Chapter 9 Energy, Power and Resistance Flashcards
Potential Differnece
Measure of transfer of energy by charge carriers
Electromotive Force
Energy transferred from chemical energy to electrical energy per unit charge
Electron Gun:
- Used to ionise particles by adding or removing electrons from atoms
Thermionic Emission
The emission of electrons from the surface of a metal
How does an electron gun work?
- Need a source of electrons
- Small metal filament heated by electric current
- Electrons gain kinetic energy
- Escape from the surface of the metal
- If heated filament is placed in a vacuum and high pd is applied between filament and anode
THEN: - Filament acts as a cathode and freed electrons accelerate towards the anode
- If anode has a hole:
- Electrons can pass through it
- Beam of electrons with a specific Ke
Electron Energy Equations:
Ke = eV
=>
eV = 1/2 m v^2
Mass of an electron:
9.11 * 10^-31
Resistance
Measure of opposition to flow in electrical circuit
Resistance Equation:
R = V / I
Ohm’s Law:
For a metallic conductor kept at a constant temp:
- Current in wire is directly proportional to the p.d. across its ends
Relationship between temp and resistance:
AS WIRE GETS HOTTER, ITS RESISTANCE INCREASES
Why:
- When temp increases, POSITIVE IONS inside the wire have more INTERNAL ENERGY and vibrate with greater amplitude about means positions
- Frequency of collisions increase
- Carriers transfer more energy
Why is filament lamp non-Ohmic?
- Increase in resistance is caused by wire getting so hot it glows
- Current increasing so rate of flow of charge increases
- More collisions between metal ions and electrons
- Collisions lead to transfer of energy from electrons to ions
- Causes ions to vibrate more
- Increasing temp
Diodes:
- Non-Ohmic Conductors
- Resistance of a diode is not constant
Factors which affect resistance:
- Material of the wire
- Temperature
- Length of the wire
- Cross-sectional area of the wire
Formula for resistivity:
R = pL / A
NTC:
- Negative Temperature Coefficient
- Resistance drops as temperature increases
Explain NTC semiconductors:
In some semiconductors, as temp increases, no. density of charge carriers increases
Thermistor
Electrical component made from semiconductor with NTC
I-V characteristic of Thermistor:
- Non- Ohmic
- Similar to Filament Lamp
LDR and light:
- As light intensity increases, resistance of LDR falls
Why does resistance of LDR change with light intensity?
When light shines onto LDR, no. density of charge carriers increases dramatically leading to a rapid decrease in resistance of component
Electrical Power:
Rate of energy transfer
Power Equation:
P = V I
What is 1 kWh is Joules?
3.6 MJ
