The current against potential difference graphs for a resistor, filament lamp and diode, why each has its shape, and how LDRs and thermistors respond.
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1.What is an I-V characteristic?
A graph showing how the current through a component varies with the potential difference across it.
2.Describe the I-V graph for a fixed resistor at constant temperature.
A straight line through the origin, showing current is directly proportional to potential difference.
3.What does a straight line through the origin tell you about resistance?
That the resistance is constant.
4.What is an ohmic conductor?
A component whose resistance stays constant, so current is directly proportional to potential difference.
5.Describe the I-V graph for a filament lamp.
A curve that becomes less steep as potential difference increases, showing resistance rising.
6.Why does the resistance of a filament lamp increase?
As current increases the filament gets hotter, the ions vibrate more, and electrons collide with them more often.
7.Describe the I-V graph for a diode.
Almost no current in one direction, and current flowing freely once the potential difference is high enough in the other direction.
8.What does a diode do?
It allows current to flow in one direction only.
9.Why is a diode useful?
It protects components from a reversed supply, and it is used to convert alternating current to direct current.
10.What is an LDR?
A light dependent resistor, whose resistance changes with light intensity.
11.How does the resistance of an LDR change with light?
It decreases as light intensity increases.
12.Give an application of an LDR.
Automatic street lights and camera light meters.
13.What is a thermistor?
A resistor whose resistance changes with temperature.
14.How does the resistance of a thermistor change with temperature?
It decreases as temperature increases.
15.Give an application of a thermistor.
Thermostats in ovens, heating systems and fire alarms.
16.Why must the temperature be kept constant when investigating a resistor?
Resistance changes with temperature, so a rise would change the results and hide the true relationship.
17.How do you reverse the potential difference in an I-V experiment?
Swap the connections to the power supply, or use a supply that can be set to negative values.
18.Why is an I-V graph plotted with both positive and negative values?
To show whether the component behaves the same way with the current in each direction, which reveals a diode.
19.How do you calculate resistance from a point on an I-V graph?
Divide the potential difference by the current at that point.
20.A point on a graph shows 6 V and 2 A. Calculate the resistance.
6 divided by 2 = 3 ohms.
21.Why is the gradient of an I-V graph not the resistance?
The gradient is current divided by potential difference, which is one over resistance rather than resistance itself.
22.What does a steeper I-V line mean?
A lower resistance, because more current flows for the same potential difference.
23.Why is a filament lamp not an ohmic conductor?
Its resistance is not constant, because it changes as the filament heats up.
24.What symbol represents a variable resistor?
A rectangle with an arrow through it.
25.In a circuit with an LDR in the dark, what happens to the current?
The resistance of the LDR is high, so the current in that branch is small.
26.Why do thermistors make good temperature sensors?
Their resistance changes markedly with temperature, so the change is easy to detect and use in a control circuit.
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