PH12. U1, A2: Electricity

Sunshine College, 2017

This table of contents is hyperlinked. Click the title you want to jump to the relevant section.

Week 4: Charge, Current, Potential Difference, Energy, and Power.        2

Week 5: Resistance and circuit analysis        9

Week 6: SAC        15

Week 7: Potential Dividers, Potentiometers, Thermistors, Diodes and LDRs.        16

Week 8: Household Electricity and Electrical Safety        23

Week 9: Practical Activity        30

Note for 2018.

Check out this article prior to re-planning this unit: https://educationechochamber.wordpress.com/2017/10/15/electrifying-engelmann/ 

Term 1, Week 4: Charge, Current, Potential Difference, Energy, and Power.

Intended Learning Outcomes: By the end of the week, SWBAT

• apply concepts of charge (Q ), electric current (I ), potential difference (V ), energy (E ) and power (P ), in electric circuits
• explore different analogies used to describe electric current and potential difference
• investigate and analyse theoretically and practically electric circuits using the relationships Q = IT, E = VQ
• justify the use of selected meters in circuits

Homework Questions

• See handout : )

Learn

• Charge (Q)

• Charge is a measure of how (electrostatically) positive or negative something is.
• The units of charge are Coulombs (C).
• The elementary charge (e), is the charge of a proton and is Coulombs. This means that to make one coulomb you need  

• You do: Calculate how many electrons make up a charge of -8 C.

• Current (I)

• Current is the movement of electrical charge (electrons moving).
• Current is a measure of how many coulombs pass a point in a circuit in one second. Units are coulombs/second=Amps (A).

• Show: Phet simulation: signal circuit.

• CONFUSING: Electrons move from the negative end of a battery around the circuit, but we define current as flowing from the positive end (because scientists used to think that protons carried the charge).

• You do: 3.2 Review, Question 8.

• Measuring Current

• We measure current with an ammeter. An ammeter is like a waterwheel, to measure the ‘flow’ it has to be in the flow of the water.

• Electric Circuit Symbols

•          Battery (Bigger side is positive)

• Question:
• 

• Potential Difference, or Voltage (V)

• Voltage just means how many joules of energy each coulomb has (units, joules/Coulomb = Volts).

• More formally, Potential difference is the work done to move a charge against an electric field between two points.

• I do: 3.4 Review, 3ai
• You do: 3.4 Review, 3aii, and 3aiii.
• You do: 3.4 Review, Q4.
• You do: 3.4 Review, Q5.

• Coulombs gaining and use joules

• A battery gives coulombs joules

• As coulombs pass things in the circuit, they use their joules of energy to do things, like light up light bulbs

• Measuring Potential Difference

• To measure potential difference (the number of joules a coulomb has lost by passing through some part of a circuit) we use something called a Voltmeter

• A voltmeter must be placed ‘in parallel’ to the thing that you want to measure, so you’re checking the coulombs before and after they pass through the item in the circuit to see the number of joules used (potential difference).

• Hint: There’s something called a ‘multimeter’ which can be an ammeter, a voltmeter, and a whole heap of other things too. Multimeters look like this:

• Discuss with a partner: 3.3 Review, Question 8.

• Energy (E)

•    (The number of joules you use depends on how many coulombs you have and how many joules you give to each of them.)

• All questions related to this you can just do by thinking about our how many joules each coulomb gets. Re-do review 3.3 questions 3, 4, and 5 now using the formula

• Power (P).

• The units of power are Joules per second
•   (The number of joules per second depends on how many joules each coulomb has, and how many coulombs are passing per second).
•  (The number of joules per second depends on how many joules you have per second… duh!).

• You do: 3.3 Review, Questions 2 and 6

• Significant figures (use handout!).

Term 1, Week 5: Resistance and circuit analysis

Intended Learning Outcomes: By the end of the week, SWBAT

• model resistance in series and parallel circuits using i) (I–V) graphs, ii) resistance as the potential difference to current ratio, including R = constant for ohmic devices and iii) resistors in series and parallel
• calculate and analyse the effective resistance of circuits comprising parallel and series resistance

Homework Questions

• See handout : )

Learn

• Resistance

• It isn’t always easy for coulombs (groups of electrons) to pass through substance, sometimes they feel ‘resistance’ when they do this.
• Resistance has the units of ohms  
• Resistance depends on three things: a material’s cross sectional area (triple the cross sectional area,  the resistance), length (double the length, double the resistance) and temperature (hotter->harder for e- to move).

