Electricity Flashcards
Define Current, its units and base units
the rate of flow of charge per unit time, amperes, [Cs-1 ]
What is the equation linking I, Q and t
-
I = Q/t
- [Cs-1]= C/s
What is the elementary charge, e ?
1.6 x 10-19C
What must the charge on any object be?
It must be an integer of the elementary charge, expressed as Q = ne ; where n is the number of electrons either added or removed
modelling electric current in metals
how can the structure of a metal be thought of as
a regular crystal structure or lattice of positive ions, surrounded by a number of delocalised electrons.
why may there be a larger current in a circuit?
- a greater number of electrons moving past a given point each second
- the same number of electrons moving faster through the metal
what is conventional current
it is when current flows from a positive terminal towards a negative one, it is opposite the flow of electrons
what is an electrolyte, and what are its charge carriers
it is a liquid, ionic solution, that can carry an electric current , the electric current is a flow of ions
what happens if salt is dossolved in water
as it is an ionic solution the salt seperates into positivley charged sodiumions, cations , Na+ , and negativey charged chlorine ions, anions, Cl-. If an anode and cathode are placed in the solution, ions are attracted to the electrode. when the Na ions reach the cathode they accept an electron and the Cl ions donate an electron
what is used to measure current
Ammeters are used to measure the electrical current at any point in a circuit, placed in series
what is an important property of ammeters
they should have the lowest possible resistance in order to reduce the effect they have on the current. the ideal ammetrer has 0 resistance.
What does the conservation of charge state
electric charge can neither be created nor destroyed, the total amount of electric charge in the universe is constant.
What is kirchhoff;s first law
- the sum of currents into a junction is equal to the sum of current leaving a junction
- ∑Iin = ∑Iout
what is number density
it is the number of free charge carriers per unit volume
what is the average number density of conductors, semi conductors and insulators
- 1029 m-3
- 1019 m-3
- 109 m-3
What is an equation for I, linking, I,A,n,e,v
I = A n e v
what is the effect of changing the cross sectional are of a wire
the narrower the wire the greater the drift velocity must be in order for the current to be the same
Why does increasing the temperature increase resistance
as temp increases the positive ions have more internal energy and vibrate with greater amplitude about their mean positions. the frequency of collisions between the charge carriers and the positive ions increases, so the charge carriers too more work to transfer more energy as they travel
what is potential difference
it is a measure of the transfer of energy by charge carrier
what is one volt equal to
- 1 V = 1 J C-1
*
What is the equation for Voltage
V = W/Q
What is used to measure voltage and how
a voltmeter is used in parallel, it should have infinite resistance so that no current passes through the voltmeter itself
potential difference Is used to describe…
when work is done by the charge carriers, they are losing energy as they pass through the component.
define electromotive force
when work is done on the charge carriers. they gain energy as they pass through a cell.
what is the formula for e.m.f
Ɛ = W/Q
What is a formula for work done
W = VQ
W = ƐQ
How does an electron gun work?
- a small metal filament is heated by an electric current
- the electrons gain enough KE to escape the surface of the material
- this process is called thermionic emission
- the filament acts asa cathode, and the freed electrons accelerate to the anode
- if the anode has a small hole in it the electrons pass through it creating a beam of energy
what is the formula for kinetic energy of an electron
- e*V = ½mv2
- e is the elementary charge and V is the accelerating p.d.
what is resistance?
when a component or material resists the flow of charge carriers through it.
What is the formula for resistance
R = V/I
what are the base units for ohm
1 Ω = 1 V A-1
What is ohm’s law
for a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d. across its ends.
what are the properties of filament lamps
- p.d is not directly proportional to the current
- it is a non-ohmic component
- the resistance is not constant
- behaves in the same way regardless of polarity
why is there a non constant resistance in filament lamps?
- the wire gets so hot it glows
- the current increases, more electrons per second pass through it
- more collisions occur between electrons and positive metal ions
- this transfers energy to the ions causing them to vibrate more
- and this results in them colliding with more electrons
what are the properties of a diode?
- p.d. is more directly proportional to current through it
- non-ohmic component
- the resistance is not constant
- diodes behaviour depends on the polarity
what is the threshold p.d of a diode?
0.7 V
What occurs once the threshold voltage of a diode is reached?
the resistance drops sharply for every small increase in p.d.
what is resistivity
a term used to describe the electrical property of a material.
what is the relationship between resistance and length?
doubling the length doubles the p.d so the resistance must have doubled.
- R ∝ L
What is the relationship between cross-sectional area and resistance?
- As area increases the resistance drops. doubling the are will double the current so the resistance must have halved
- R ∝ A-1
What is the formula for resistivity?
- since R ∝ L and R ∝ A-1, it can be combined to produce
- R ∝ L/A
- R = pL / A
What is a thermistor with a negative temperature coefficient?
the resistance drops as the temperature increases, as the temperature increases in some semi-conductors the number density of the charge carriers also increase.
what is a thermistor?
An electrical component made from a semiconductor with a negative temperature coefficient
What are uses of thermistors?
- thermometers
- thermostats
- monitor temperature of components inside electrical devices
- measure temperatures
what is an LDR
a light dependant resistor changes their resistance depending on the light intensity.
What is an LDR made of and how does it work
it is made from a semiconductor in which the number density of charge carriers depends on the intensity of the incident light.
what is power
the rate of energy transfer by each electrical component
what does power depend upon?
the current in the component and the voltage across it
What is the electrical equation for power
- P = VI
- P = I2R
- P = V2 / R
How to derive P = VI
- P = W / t
- V = W/Q rearrange W = VQ
- substitute P = VQ / t
- Q / t = I therefore P = VI
how do you calculate the energy transferred
W = VIt
- because W =Pt, sub P = VI
what does the energy transferred to each electrical device depend upon
- the power of the device
- how long the device is used for
What is a Kilowatt-hour
the energy transferred by a device with a power of 1kW operating for a time of 1 hour. = 3.6MJ
formula for energy transferred
J = Wt
What is kirchhoffs second law
in any circuit, the sum of the electromotive forces is equal to the sum of the p.d.s around a closed loop.
- essentially the total energy transferred to the charges in a circtuit is always equal to the total energy transferred from the charges as they move around the circuit
Derive R = R1 + R2 … from adding resistance in series
- kirchoffs second law, V = V1 + V2 ……
- this can be written as IR = IR1 + IR2 ….. as V = IR
- according the kirchhoffs first law the current through each resistor must be the same so I is a constant giving R = R1 + R2 ….
derive 1/R = 1/R1 + 1/R2 … from adding resistance in parallel
- kirchoffs first law I = I1 + I2 + ….
- Kirchoffs second law. the p.d. across each resistor is constant and must be equal to V. dividing the 1st equation by V gives
- I/V = I1/V + I2/V + ….
- as V = IR 1/R= 1/R1 + 1/R2
Explain the lost volts
- when there is current in a power sourcce work has to be done by the charges as they move through the power source
- hence some energy is transferred into heat when there is a current in the power source and not all the energy is transferred to the charge is available for the circuit
formula for emf
- ε = V + Ir
- ε = IR +Ir = I(R+r)
- where V is the terminal p.d
- I is the current through the powersupply
- r is the internal resistance of the power supply
