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© Boardworks Ltd 2003 IGCSE Electricity – Resistance

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© Boardworks Ltd 2003 Resistance State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current Recall and use the equation R = V/I Describe an experiment to determine resistance using a voltmeter and an ammeter Relate (without calculation) the resistance of a wire to its length and to its diameter

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© Boardworks Ltd 2003 Aims Know the relationship between resistance, p.d. and current Recall and use R=V/I Describe an experiment to determine resistance Understand the concept of resistivity Use R=***l/A

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© Boardworks Ltd 2003 Resistance Supplement Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire

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© Boardworks Ltd 2003 How are current and voltage related for a resistor? V A Set up the circuit as shown below: Slowly move the variable resistor and for each setting record the current in amps and the voltage in volts. Plot a graph of your results. P.D. (V) I (A)

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© Boardworks Ltd 2003 Ohm’s Law: The current flowing through a wire is proportional to the potential difference (voltage) across it provided the temperature remains constant. Georg Ohm 1826 Current, I / amps Potential difference / V What does proportional mean? Current and voltage If you double the voltage then the current doubles. x x x x x x x x Plot your points on the graph. Draw a line of best fit for your graph. What does your graph look like? If you get a straight line it means that the two quantities current and voltage are proportional. This fact was put into a law:

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© Boardworks Ltd 2003 Electron flow and Resistance Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter. Resistance is a measure of how much a material tries to stop electricity passing through it.

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© Boardworks Ltd 2003 V=IR We can express Ohm’s Law mathematically using the equation: Voltage = Current x Resistance V=IR Voltage measured in Volts (V) Current measured in Amps (A) Resistance measured in Ohms ( )

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© Boardworks Ltd 2003 Formula triangles V I RR x Formula triangles help you to rearrange formula, the triangle for the Ohm’s Law is shown below: Whatever quantity you are trying to find cover it up and it will leave you with the calculation required. So if you were trying to find current, I….. …you would cover I up… …and you are left with the sum… I = V R

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© Boardworks Ltd 2003 Formula triangles

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© Boardworks Ltd 2003 Resistance for a bulb If you have a filament bulb and it has a current of 20A running through it, with a potential difference of 100V across it, what is the resistance of the bulb? V = IR R= V/I R= 100V/20A R= 5

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© Boardworks Ltd 2003 Current for a diode A diode has a current of 5A running through it, and a resistance of 5 . What is the potential difference across the diode? V = IR V= 5A x 5 V= 25V

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© Boardworks Ltd 2003 To do P179 answer all questions.

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© Boardworks Ltd 2003 Resistivity Resistance of a wire depends on 3 things. –Length –Cross sectional area –Material (resistivity)

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© Boardworks Ltd 2003 Length

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© Boardworks Ltd 2003 Area

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© Boardworks Ltd 2003 Resistivity

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© Boardworks Ltd 2003 To do

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© Boardworks Ltd 2003 Use a textbook or other resource to fill in the table below: ComponentCircuit symbol Diode Light Dependent Resistor Variable Resistor Thermistor

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© Boardworks Ltd 2003 Voltage/current graphs 1. A wire or resistor. 2. A filament lamp. 3. Wires of different materials. 4. A diode. Which of the components obeys Ohms Law? I V I V I V I V partly x

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© Boardworks Ltd 2003 To do Sketch I-V graphs for An Ohmic conductor at constant temperature. A filament bulb A diode

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© Boardworks Ltd 2003 What are the units of resistance? A.Amps B.Ohms C.Volts D.Watts

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© Boardworks Ltd 2003 What does the circuit symbol shown represent? A.Voltmeter B.Variable resistor C.Light dependent resistor D.Thermistor

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© Boardworks Ltd 2003 What causes resistance? A.Inertia B.Friction C.Buoyancy D.Viscosity

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© Boardworks Ltd 2003 If two resistors of 6 and 4 are placed in parallel, what is their total resistance in the circuit? A.10 B.2 C.2.4 D.24

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© Boardworks Ltd 2003 If a resistor that obeys Ohm’s Law has a potential difference of 10V across it and a current of 5A running through it. What is its resistance? A.50 B.2 C.0.5 D.15

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