9 - Energy, power, and resistance Flashcards
What is Thermionic emission?
Thermionic emission is when a heated object emits electrons (thermal energy is transferred to the electrons)
How does an electron gun work?
A small metal filament is heated, known as a cathode.
Some gain enough (thermal) energy to escape the surface of the metal (thermionic emission).
A high voltage is applied between the filament and the anode inside a Vacuum.
The electrons in the wire gain kinetic energy as they accelerate towards the anode.
A small hole in the anode allows a specific line of electrons to pass through, creating a beam.
What is eV?
eV is a derivation of the formula for work done:
V = W/Q —> W = QV
QV = eV
(eV = ½mv²)
Describe the symbol and function of a Light-dependent resistor (LDR).
Resistance value changes based on light exposure.
(Resistance inversely proportional to light level)
Describe the symbol and function of a Fuse.
‘Blows’ open at high fault currents.
Describe the symbol and function of an LED.
Emits light when powered.
Describe the symbol and function of a Thermistor.
Resistance value based on temperature
(Resistance inversely proportional to temperature)
Describe the symbol and function of a Variable resistor.
Resistor that has a variable resistance.
What is voltage?
Voltage is the energy transferred by the charge carriers.
V = W/Q
1V = 1JC⁻¹
What is EMF (Electromotive force)?
EMF is the energy transferred to the charge carriers (by the cell/ battery)
It is also the term used to describe when work is done
ε = W/Q
What is resistance?
Resistance is the ratio between the voltage across a component and the current flowing through it.
R = V/ I
1Ω = 1VA⁻¹
What is Ohm’s law?
For a metallic conductor at constant temperature:
Voltage ∝ Current
V = IR
What is the I-V characteristic for a fixed resistor?
The I-V characteristic for a fixed resistor is a straight line through the origin.
As the resistor obeys Ohm’s law, it is an ohmic conductor.
Polarity is independent to behaviour.
What is the I-V characteristic for a filament lamp?
A filament lamp is a non-ohmic component; it does not obey Ohm’s law.
Polarity is independent to behaviour.
What is the I-V characteristic for a diode?
A filament lamp is a non-ohmic component; it does not obey Ohm’s law.
The diode’s behaviour is dependent on polarity
What is the I-V characteristic for a thermistor?
How does the resistance of an LDR change with light intensity?
What is the resistivity?
Resistivity is the measure of how much a particular material opposes electron flow.
resistivity = (resistance x cross-sectional area)/length
ρ = RA/L
Units: Ωm (ohm-metre)
What is the relationship between resistance and cross-sectional area (of a wire)?
Resistance (R) is inversely proportional to it’s cross sectional area (A)
R ∝ 1/A
What is superconductivity?
When some materials are cooled, their resistivity drops.
However, at a critical temperature, the resistivity suddenly drops to zero.
What is electrical power?
Electrical power is the rate of energy transfer by each electrical component.
electrical power = p.d. x current
P = VI
What are the 3 alternate equations for electrical power?
P = VQ/t
V = IR
I = V/R
∴ P = I²R
∴ P = V²/R
How is energy transferred over time calculated?
P = W/t
∴ W = Pt
P = VI
∴ W = VIt
What is the formula for energy transferred by an electrical device?
W = Pt
What is 1 kWh in joules?
1 kWh = 3.6MJ