• You do 3.4 Review, question 8

• Ohm’s Law

• The rate at which coulombs can pass through a resistor depends on two things, how many joules each coulomb has, and what the resistance is.
• Georg Ohm (1789-1854) found that voltage, current, and resistance are related to each other through the equation:

• I do: The maths related to the examples above.
• You do 3.4 Review, question 4, part a.

• Ohmic resistors

• An ohmic resistor is just a resistor that follows Ohm’s law, . So, if you double the voltage, you double the current, etc.

• I do: 3.4 Review, Question 6.
• You do 3.4 Review, question 4, part b.
• You do 3.4 Review, question 7.

• Voltage vs. Current graphs

• We know a linear graph has the form y=mx+b, but if we draw a graph with I  on the y axis and V on the x, axis, we can graph Ohm’s law as: . Such a graph would look like this.

• If we see a straight line on a current vs. voltage graph, we have an ohmic resistor, if it isn’t a straight line, it’s a non-ohmic resistor.

• I do, 3.4 Review, Q3.
• You do, 3.4 Review, Q1.
• You do, 3.4 Review, Q2.
• If you finish early: You do, 3.4 Review, Q9.

• Resistors in Series and Parallel

• When we combine resistors in a circuit we get a ‘total resistance’ (RT).
• When resistors are in series we find RT by simply adding up the resistances of each resistor.

• I do: Two resistors in series
• You do: see below.

• When we combine resistors in parallel, we find the effective resistance with the following formula:

• I do: Two resistors in parallel
• You do: See below.

• Here’s a trickier one…determine the total resistance between points A and B.

• Circuit analysis: (Resistance, Current, and Voltage drop around circuits)

• Steps for calculating voltage and current for different parts of circuits:

1. Find the total resistance for each group of parallel resistors
2. Find the total resistance for the whole circuit
3. Use the total resistance and Ohm’s law to calculate the current in the circuit
4. Use Ohm’s law to calculate the voltage drop across each resistor (or parallel group of resistors) in series

1. Kirchoff’s voltage law: Each coulomb must use up all of its joules as it goes around a circuit.

5. Use Ohm’s law where necessary to calculate the current through individual resistors in parallel (based on the know voltage drop across them).

1. Kirchoff’s current law: The amount of current flowing into a junction must be the same as the current flowing out of that junction.

Can use to answer questions like: Calculate the current through each resistor as well as the voltage drop (how many joules each coulomb uses as it passes through) over each resistor.

• I do:
• You do:

Week 6: SAC

Week 7: Potential Dividers, Potentiometers, Thermistors, Diodes and LDRs.  

Intended Learning Outcomes: By the end of the week, SWBAT

• calculate and analyse the effective resistance of circuits comprising voltage dividers and potentiometers
• describe energy transfers and transformations with reference to transducers
• investigate and apply theoretically and practically concepts of current, resistance, potential difference and power to the operation of electronic circuits comprising resistors, light bulbs, diodes, thermistors, light dependent resistors (LDRs), light-emitting diodes (LEDs)
• investigate practically the operation of simple circuits containing resistors, variable resistors, diodes and other non-ohmic devices

Homework Questions

• See handout : )

Learn

• Potential Dividers (aka: voltage dividers)

• When you want to have control of how much voltage is delivered to a particular part of your circuit you can use a ‘potential divider’.
• Potential dividers are most often used to deliver a potential difference to a ‘switch’ or an ‘op-amp’  (op-amps are beyond the scope of this course, but you can find out about them here : )

• I do: Find the output voltage of the following potential divider circuit

• You do: Find the output voltage of the following potential divider circuit.
•  

• The voltage divider formula.

• Try to re-do the question using this formula now!

• Potentiometers (aka: Variable resistors)

• Potentiometers are simply a resistor that has a resistance that a user can change.

• Potentiometers are often used in such things as volume controls and light dimming switches. Like this dude does : )

• Demo: Here’s what a potentiometer looks like in action.

• Thermistors and Light Dependent Resistors (in potential divider circuits!!!)

• Thermistors and Light Dependent Resistors (LDRs) are potentiometers that aren’t controlled by an operator, but by temperature or light.

• Usually an LDR will have very high resistance in the dark (a few million Ohms), and a lower resistance in the light (a few hundred Ohms)
• PTC Thermistors (Positive Temperature Coefficient) have a resistance that increases as temperature increases. NTC thermistors are opposite.

