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">1. When the internal resistance is large in comparison to the external resistance, the terminal potential difference falls to a small value. This is used to make high voltage supplies safe for use in a laboratory. Explain how this makes the supply safe. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">ANALYSIS 1. When a human body is connected across the terminals the resistance is about 25kΩ. If the internal resistance of the supply is 5MΩ, the terminal potential difference falls to a low value with very little current flowing, making it safe. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">2. It should not matter whether the voltmeter is connected across //R// or across the cell. This is partly because of the low resistance of the ammeter. Explain why. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">ANALYSIS <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13.3333px;">2. There will be a very small potential difference across the ammeter. It is assumed to be so small that it can be ignored. This means that the potential difference across R and across the cell is the same. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">3. The intercept of your graph will be very close to the true value for the emf of the cell. Account for any difference. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">ANALYSIS <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13.3333px;">3. The voltmeter does not have an infinite resistance. Any small current will cause a pd across the internal resistance, reducing the terminal pd below the emf. <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">4. Explain any difference between your value for //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">**r** //<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;"> and the manufacturer’s value. ANALYSIS <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 10pt;">4. The cell itself has an internal resistance but this is very small.
 * <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13px;">A ****<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13px;">n ****<span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13px;">alysis of results ( Complete your own before reading this! ) **
 * <span style="background-color: #ffffff; font-family: Arial,sans-serif; font-size: 13px;">Questions for your analysis, in more depth **

Don't forget to include Bibliography! ([|MLA] format)
 * Refer to the Ohm's law investigation guideline above, complete your evaluation and conclusion. **

Discharging a cell
CAPACITY: The amount of charge it can deliver to an external circuit in its lifetime. The bigger the current, the faster the cell discharges. Graph taken from K.A.Tsokos IB Text

5.4 – Magnetic effects of electric currents Essential idea: The effect scientists call magnetism arises when one charge moves in the vicinity of another moving charge.

//** Nature of science: **// Modells and visualization: Magnetic field lines provide a powerful visualization of a magnetic field. Historically, the field lines helped scientists and engineers to understand a link that begins with the influence of one moving charge on another and leads onto relativity. (1.10)

//** Understandings: **// • Magnetic fields • Magnetic force

//** Applications and skills: **// • Determining the direction of force on a charge moving in a magnetic field • Determining the direction of force on a current-carrying conductor in a magnetic field • Sketching and interpreting magnetic field patterns • Determining the direction of the magnetic field based on current direction • Solving problems involving magnetic forces, fields, current and charges

//** Guidance: **// • Magnetic field patterns will be restricted to long straight conductors, solenoids, and bar magnets

//** International - mindness: **// • The investigation of magnetism is one of the oldest studies by man and was used extensively by voyagers in the Mediterranean and beyond thousands of years ago

//** Theory of knowledge: **// • Field patterns provide a visualization of a complex phenomenon, essential to an understanding of this topic. Why might it be useful to regard knowledge in a similar way, using the metaphor of knowledge as a map - a simplified representation of reality?

• Only comparatively recently has the magnetic compass been superseded by different technologies after hundreds of years of our dependence on it • Modern medical scanners rely heavily on the strong, uniform magnetic fields produced by devices that utilize superconductors • Particle accelerators such as the Large Hadron Collider at CERN rely on a variety of precise magnets for aligning the particle beams
 * // Utilazation: //**


 * // Aims: //**
 * • ** **Aims 2 and 9:** visualizations frequently provide us with insights into the action of magnetic fields; however, the visualizations themselves have their own limitations
 * • ** **Aim 7:** computer-based simulations enable the visualization of electromagnetic fields in three-dimensional spacet

**//Data booklet reference://** • //F = qvB sin// <span style="color: #ad13da; font-family: Symbol,sans-serif;">q • //F = BIL sin// <span style="color: #ad13da; font-family: Symbol,sans-serif;">q

Magnetic fields
The pattern of the magnetic field (including direction) due to currents in straight wires and in solenoids

The magnetic field is tangent to the imaginary curves around a current carrying conductor. A magnetic field pattern of a solenoid resembles that of a bar magnet. || **Right-hand grip rule** (Figures taken from K.A.Tsokos IB Text 6th edition.) || EXAMPLE 1) State the direction of the magnetic field at point **P**. Question taken from K.A.Tsokos IB Text 6th edition SOLUTION:
 * <span style="background-color: #ffffff; color: #555555; font-family: &#39;Helvetica Neue&#39;,Helvetica,Arial,sans-serif; font-size: 12px;">Magneti field around a flat coil

media type="youtube" key="UV0wtX9AXq4" width="560" height="315"

Current and Magnets - [|Sixty Symbols] Published on 22 Aug 2012 youtube.com
 * Magnetic force on a moving charge **

A charge //q// moving with speed //v// in a region of magnetic field of magnetic flux density //**B**// will experience a magnetic force F ; //**F**// = //qv**B**// sin <span style="font-family: Symbol,sans-serif;">q

<span style="font-family: Arial,Helvetica,sans-serif;">There is no magnetic force on a moving charge if the charge moves along the field direction. The direction of force is at right angle s to both the velocity vector and the magnetic field.

