P9 - Energy, Power and Resistance Flashcards
Draw the 14 circuit symbols.
Look in folder.
Define polarity.
Order of terminals in cells and batteries.
Define Potential Difference.
Energy transferred from electrical energy to other forms per unit charge - charge carriers losing energy.
Define a volt.
The P.D. across a component when 1J of energy is transferred per unit charge passing through a component. 1V=1JC^-1.
What is the P.D. equation?
V = W/Q
What is the emf equation?
E = W/Q
Define EMF.
The energy supplied by the battery to each charge carrier passing through. The PD of a cell when no current flows. Voltage without charge? When charge carriers gain energy.
What is the difference between P.D. and EMF?
P.D. is word done by charge carriers and EMF is work done on charge carriers.
What is eV?
Energy gained when an electron accelerates through 1V of potential difference.
How much is 1 electron volt equal to?
1eV = 1.6x10^-19 J
(derived from e = QV
Define electron gun.
A device used to produce a narrow beam of electrons with precise kinetic energies.
Define thermionic emission.
The process where free electrons are emitted from the surface of a metal when heat is applied.
Describe the process of using an electron gun.
- Electron source is small metal filament.
- Electrons gain energy when filament is heated.
- Some gain enough energy to escape the metal’s surface.
- They enter a vacuum with a high P.D. applied between anode and filament.
- They gain kinetic energy from P.D. as they travel through the vacuum.
- They form a narrow beam through a hole in the anode.
How do you derive eV = 1/2 mv^2
V = W/Q
W = QV
W = eV
W = 1/2mv^2
eV = 1/2mv^2
Why do electrons have a greater velocity than protons?
Protons have larger mass so the velocity decreases (eV = 1/2mv^2)
Define Ohm.
The resistance of a component when 1V is produced per ampere of current.
1 ohm = 1VA^-1
Define electrical resistance.
Components resist charge that flows through it. Energy is needed to move charge carriers so the higher the resistance the more energy required.
What do we know about ohmic conductors?
Fixed resistance - voltage and current are directly proportional. Straight line passes through origin.
What us the resistance if an IV graph with low gradient
Hgh resistance
What does the bar do in a diode
Stops conventional current flowing
What do you call the point when resistance increases in a diode
Threshold voltage
What are the key features of a diode
.Semiconductor
.Current flow one direction
.ones that emit light don’t get hot
.non-ohmic so v not dp to i
.resistance not constant
.dependent on polarity
Leds different colours due to different thresholds
What is resistance proportional to?
Length and resistivity - inversely to area
Define resistivity
Intrinsic property of how string a material opposed current flow whereas resistance is about one component
What is the equation for resistivity
P =RA/L in Ohmeters
What are the key features of a thermistor
.semiconducting material
.decrease resistance when temperature increases so inversely proportional in thermistor.
What is an NTC
Negative temperature coefficient
How do you measure resistivity of a wire?
Take a wire and measure area with a micrometer of vernier calliper. Then place on a ruler. Measure current and voltage across it to work out resistance and plot this against length. Multiply gradient by area to get resistivity.
What are the 4 power equations?
P = W/t
P = VI
P = I²R
P=V²/R
What is the equation for work done?
W = VIt
What is the relationship between resistance and light intensity?
Indirectly proportional
What is the LDRs properties
Semiconductor
Number of free electrons increases with light intensity.
Cricket, phones, laptops, street lights.
How do you investigate LDRs?
Move it away from light source or change p.d.
What is the graph of R against LI?
Non linear curve going down, as LI increased, the rate of change of resistance decreases.
What is electricity measured in?
KWh
What is 1 kWh in J?
3.6 MJ
