2. NCEA L2 D.C. ELECTRICITY 2014

3. CIRCUITS  An arrangement of electrical components which allow movement of electrical charge.  Conductor = an object which allows charge to flow through it.  Metals are good conductors as they contain free outer electrons loosely held by the nucleus’ attraction. They move randomly so when no charge is applied the net movement/current is 0.  Apply a voltage or potential difference i.e. the net movement moves towards the terminal with least number-the positive. The more electrons pushed through the faster the current (Note current = Coulombs per second).

4. CIRCUITS….  Current can only flow when the circuit is closed.  Conventional current is where +ve charge travels from +ve to –ve. Natural current is where it travels from a build up of –ve charge to an area of +ve charge.

5. CURRENT & VOLTAGE Current: flow of electrons Measure with: ammeter in series Voltage (Potential difference): difference inn energy carried by electron before and after a component Measure with: voltmeter in parallel

6. CIRCUITS  1 bulb in series  2 bulbs in series  2 bulbs in parallel  2 bulbs in parallel with 1 bulb in main loop How does voltage and current behave in each?

7. WHAT IS ELECTRICAL CURRENT? Electric current is the flow (movement) of electric charge. I = q/t Symbol : IUnits: Amperes (A) Where I = current in amps (A) q = charge in Coulombs (C) t = time (s) Current is measured with an ammeter placed in series in the circuit

8. ELECTRICAL CURRENT 1 Amp = 1 Coulomb per second 1 Coulomb = 1/1.6x10¯¹ ⁹ = 6x10 ¹ ⁸ electrons A total charge of 50 C passes a point in 5 seconds.  I= Q/t  = 50/5  = 10 A

9. WHAT IS ELECTRICAL ENERGY? Voltage is the difference of electric potential between two points in an electrical circuit, expressed in volts. Symbol : VUnits: Volts (V) Often thought of as the change in energy as the charge moves between two points. 1 Volt = 1 Joule of energy per Coulomb of charge.

10. Where  E = change in energy (J)  V = voltage in volts (V)  q = charge in Coulombs (C) Where E = electric field strength  F = force on the charge (N)  q = charge in Coulombs (C) The voltage between two points, distance apart ‘d’, in an electric field ‘E’  E = Vq V = Ed F = Eq VOLTAGE

11.  Potential difference  Each electron carries energy, which can be released due to the object it is flowing through.  Thin wires offer large resistance to a flow of charge and so more work needs to be done by the current to push the charge through, The result is that heat is produced and energy is lost.  Therefore a potential energy difference exists over the wire, this is the voltage.

12. HOW TO MEASURE VOLTAGE....  Measured using a voltmeter that has to be connected in __________ across the object whose voltage is being measured.

13. WHAT IS ELECTRICAL POWER? The rate at which electrical energy is transferred by an electric circuit. Dictates brightness. Symbol : PUnits: Watts (W) W= Joules per second P = IV Where P = power in watts (W) I = current in amps (A) V = voltage in volts (V)

14. WHAT IS RESISTANCE? Is defined as a measure of the degree to which an object opposes an electric current through it Symbol : R Units: Ohms (Ω) V = IR Where R = resistance in ohms (Ω) I = current in amps (A) V = voltage in volts (V) Resistance occurs in all conductors. A good conductor has low resistance. The current in a conductor depends on the resistance and the voltage applied across it-Ohm’s Law. Resistance often very large kΩ or MΩ

15. CURRENT, VOLTAGE & RESISTANCE IN SERIES & PARALLEL R1R1 R2R2 A V1V1 A2A2 V2V2 A V R2R2 R3R3 A3A3 A1A1 I1I1 I2I2 I3I3 V I I I SERIESPARALLEL The current, I, measured by each ammeter has the same value in each part of the circuit. The voltages across the series resistors add up to the supply voltage V s = V 1 + V 2. The equivalent resistance, R s, of the whole circuit is given by: R s = R 1 + R 2. The currents in the parallel components add up to the current from the supply, ie I 1 = I 2 + I 3. The voltage is the same across all branches in parallel The equivalent resistance, R s, of the whole circuit is given by: 1/R p = 1/R 2 + 1/R 3.

16. EXAMPLE…  In the circuit shown:  1. Find the total resistance of the circuit  2. The total current  3. The voltage across each resistor 2V 50 Ω 150 Ω I=V/R

17. EXAMPLE…..  Find..  The resistance of the circuit  The total current in the circuit  The current that flows through each resistor 2V 50 Ω 150 Ω

18. HOOKE’S LAW

19. WHAT IS RESISTANCE? Is defined as a measure of the degree to which an object opposes an electric current through it Symbol : R Units: Ohms (Ω) V = IR Where R = resistance in ohms (Ω) I = current in amps (A) V = voltage in volts (V) Resistance occurs in all conductors. A good conductor has low resistance. The current in a conductor depends on the resistance and the voltage applied across it-Ohm’s Law. Resistance often very large kΩ or MΩ

