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IV Characteristics Electricity Lesson 4

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Learning Objectives To recall the symbols for different circuit components. To investigate the characteristics of different components. Draw and explain the characteristics for a wire, a filament bulb, thermistor and diode. Explain how and why resistance depends on temperature for metals and semiconductors.

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**Circuit Symbols? Variable Resistor Ammeter Voltmeter Thermistor Cell**

Indicator or Light Source Diode Light Emitting Diode Resistor Variable Resistor Thermistor Light Dependent Resistor Heater Electric Motor

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**Complete the Table V I R (a) 0.30 A 18 Ω (b) 12 V 88 Ω (c) 14.4 V**

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Ammeter An ammeter measures the current through part of the circuit. It is always connected in series.

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Voltmeter A voltmeter measures the potential difference between two points. It is always connected in parallel.

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Cell A cell provides voltage to a circuit. This pushes the charge around the circuit.

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Lamp The symbol for an indicator or any light source is the same (not including a light emitting diode).

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Diode A diode only allows current to flow in one direction. (In the way the triangle points) The diode has a very high resistance in the reverse direction. (For LED add the two little arrows)

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Resistor A resistor impedes the flow of charge around a circuit. As it does so, it heats up.

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Variable Resistor A resistor that's value can easily be changed.

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Thermistor The resistance of a thermistor decreases as the temperature increases.

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**Light Dependent Resistor**

An LDR is a light dependent resistor. Its resistance decreases as the amount of light falling on it increases.

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**Investigating I-V Characteristics**

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**Wire (Ohmic Conductor)**

For a resistor, the current-voltage graph is a straight line through the origin. The current through a resistor is directly proportional to the potential difference (voltage) across the resistor.

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Ohmic Conductor Note that for an Ohmic conductor the gradient of the line an I-V graph is equal to 1/R. But this is for an Ohmic conductor only. This is because R for a metallic conductor is constant under constant physical conditions. It does not apply for the filament bulb.

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Filament Bulb

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Filament Bulb For a filament lamp, as the voltage increases, so does the current. But the increased current raises the temperature of the filament bulb which also increases the resistance. This is why the curve bends toward the end as it is increasingly difficult for more current to flow.

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**Thermistor At constant T, it gives a straight line.**

The higher T is, the greater the gradient of the line as the resistance falls with increasing T.

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Diode

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Diode Notes The diode only allows current to flow in one direction (forward direction). The diode has a high resistance in the opposite direction (reverse direction). Needs a certain pd to conduct (typically about 0.6 V for silicon diode.)

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Applications Why does the resistance of metals increase with temperature? Positive temperature coefficient. Thermistor Temperature sensors Negative temperature coefficient. % change of resistance. Why? number of charge carriers.

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