- When a current flows through the metal conductor, some of the electrical energy is transferred into heat energyand this causes the metal to heat up.
- This extra heat energy causes more vibrations which make it more difficult for the charge-carrying electrons to get through the resistor - the current can't flow as easily and the resistance increases.
- More current means an increase in temperature, which means an increase in resistance, which means the current decreases again.
- This is why an I-V graph levels off at high currents.
Why Filament Lamps Blow
- When you first switch a bulb on, the filament has a lower resistance because it's cold.
- This means that the initial current flowing through the filament will be larger than the normal current, so the filament is more likely to burn out at this time.
- The filament also heats up very quickly from cold to its operating temperature when it's switched on.
- This rapid temperature change could cause the filament wire to blow too.
Semiconductors and Diodes
- A group of materials which conduct electricity (but not as well as metals). When their temperature rises, they can release more charge carriers and their resistance decreases.
- This means they make excellent sensors for detecting changes in their environment.
- A component designed to allow current to flow in one direction only.
- Forward bias is the direction in which the current is allowed to flow.
- Most diodes require 6.0 V in the forward direction before they conduct - the threshold voltage.
- In reverse bias the resistance is very high and current flow is tiny.
- A resistor with a rsistance that depends on its temperature - it is a type of semiconductor.
- NTC - Resistance increases as the temperature goes up.
- Increasing the current through the thermistor increases its temperature.
- Its gradient of the I-V shows its resistance decreases.
- Warming the thermistor means more energy to its electrons, so more charge-carriers are availible so its resistance decreases.
- Their sensitivity to temperature makes them really good temperature sensors.
Producing an I-V Graph
1. Use a variable resistor to increase or decrease the resistance of the circuit in small, equal steps. Changing the resistance changes the amount of current flowing through the circuit.
2. For each change in resistance take a reading from the ammeter and the voltmeter.
3. Reverse the direction of the electricity flow by switching the wires connected to the power pack. Then repeat steps 1 and 2 to collect negative data.
- Need to measure resistance and temperature.
- Use a water bath to very the temperature.
- Use a digital thermometer to measure the temperature.
- Then follow the method above.