Chapter 9 Flashcards
What is a battery?
2 or more cells connected in series
What is the resistance of ammeters and voltmeters?
Ammeter have a resistance of 0 or near zero
Voltmeters have a resistance on infinity
Define potential difference
Work done by charge carriers per unit of charge
Measures the energy transferred by charge carriers as they pass through component which make them transfer the energy
Define 1 volt
The PD across a component when 1 J of energy is transferred per unit charge passing through a component
What is the formula of PD
V = W/Q
Define electromotive force
The work done to charge carriers per coulomb as they pass through a cell or battery
Energy transferred to electrical energy from chemical per unit charge
AKA voltage in
What is the formula for electromotive force?
E = W/Q
What does an electron gun do?
Produces and accelerates a narrow beam of electrons which have precise kinetic energies
How does an electron gun work?
Metal filament is heated by electric current(cathode)
Electrons gain kinetic energy and some have enough to leave the surface of the metal (THERMIONIC EMISSION)
The circuit is in a vacuum tube with a high PD between cathode and anode so electrons accelerate(and gain energy)
The anode has a hole in it so electrons can pass as a beam with a specific kinetic energy
What is the formula for velocity of electrons?
eV = 1/2 mv^2
Charge of electron x PD = 1/2 mass x velocity^2
What does a higher PD mean?
The electrons have more energy so the beam moves faster
Define resistance
The PD across a component divided by the current passing through the compound
A component has a resistance of 1ohm if a potential difference of 1V makes a 1A current flow
What does higher resistance mean?
Higher resistance means that it takes higher energy to push charge carriers
How can resistance be determined experimentally?
Set up a circuit with a variable power supply, ammeter in series and voltmeter in parallel
Draw a graph of IV
The reciprocal of the gradient is the resistance
What is Ohms law?
For a metallic conductor at a constant temperature, current is directly proportional to PD across the ends
Components are ohmic if they follow this law
What is the effect of temperature on resistance?
As a wire gets hotter:
Positive ions vibrate with greater amplitude about a fixed position
Frequency of collisions with charge carriers and ions increase
Charge carriers have to do more work to pass
This increases resistance so more energy must be transferred for charge carriers to travel
What does a shallow gradient of an IV graph show?
Greater resistance
What do the IV graphs of resistors and filament lamps look like?
Straight line and wavy line
What are diodes?
Components made from semiconductors which allow current to flow in only one direction
They have forward bias and infinite resistance in the opposite direction
What are LED’s?
Diodes which emir a single wavelength if light when they conduct
What is the graph characteristics of a diode?
It is a non ohmic conductor as PD is not directly proportional to I
Explain resistance in a diode
Has a threshold voltage of 0.69 v where resistance drops. This happens because number density of charge carriers increase
Before 0.69v the resistance is infinite
What is a thermistor? What do they do and how do they work?
A component made from a semiconductor which has a negative temperature coefficient.
Resistance decreases as temperature increases because increasing temperature causes bonds to break which releases electrons so number density increases
What does ntc mean?
Negative temperature coefficient: resistance drops as temperature increases because of bonds breaking which increase number density
What is the characteristic graph of a thermistor?
What is the graph of resistance vs temperature
Same as filament lamp
Non ohmic
PTC is wavy the other way
Goes from top left to bottom right as a curve and asymptote
What is a LDR?
A light dependent resistor where resistance changes depending on light intensity
It is made from semi conductors
As light intensity increases, number density increases so resistance decreases
Non ohmic relationship(same as resistance temp graph for thermistor)
What factors affect resistance?
Material of wire and it’s resistivity
Length of wire
Cross sectional area
Define resistivity
The physical property of a material. It is the resistance of a 1m length of wire with a 1m2 cross sectional area
Unit: ohm meters
Same for any shape at a given temperature
What are the relationships between resistance, length, area
R is directly proportional to length(longer wire means larger pd)
R is inversely proportional to the area
What is the formula for resistivity?
Resistivity(ohm meters) = RA/L
Resistivity at a given temperature is the product of resistance and cross sectional area divided by length
What is resistivity from a graph?
Gradient of a resistance length graph multiplied by area
Define power
The rate of energy transfer or work done per unit time
P=W/t
What are the formulae one must know for power?
P=W/t
P=I^r
P=V^2/R
P=IV
How do you do you find total energy?
Just multiply by time
W=IVt
W=I^2 R t
W=v^2/R t
What is a kilowatt hour?
Energy transferred by a device with a power of 1 kW operating for an hour
How do you calculate energy in KwH
E(kWH)=P(kW)xt(Hours)
