Current-Voltage Characteristics Flashcards
1
Q
What is current?
A
- The rate of flow of positive charge carriers
2
Q
What is potential difference?
A
- the work done per unit charge
3
Q
What is resistance?
A
- Opposition to current
- The higher the resistance the lower the current
4
Q
What is Ohm’s Law?
A
- For a conductor at a constant temperature, the current through it is directly proportional to the potential difference across it
- Ohm’s law only true for ohmic conductors under constant physical conditions
- Ohm’s law is special case, lots of components aren’t conductors and have characteristic current-voltage graphs of their very own
5
Q
What is the gradient of a directly proportional I-V graph?
A
- Change in I / Change in V = 1/R
- Resistance is constant if it’s a straight line graph
6
Q
What are the ideal voltmeters and ammeters?
A
- Voltmeters with infinite resistance so no current flows through it
- Ammeter with no resistance so no potential difference across it
7
Q
What factors have a significant effect on resistance?
A
- Light level and Temperature
8
Q
Describe the I-V characteristics for an ohmic conductor
A
- I–V graph is a straight line through the origin
- The current is directly proportional to the potential difference
- E.g. a fixed resistor
- The steeper the straight line, the lower the resistance
9
Q
Describe the I-V characteristics for a semiconductor diode
A
- I–V graph is a horizontal line that goes sharply upwards
- A diode is used in a circuit to allow current to flow only in a specific direction
- If current is in the direction of the arrowhead symbol, this is forward bias. Shown via sharp increase in potential difference and current on the right side of the graph
- When diode is switched around, it does not conduct and is called reverse bias. Shown via zero reading of current or potential difference on the left side of the graph
10
Q
Describe the I-V characteristics for a filament lamp
A
- I–V graph has an ‘S’ shaped curve
- Shows the current increasing at a proportionally slower rate than the potential difference
- As current increases, the temperature of filament in the lamp increases
- filament is a metal, the higher temperature causes an increase in resistance
- Resistance opposes the current, causing the current to increase at a slower rate
- Resistance constant where graph is straight line
- Resistance increases as graph curves
- Filament lamp obeys Ohm’s Law for small voltages
11
Q
What are charge carriers?
A
- Negatively charged electrons that move under the influence of a voltage to create an electric current.
12
Q
Describe I-V characteristics of a thermistor
A
- i-V graph opposite of filament lamp
- Thermistor is resistor with resistance that depends on its temperature
- Negative Temperature Coefficient thermistor only need to know, meaning resistance decreases as temperature goes up
13
Q
Why does resistance of thermistor decrease as temperature rises?
A
- Warming thermistor gives more electrons enough energy to escape from their atoms
- More charge carriers can be released and is available, so resistance is lower as more current is flowing
14
Q
Explain why the I-V graph of a filament lamp is an S curve
A
- Current flows causing there to be heat energy released from filament metal, as current increases, temperature increases, and resistance increases
- Particles in metal vibrate more due to increased temperature
- Vibrations make it more difficult for charge carriers to get through, decreasing current and increasing resistance
15
Q
How does the temperature affecting resistance vary in a filament lamp and thermistor?
A
- For most resistors there is a limit to the amount of current that can flow through them
- In a filament lamp, more current means increase in temperature which causes an increase in resistance, causing current to decrease. Reason why I-V graph of filament lamp levels off at high currents
- In an NTC thermistor, as current increases, increasing the temperature, resistance decreases
