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**IGCSE Electricity – Resistance**

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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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**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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**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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**How are current and voltage related for a resistor?**

Set up the circuit as shown below: P.D. (V) I (A) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 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. 0.6 1.1 1.8 A 2.5 3.0 3.5 4.2 V

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**Current and voltage Draw a line of best fit for your graph.**

Plot your points on the graph. What does your graph look like? If you get a straight line it means that the two quantities current and voltage are proportional. x x Current, I / amps x What does proportional mean? x x If you double the voltage then the current doubles. x x x This fact was put into a law: Potential difference / V 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

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**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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**Voltage = Current x Resistance**

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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** x V I Formula triangles**

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. …and you are left with the sum… So if you were trying to find current, I….. I = V R V …you would cover I up… I R x

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Formula triangles This is a flash movie – click on the letter you want to cover to start the animation

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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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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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To do P179 answer all questions.

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**Resistivity Resistance of a wire depends on 3 things. Length**

Cross sectional area Material (resistivity)

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Length

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Area

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Resistivity

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To do

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**Use a textbook or other resource to fill in the table below:**

Component Circuit symbol Diode Light Dependent Resistor Variable Resistor Thermistor

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** x ..partly Voltage/current graphs I 1 I 3 I 2 4**

1. A wire or resistor. 2. A filament lamp. 3. Wires of different materials. 4. A diode. x ..partly Which of the components obeys Ohms Law?

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**To do Sketch I-V graphs for**

An Ohmic conductor at constant temperature. A filament bulb A diode

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**What are the units of resistance?**

Amps Ohms Volts Watts

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**What does the circuit symbol shown represent?**

Voltmeter Variable resistor Light dependent resistor Thermistor

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**What causes resistance?**

Inertia Friction Buoyancy Viscosity

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If two resistors of 6 and 4 are placed in parallel, what is their total resistance in the circuit? 10 2 2.4 24

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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? 50 2 0.5 15

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