P2 Resistance and I-V Characteristics (page 181) Flashcards
What do ‘Ohmic’ Conductors have?
they have a constant resistance.
For some components, the current through them is changed, what else changes too?
The resistance of the component changes as well.
The resistance of Ohmic Conductors (e.g. a wire or a resistor) does or doesn’t change with the current, and why?
It doesn’t change with the current. It is at a constant temperature, so the current flowing through an ohmic conductor is directly proportional to the potential difference across it (R is a constant in V = IR, see page 180).
The resistance of some resistors and components DOES change, give examples?
e.g. a diode or a filament lamp.
Explain why a filament lamp DOES have a resistance?
When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament (see page 167), which is designed to heat up. Resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases.
Explain why Diodes DOES have a resistance?
Diodes resistance depends on the direction of the current. They will happily let current flow in one direction, but have a very high resistance if it is reversed.
What do the term I-V characteristic refer to?
it refers to a graph which shows how the current (I) flowing through a component changes as the potential differences (V) across it is increased.
Do Linear components have an I-V characteristic that’s a straight line or curved line?
a straight I-V characteristic line (e.g. a fixed resistor).
Do a Non-Linear component have a straight or curved I-V characteristic?
a curved I-V characteristic line (e.g. a filament lamp or a diode).
Tell me the process of an experiment to find out a component’s I-V characteristic?
1) Set up a test circuit shown on diagram 1 page 181.
2) Begin to vary the variable resistor. This alters the current flowing through the circuit and the potential difference across the component.
3) Take several pairs of readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes. Repeat each reading twice more to get an average pd at each current (pd - potential difference).
4) Swap over the wires connected to the battery, so the direction of the current is reversed.
5) Plot a graph of current against voltage for the component.
6) The I-V characteristics you get for an ohmic conductor, filament lamp and diode should look like the diagrams on page 181).
What example is a Ohmic Conductor?
e.g. resistor at a constant temperature.
The current through an ohmic conductor (at constant temperature) is directly proportional to potential difference so you get a straigh line.
What example is a Filiment Lamp graph?
As the current increases, the temperature of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower - hense the curve. (middle diagram on page 181).
What example is a Diode graph?
Current will only flow through a diode in one direction as shown in diagram 3 page 181. The diode has a very high resistance in the reverse direction.
Make sure you undestand the graphs for your exam.
Since V = IR, you can calculate the resistance at any point on the I-V characteristic, how?
by calculating R= V/I.
Explain the shape of the filiment lamp (I-V characteristic on page 181, for the quadrant where I and V are positive (3 marks)
As the current through the lamp increases, the temperature of its filament increases (1 mark), causing its resistance to increase (1 mark). A larger resistance means less current can flow per unit potential difference, and so the graph gets shallower (1 mark).