• I do: Checkpoints Chapter 5, Question 26 (parts A and b)

• You do: Checkpoints Chapter 5, Question 30, part A.
• I do: The above, part C
• You do: Checkpoints Chapter 5, Question 30, part B and C.
• And now for another thermistor question!!!

• Transducers

• ‘Input Transducers’ are devices that change non-electrical signal into electrical signal (Thermistors and LDRs are both input transducers)

• Question: What might an ‘output transducer’ do?
• Question: Can you think of any examples of an output transducer? (See heaps of transducer examples here!)

• Diodes and LEDs (Light emitting Diodes)

• Diodes are a device that only allow current to pass through it in one direction
• Diodes are non-Ohmic, and they generally have a ‘forward’ or ‘switch on’ voltage (they won’t turn on unless the coulombs coming to them have a certain number of joules).

• I do:

If the resistance of R1 is 1200 Ohms, what is the current flowing in the circuit?

• You do: Checkpoints Chapter 5, Question 27.

Variable resistors, also referred to as potential dividers, are used in household applications such as volume controls and light dimmers.

Week 8: Household Electricity and Electrical Safety

Intended Learning Outcomes: By the end of the week, SWBAT

• model household (AC) electrical systems as simple direct current (DC) circuits
• apply the kilowatt-hour (kW h) as a unit of energy
• explain why the circuits in homes are mostly parallel circuits
• model household electricity connections as a simple circuit comprising fuses, switches, circuit breakers, loads and earth
• compare the operation of safety devices including fuses, circuit breakers and residual current devices (RCDs)
• describe the causes, effects and treatment of electric shock in homes and identify the approximate danger thresholds for current and duration.

Homework Questions

• See handout : )

Learn

• AC and DC current.

• AC stands for ‘alternating current’ and it means that the direction of the current ‘alternates’. DC stands for ‘Direct Current’.

• Animation.

• Batteries give DC current whilst current from power points is AC.

• Wiring of houses, parallel or series?

• 
• Houses are wired in parallel so that switching one circuit won’t influence other circuits.

• The Kilowatt-hour

• A Kilowatt-hour (kWh) is simply one kilowatt of power used for one hour (think units!).

• Question: Kilowatt-hours can be expressed in another unit, what unit is this?
• Question: How many Joules in a kWh?
• I do:
• You do:

• Electrical Safety Devices (Fuses and Circuit Breakers)

• When a circuit is ‘overloaded’ (not enough resistance, too high current) this can start fires
• The most common cause of overloading is short circuits
• 

• Beware short circuits, see here.

• Safety devices: Fuses will burn through if overloading occurs and circuit breakers will disconnect circuit if overloading occurs.

• Insulation of electrical wires also helps to prevent short circuits

• Electric Shocks and their effects.

• Resistance of the human body ranges from around 500 to 100,000 Ohms.

• A little more info: "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms." source.

• Whether or not an electric shock is deadly also depends on the duration of the shock. For a ‘severe’ 50mA current

• You do: When my Dad was building his house he brushed his hand against a live wire and it gave him an electric shock, knocking him to the ground and making him feel incredibly sick. If the voltage in Australia is 240V and we assume that he received a ‘severe shock’ what was his resistance?

• Here’s what an electric shock looks like. (language warning)

• The three wires in our appliances

• Most appliances in Australia have three wires. The live wire (brown) carries current to the appliance. Neutral wire (blue) completes the circuit. The green/yellow wire is earth (for safety).
• An ‘earth’ wire connects the case of an electric device to the earth. This protects users against electric shock (is connected to ‘earth stake’ outside the house’)

• source
• Ps: This is an ‘earth stake’.
• 

• Residual Current Devices

• A residual current device monitors the current in the ‘live’ and ‘neutral’ wires and will switch off the current if there is a difference (because a difference suggests a current ‘leak’).
• 

For Ollie to maybe check out next time?

Week 9: Practical Activity

Intended Learning Outcomes: By the end of the week, SWBAT

• Students gain familiarity with building and analysing simple circuits, then recording results in the format of a Prac Report.

Learn

• Prac Report Template here: http://tiny.cc/vcephysicspracguide
• Practical activity to be desrcibed in class.  

Week x: Template

Intended Learning Outcomes: By the end of the week, SWBAT

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Learn

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