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Current-voltage characteristics questions
Plot current against potential difference for a component and the shape of the graph identifies it. Three characteristics are required: a resistor at constant temperature, a filament lamp and a diode. Only the first obeys Ohm's law.
19 original questions · 58 marks · the current-voltage characteristics notes · Electricity
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State Ohm's law.
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For a metallic conductor at constant temperature (1), the current through it is directly proportional to the potential difference across it, I ∝ V (1).A point on a component's I–V graph reads V = 4.0 V and I = 0.20 A. Calculate the resistance at that point.
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R = V/I = 4.0/0.20 (1)
R = 20 Ω (1)Sketch the current–voltage characteristic of an ohmic conductor at constant temperature.
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A straight line passing through the origin, current proportional to voltage (1), with the same gradient for positive and negative pd (1).When the I–V characteristic of a component is measured, the meters are assumed to be ideal. State the resistance of an ideal ammeter and of an ideal voltmeter.
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Ideal ammeter: zero resistance (1). Ideal voltmeter: infinite resistance (1).State how the resistance of a component is found from a point on its I–V characteristic.
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R is the ratio V/I of the coordinates at that point, not the gradient of the curve (1).A student plots the characteristic of an ohmic conductor with V on the vertical axis and I on the horizontal axis. Describe the graph obtained and state what its gradient represents.
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A straight line through the origin (1). The gradient is V/I, which for an ohmic conductor is its (constant) resistance (1).Describe the shape of the I–V characteristic of a filament lamp and explain its shape.
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The curve is S-shaped through the origin (1) and gets shallower as the voltage rises: for a given increase in V the current increases less (1). As current rises the filament heats up, so its resistance increases, curving the graph away from a straight line (1).Describe the I–V characteristic of a semiconductor diode.
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In forward bias almost no current flows until about 0.6 V (1), after which the current rises steeply (1). In reverse bias only a negligible current flows, so the diode conducts in only one direction (1).A filament lamp gives the readings V = 2.0 V, I = 0.40 A and V = 8.0 V, I = 0.80 A. Calculate the resistance at each point and comment on the change.
A component gives currents of 0.10 A, 0.20 A and 0.30 A at 2.0 V, 4.0 V and 6.0 V. Show that it is ohmic.
In forward bias a diode passes 35 mA when the pd across it is 0.62 V. In reverse bias a pd of 5.0 V drives a current of only 1.0 μA through it. Calculate the resistance of the diode at each point, and comment on the values.
A student measures the I–V characteristic of a metal wire at 20 °C, then repeats the measurement with the wire held at a steady 80 °C in an oven. Describe and explain how the two graphs differ.
The characteristic of a filament lamp is unchanged in shape when the pd is reversed, but the characteristic of a diode is not. Explain both observations.
Describe how the resistance of a negative temperature coefficient (NTC) thermistor changes with temperature, and explain this in terms of charge carriers.
A filament lamp operates at 12 V with a current of 2.0 A. Calculate (a) its operating resistance and (b) its power. When barely lit it reads 1.0 V and 0.50 A; calculate its resistance then.
Explain why a filament lamp does not obey Ohm's law.
A control circuit needs a component whose resistance changes by less than 5% between pds of 2.0 V and 6.0 V. Component P passes 0.40 A at 2.0 V and 1.00 A at 6.0 V. Component Q passes 0.100 A at 2.0 V and 0.294 A at 6.0 V. Deduce which component is suitable.
Describe an experiment to obtain the full I–V characteristic of a filament lamp, including negative values of pd. Include the circuit used, the measurements taken and how the results are processed.
A student says: 'Below its threshold a diode has no resistance, because there is no current.' Explain the student's error and state what actually happens to the diode's resistance below the threshold.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
Practise current-voltage characteristics one question at a time
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