Chapter 9 Energy, Power and Resistance Flashcards
What is a battery?
It is a group of cells in series or parrallel
LEARN THE SYMBOLS FOR ELECTRIC CIRCUITES
DO IT
What is a volt?
It is the potential difference between two points on a circuit
What is the unit and SI derivation of a volt?
The unit is Volts (V) and si units JC^-1
What is the equation for a volt?
Volt = Work / Coulombs
How is a volt meter used?
It is used in parallel with the circuit
What is electromotive force? What is the symbol?
This is used to describe when work is being done on the charge carriers as they pass through a battery. It is the energy gained per coulomb passing through a battery so has the units Volts.
The symbol is ε
What is the equation for electromotive force?
ε = W / Q
How is energy transfer calculated from voltage?
Work = Voltage x coulombs
or
Work = Electromotive force x coulombs
How does an electron gun work?
A hot filament in a vacuum is heated and given a negative charge (cathode) then an anode is put at the other end of the vacuum and the electrons released from the cathode are attracted to the anode and accelerate
What is the equation for an electron gun?
eV = 1/2mv^2 e = elementary charge V = potential difference of the cathode and anode m = mass v = velocity
How can the acceleration of electrons be changed?
By changing the potential difference of the cathode and anode
What is the most important equation for electricity?
V = IR
What is resistance?
It is a measure of how hard it is for current to flow through a material
What is the equation for resistance?
R = V / I
What is the unit for resistance? and SI derivative and symbol?
The ohm
V A^-1
Ω
What is ohms law?
As long as temperature remains constant, the current through a material is directly proportional to the voltage across it.
How does temperature affect resistance?
The wire gets hot because of collisions between electrons and the positive ions in the wire. This heat causes to positive ions to vibrate more and this causes the frequency of collisions to increase causing more collisions. So as temperature increases so does the reisistance
What material is used for fillaments?
Tungsten
What does the IV graph for a resistor look like?
It is a straight line through the origin where the gradient is 1/resistance
It is ohmic because it obeys ohms law
Filament lamps
The gradient starts even (middle e.g. 1 ) and then flattens out as the voltage increases. As the voltage increases the resistance increases
It is non ohmic because the gradient is not constant
What is a diode?
It is a component that only allows current to flow in one direction. The resistance to current in the reverse direction is very high
What is an LED?
It is light emitting diode that is made from a material the releases light when electricity passes through it.
What is special about the light from an LED?
It is of a specific single wavelength
What does the IV curve for a diode look like?
It starts flat (high resistance) and then past the threshold voltage the gradient increases and becomes much steeper (lower resistance)
Then when the current is in the reverse direction the resistance is almost infinate.
What are factors that affect the resistance of a wire?
Material
Length of wire
Cross-sectional area
Temperature
What are praportionalities that can be drawn from the resistance of a wire?
R ∝ L
R ∝ 1/A
What is resistivity?
resistance refers to a specific component and resistivity is used to describe the electrical properties of a material
It is the resistance of a component multiplied by its cross sectional area per unit length
What is the symbol of resistivity? and what is its unit?
ρ and units Ωm
What is the equation for resistivity?
R = ρL / A
What affects resistivity?
The temperature
What is the order of resistivity for conductors?
10^-8 Ω
What is the order of resistivity for insulators?
10^16 Ω
What is a negative temperature coefficient?
It means that as the temperature increases, the resistance drops
What is a thermistor?
It is a component thats resistance changes with temperature
What are thermistors made from?
They are made from semiconductors with a negative temperature coefficient
What are uses for thermistors?
Thermometers
Thermostats and air conditioning
Inside computers
Car engines
What graph can be made from placing a resistor in hot water with a thermometer?
The Resistance of the thermistor can be calculated with a ammeter and a voltmeter and then plotted against temperature as the water cools
What are the IV characteristics of a thermistor?
They are non ohmic and produce a graph oposite to a filament lamp. The gradient increases (so resistance decreases) as the voltage increases.
What is an LDR and what does it do?
Light dependent resistor
A component thats resistance changes as light is shone on it
What are the properties of an LDR?
In the dark they have a high resistance
In the light they have a low resistance
How does distance from a light source affect an LDR?
The further the distance the lower the light intensity so the further the distance the higher the resistance
What is the equation to work out electrical power?
P = VI
What is the unit for power?
Watts (W)
What are alternative equations for electrical power?
P = I^2R P = V^2 / R
How is the P = IV derrived?
P = Work / Time P = Voltage x Coulombs / Time P = Voltage x Current
How is energy transferred calculated?
Watts x time = Joules transferred (Work)
Work = VIt
What is the definition of a kilowatt hour?
It is the energy transferred by a 1 kilowatt device operating for 1 hour
What is the symbol for kilowatt hour?
kWh