20. RESISTANCE...  A resistor is a component designed to resist the flow of current through it.  From the power equation and the resistance equation the two can be substituted to give the following power equations involving resistance, voltage or current:  P = IV&V = IR   P = I 2 R  P = V 2 /R

21.  Pages:  127-128  131-133 Resistors in combination  134 Simple Circuits

22. RESISTANCE  Resistance is directly proportional to length ………..the longer the wire, the more resistance  Resistance is inversely proportional to cross sectional area  area of circle πr²……  If the radius doubles, the cross section increases by 4 times and the resistance decreases by 4 times

23. WORKBOOK  Electric Current Pg 150 # 1,3  Electrical Energy Pg 151 # 1,2,3  Electric Power Pg 152 # 1,3,4

24. INTERNAL RESISTANCE… CurrentEffect 1mAMax safe current 2-5mAFelt by most 10mAMuscle spasm, can be fatal 100mAProbably fatal Resistance of skin is often about 10k Ω -100k Ω but can drop to less than 1.5k Ω when the skin is wet. E.g. 10 V with resistance of 10,000 = 1mA 240 V with resistance of 1200 = 200mA

25. OHMS LAW…  http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html

26. OHMIC CONDUCTOR  This is a conductor that obeys Ohms law of V=IR, and so a graph of V against I should…..  Non Ohmic conductors will not …….  Diodes: only allow current to flow in the direction of the arrow.

27. PRACTICAL TO DETERMINE IF A BULB IS AN OHMIC CONDUCTOR….  Set up a series circuit with a bulb and an ammeter in it.  Record 3 measurements for V and I when the bulb is glowing and 3 when it isn’t.  Plot V against I  Determine whether it is an Ohmic or Non-Ohmic conductor.  What happens to the resistance of the bulb as it starts to heat up??

28. CURRENT, VOLTAGE & RESISTANCE IN SERIES & PARALLEL R1R1 R2R2 A V1V1 A2A2 V2V2 A V R2R2 R3R3 A3A3 A1A1 I1I1 I2I2 I3I3 V I I I SERIESPARALLEL The current, I, measured by each ammeter has the same value in each part of the circuit. The voltages across the series resistors add up to the supply voltage V s = V 1 + V 2. The equivalent resistance, R s, of the whole circuit is given by: R s = R 1 + R 2. The currents in the parallel components add up to the current from the supply, ie I 1 = I 2 + I 3. The voltage is the same across all branches in parallel The equivalent resistance, R s, of the whole circuit is given by: 1/R p = 1/R 2 + 1/R 3.

29. EXAMPLE…  In the circuit shown:  1. Find the total resistance of the circuit  2. The current  3. The voltage across each resistor 2V 50 Ω 150 Ω

30. EXAMPLE…..  Find..  The resistance of the circuit  The total current in the circuit  The current that flows through each resistor 2V 50 Ω 150 Ω

31. PROJECTILE MOTION

32. ELECTRICITY  Pg 157 all  Pg 158 all  Pg 159 Q 3  Pg 160 Q1,2,3

33. RESISTOR PRACTICAL  In series….. different resistors.  1: 2 identical resistors  2: 2 different resistors  3: 3 different resistors  Put voltmeters across the resistors and record the readings for each circuit.  Why are the 2 voltages different? Compare their voltage to the voltage across the battery.

34. RESISTOR PRACTICAL  In Series….. 2 different resistors.  Investigate current and voltage.  Firstly, put 2 ammeters in the circuit-record their readings.  Then put the 2 voltmeters across the resistors and record the readings.  Why are the 2 voltages different? Compare their voltage to the voltage across the battery.

35. READ INFORMATION PAGE 162 - 163 COMPLETE RELEVANT EXERCISES FROM RUTTER READ INFORMATION PAGE 162 - 163 COMPLETE RELEVANT EXERCISES FROM RUTTER

36. CIRCUIT….  Set up basic parallel circuit with 2 identical bulbs in parallel  Connect an ammeter in the main part of the circuit and another one in a branch.  What should be the relationship between these readings?  Does this work out practically?

37. RUTTER….  Series and Parallel resistors 164  Circuit exercises 166 1,3,5,6.

38. POTENTIAL DIVIDER The idea behind the potential divider is that resistors can be used to control the voltage in parts of a circuit.

39. FORMULA  Output voltage = input voltage x R2  (R1 + R2)

40. EXPT....  http://tap.iop.org/electricity /circuits/118/file_46040.pdf http://tap.iop.org/electricity /circuits/118/file_46040.pdf  Video PD  http://www.youtube.com/w atch?v=KL08eX9aaVk http://www.youtube.com/w atch?v=KL08eX9aaVk +9V 0V Supply rail Ground rail

41. 20V 3.3k Ω 2.2k Ω V=?  A potential divider is made from 2 resistors and connected to a 20V supply voltage. Calculate the voltage across the 2.2k Ω resistor. Output voltage= input voltage x R2 (R1 + R2)