<span style="font-family: Arial,Helvetica,sans-serif;">The unit of the magnetic flux density is the tesla (T): A magnetic flux density of 1 T produces a force of 1 N on a charge of 1 C moving at 1 m/s at right angles to the direction of the field. //**B**// = //**F /**// //qv// sin <span style="font-family: Symbol,sans-serif;">q

<span style="font-family: Arial,Helvetica,sans-serif;">There is no magnetice force if the charge is not moving. The electric force on a charge is always non-zero whether the charge moves or not. A change in the direction of the magnetic field will cause a change in the deflection of the charged particles, with the direction of the particles staying constant.
 * <span style="font-family: Arial,Helvetica,sans-serif;">Difference between the magnetic force and the electric force: **


 * Force on charged particles **

The magnetic force on a current-carrying wire
The magnetic force on a current carrying wire with the length L is ( L is the length of the wire in the region of the magnetic field.) : <span style="font-family: Symbol,sans-serif;">q is the angle between the current and the direction of the magnetic field.
 * [[image:mit 1.PNG width="443" height="276"]] || [[image:mit 2.PNG width="418" height="275"]] || [[image:sciencelanguagegallery/Force on current carrying conductors experiment.jpg]] ||
 * MIT Physics [|Forces on a Current-Carrying Wire] [|mittechtv] published 8 Aug 2008 || MIT Physics [|Jumping Wire] [|mittechtv] published on 8 Aug 2008 ||  ||
 * Force on current-carrying wire **
 * //F// = //BIL//** sin <span style="font-family: Symbol,sans-serif;">q

Force on a current in magnetic field F = BIL sin <span style="font-family: Symbol,sans-serif;">q (theta) where the units of B are T, tesla. The flux density of a field of one tesla is therefore defined as the force per unit length on a wire carrying a current of one ampere at right angles to the field. To have an induced e.m.f, a coil must cut through the magnetic field lines or a magnet must cut through the magnetic field lines of a coil. An emf is induced in a conductor whenever magnetic field lines (magnetic flux) are crossed.

The direction of Force on current carrying conductors can be found using Fleming's left hand rule. __ **Fleming's left hand rule** __ for the relative directions of force, field and current EXAMPLE 2) State the diretion of the force on wire Z. The currents of X, Y and Z are equal magnitude. The directions of the currents are shown as SOLUTION: X attracts Z and Y repels it. Y is closer to Z so the force it exerts is larger than wire X. Hence the force is to the right.

Magnetic force and field
<span style="background-image: url(">[|ELECTRIC AND MAGNETIC FIELDS IN A COLLIDER -] link to Starts with a Bang blog (Dec 2011)

**Electromagnetic induction** [|IB Physics Electricity Definitions and Concepts Google doc]

Recent practice SL and HL questions from IB papers [|Combined IBHL Electricity Practice questions 0910] [|Combined IBHL Electricity Practice questions Markscheme 0910] [|Combined IBSL Electricity Practice questions 0910] [|Combined IBSL Electricity Practice questions Markscheme 0910]

<span style="display: block; font-family: Roboto,Arial,sans-serif; font-size: 10px;">A basic derivation of the four Maxwell equations which underpin electricity and magnetism. ||
 * media type="youtube" key="IANBoybVApQ" width="560" height="315" || media type="youtube" key="LcyqJWvZioM" width="560" height="315" || media type="youtube" key="AcRCgyComEw" width="560" height="315" || media type="youtube" key="AWI70HXrbG0" width="560" height="315" ||
 * Mind-Blowing Magic Magnets, by <span style="background-image: url(">[|SmarterEveryDay], Published on 20 Mar 2016 || [|How to Build a Homopolar Motor] [|Wayne Schmidt] Published on 30 Sep 2015 || Hall Effect - [|Sixty Symbols] Published on 25 Oct 2010 || Maxwell's Equations - Basic derivation <span class="view-count style-scope yt-view-count-renderer" style="color: var(--yt-metadata-color);"><span style="font-family: Roboto,Arial,sans-serif; font-size: 10px;">[|DrPhysicsA] Published on 7 Sep 2012

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 * Full Name ||
 * Quang Huy NGUYEN ||
 * Le Nguyen Duc ||
 * WonJun CHOI ||
 * Chen Cheng Chun ||
 * Le Nguyen Duc ||
 * WonJun Choi ||
 * Kei Yokokura ||
 * Woo Jin SONG ||
 * Dao Trong Tuan Hung ||
 * Han Ji Hun ||
 * Naoki Higuchi ||
 * Tráº§n Kim Tháº¡ch ||