NCEA L2 D.C. Electricity

NCEA L2 D.C. Electricity2014CircuitsAn 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).

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.Current & voltageCurrent: flow of electronsMeasure with: ammeter in series

Voltage (Potential difference): difference inn energy carried by electron before and after a componentMeasure with: voltmeter in parallel

circuits1 bulb in series2 bulbs in series2 bulbs in parallel2 bulbs in parallel with 1 bulb in main loop

How does voltage and current behave in each?What is electrical current? Electric current is the flow (movement) of electric charge.I = q/tSymbol : 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 circuitElectrical Current1 Amp = 1 Coulomb per second

1 Coulomb = 1/1.6x10 = 6x10 electrons

A total charge of 50 C passes a point in 5 seconds.

I= Q/t = 50/5 = 10 AWhat 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.

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 = VqV = EdF = EqvOLTAGEVoltagePotential 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.

How to measure voltage....Measured using a voltmeter that has to be connected in __________ across the object whose voltage is being measured.

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 = IVWhere P = power in watts (W) I = current in amps (A) V = voltage in volts (V)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 ()

V = IRWhere R = resistance in ohms () 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-Ohms Law. Resistance often very large k or MCURRENT, VOLTAGE & RESISTANCE IN SERIES & PARALLELR1R2AV1A2V2AVR2R3A3A1I1I2I3VIIISERIESPARALLELThe 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 Vs = V1 + V2.The equivalent resistance, Rs, of the whole circuit is given by: Rs = R1 + R2.The currents in the parallel components add up to the current from the supply, ie I1 = I2 + I3.The voltage is the same across all branches in parallelThe equivalent resistance, Rs, of the whole circuit is given by: 1/Rp = 1/R2 + 1/R3.ExampleIn the circuit shown:

1. Find the total resistance of the circuit2. The total current3. The voltage across each resistor

2V50 150 I=V/RA= 200 ohm, 2/200 = 0.01 A, V= 0.5 V V= 1.5 V 15Example..Find..

The resistance of the circuitThe total current in the circuitThe current that flows through each resistor

2V50 150 V= 37.5 I=0.053A I= 0.04 I= 0.013316Hookes law

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 ()

V = IRWhere R = resistance in ohms () 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-Ohms Law. Resistance often very large k or MResistance...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 = IRP = I2RP = V2/R

Page 147 19Pages:

127-128131-133 Resistors in combination134 Simple CircuitsResistanceResistance is directly proportional to length ..the longer the wire, the more resistance

Resistance is inversely proportional to cross sectional area area of circle r If the radius doubles, the cross section increases by 4 times and the resistance decreases by 4 times WorkbookElectric Current Pg 150 # 1,3

Electrical Energy Pg 151 # 1,2,3

Electric Power Pg 152 # 1,3,4

Internal resistanceCurrentEffect1mAMax safe current2-5mAFelt by most10mAMuscle spasm, can be fatal100mAProbably fatalResistance of skin is often about 10k-100k but can drop to less than 1.5k when the skin is wet.

E.g. 10 V with resistance of 10,000 = 1mA 240 V with resistance of 1200 = 200mAOhms Lawhttp://phet.colorado.edu/sims/ohms-law/ohms-law_en.htmlOhmic ConductorThis 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.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 isnt.

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??CURRENT, VOLTAGE & RESISTANCE IN SERIES & PARALLELR1R2AV1A2V2AVR2R3A3A1I1I2I3VIIISERIESPARALLELThe 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 Vs = V1 + V2.The equivalent resistance, Rs, of the whole circuit is given by: Rs = R1 + R2.The currents in the parallel components add up to the current from the supply, ie I1 = I2 + I3.The voltage is the same across all branches in parallelThe equivalent resistance, Rs, of the whole circuit is given by: 1/Rp = 1/R2 + 1/R3.ExampleIn the circuit shown:

1. Find the total resistance of the circuit2. The current3. The voltage across each resistor

2V50 150 A= 200 ohm, 2/200 = 0.01 A, V= 0.5 V V= 1.5 V 28Example..Find..

The resistance of the circuitThe total current in the circuitThe current that flows through each resistor

2V50 150 V= 37.5 I=0.053A I= 0.04 I= 0.013329Projectile motion

ElectricityPg 157 all

Pg 158 all

Pg 159 Q 3

Pg 160 Q1,2,3Resistor PracticalIn series.. different resistors.

1: 2 identical resistors2: 2 different resistors3: 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.Resistor PracticalIn 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.READ INFORMATION PAGE 162 - 163COMPLETE RELEVANT EXERCISES FROM RUTTERCircuit.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?Rutter.Series and Parallel resistors 164

Circuit exercises 166 1,3,5,6.Potential DividerThe idea behind the potential divider is that resistors can be used to control the voltage in parts of a circuit.FormulaOutput voltage = input voltage x R2 (R1 + R2)

Expt....http://tap.iop.org/electricity/circuits/118/file_46040.pdf

Video PDhttp://www.youtube.com/watch?v=KL08eX9aaVk

+9V0VSupply railGround rail20V3.3k2.2kV=?A potential divider is made from2 resistors and connected to a 20Vsupply voltage.

Calculate the voltage across the 2.2k resistor.

Output voltage= input voltage x R2 (R1 + R2)

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