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15 Cards in this Set
- Front
- Back
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What is a Non-ohmic conductor? |
A Non-ohmic conductor is a component in which the potential difference and current are not proportional to each other.
Its voltage-current graph does not follow a straight line. Instead, it gives a curve with an increasing gradient. |
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Give an example of a Non-ohmic conductor. |
A filament bulb is a non-ohmic conductor. |
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What does the graph show is happening to the resistance? |
It shows that the resistance increases as the current increases.
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Why does the resistance change? |
As the voltage across a non-ohmic conductor such as a bulb increases, the electrons carry more energy.
When they collide with metal atoms in the conductor, they transfer more energy. This makes the atoms vibrate more and more. This increases the resistance and the temperature, and causes the shape of the graph. |
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What is a Diode? What is its symbol? |
A diode is a component which has a very high resistance in one direction. This means that current can only flow in the other direction. |
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Sketch a graph of current against potential difference for a diode: |
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What is a LED? What is its symbol? |
Light-emitting diodes (LEDs) produce light when a current flows through them in the forward direction. |
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Describe what happens to the temperature and the resistance of a filament bulb as the current flowing through it increases. Use the graph in the Figure to produce a table that shows p.d.,current and resistance for the filament bulb. |
As current gets larger, the temperature of the filament gets higher and the resistance increases. |
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Describe why a diode could be used to stop damage being done to an electronic circuit by somebody putting the batteries in the wrong way round. |
As a diode only allows current to flow in one direction, connecting the battery the wrong way round would not damage the circuit. |
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Describe the most striking difference between the current–p.d. graph for a diode compared with other components. |
The graph is not symmetrical. Current in one direction is zero but in the other rises quickly. Other components are usually symmetrical. |
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Explain why the batteries in a torch with an LED will last longer than in a torch with a filament bulb. |
The graph is not symmetrical. Current in one direction is zero but in the other rises quickly. Other components are usually symmetrical. |
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When lampshades are used, care needs to be taken to make sure that the bulb does not overheat the lampshade. Explain why this is more of a problem with a filament bulb than with compact fluorescent bulbs or LEDs. |
Filament bulbs get very hot during operation: they could scorch or burn a lampshade. Fluorescents and LEDs are more efficient and produce less heat. |
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Describe how these two statements can both be true: ‘If a larger p.d. is placed across a bulb, more current flows.’ ‘The current flow is not proportional to the applied p.d. for a filament bulb.’ |
Although increasing voltage does result in increasing current, the rise in current gets less for similar rises in voltage so the changes are not proportional. |
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Use the information in the Table to describe the arguments for and against replacing an ordinary filament bulb with either a compact fluorescent bulb or an LED. The light output from the different bulbs is roughly the same. |
Althoughthe LED bulb is the most energy efficient, the purchase costs for an LED bulbare very high compared to the filament lamp and it would take about five years for the investment in an LED bulb to pay back. The compact fluorescent bulb is a little more expensive than a filament bulb, but the pay back time is less than a year. Of the three the LED has the longest life but is still veryexpensive to replace. |
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What are the key points to remember? |
The flow of current through some components is proportional to the voltage applied to them. Diodes only let current pass in one direction. Old-style filament bulbs are mush less efficient than LEDs or compact fluorescent devices. Electrons convert energy from electrical energy to heat energy when they pass through a resistance. |
