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Current-voltage characteristics questions
Put a component through its paces, plotting current against pd, and the shape of the graph names the component. Three characteristics are required. Only one of them 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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