Electricity Flashcards
Electric field
A region in which an electric charge will experience a force.
Electric current
The rate of flow of charge.
Kirchoff’s 1st Law
The total current into a junction is equal to the total current of the junction.
Mean drift velocity
-> Average distance travelled by the electrons along the wire per second.
They move slowly in one direction through the metal lattice, because they are continually colliding with other electrons and the ions in a short distance.
Number Density
The number of charge carriers per cubic metre. (m^-3)
+How does number density vary for conductors, semi-conductors and insulators?
-> Conductors: Will have a much larger value of n as metals
-> Semi-conductors: Will have a lower value of n compared to metals but higher than insulators
-> Insulators: Will have the smallest value of n compared to semi-conductors and conductors
Variation of number density with temperature:
-> A wire can become hot due to the the heating effect of an electric current. For a material such as a conductor the number density will not be changed. But for a semi-conductor (such as a thermistor) if the temperature increases, the agitation of the ions in the material actually releases electrons thus the number density increases. This actually results in the resistance of the thermistor decreasing, for the same applied p.d.
Kirchoff’s 2nd Law
Sum of e.m.f.’s is equal to the sum of p.d.’s in a closed loop.
- In Kirchoff’s 2nd Law energy is conserved so this law is called a conservation of energy law.
E.m.f.
Work done per unit charge. The electromotive force of a cell is the amount of energy transformed from other forms into electrical per unit charge.
P.d.
Work done per unit charge. The potential difference across a component is the amount of energy transformed from electrical into other forms per unit charge.
Volt
1 Volt = 1 Joule per Coulomb
Resistance
Resistance of a component is the ratio of potential difference across the component through it. Resistance opposes current.
Ohm
1 Ohm is 1 Volt per Amp
Ohm’s Law
A component obeys Ohm’s law if the current through the component is directly proportional to the potential difference across it provided the temperature of the component remains constant.
Threshold Voltage
This is the voltage at which the diode conducts as the current starts to increase.
Resistivity
The resistivity of a material is = resistance x cross sectional area of material/ length of material
Power
The rate at which energy is transferred.
Watt
1 Watt is 1 Joule per second
Why does a NTC thermistor’s resistance decrease when temperature increases?
When temperature increases, the amplitude of vibration of ions in the material increases, whuch causes the release of electrons, thus the number density increases. The resistance therefore decreases for the same p.d.
Why does an LDR’s resistance decrease as light intensity increases?
The LDR has a resistance affected by light intensity. As the light intensity exposed to the LDR increases, the resistance decreases. This is because the light provides energy to release electrons from the sensitive material thus the number density of the LDR material increases and resistance decreases for the same applied p.d.
1kWh
-> 1 kWh is the energy used by a device of power 1kW used for 1 hour
-> 1kWh is equal to 3.6 million Joules
-Find energy transferred (kWh)
-Find cost for kWh
-> Energy transferred (kWh) = Power (kW) x Time (hours)
-> Cost = Units used (kWh) x cost per unit
How to calculate total resistance in a series circuit?
R = R1 + R2 + R3 +…
Internal Resistance
Resistance at the source of emf (e.g. cell) due to its construction.
How to calculate total resistance for parallel circuits?
1/R = 1/R1 + 1/R2 + 1/R3 +…
Terminal P.d.
The potential difference across the terminals of a cell.
If there was no internal resistance, the terminal p.d. would be equal to the emf
What does ‘lost volts’ mean?
Work done per unit charge from electrical to thermal energy inside a power source (cell) with internal resistance. Lost volts = E-V. Where E is emf and V is terminal p.d.
Under what circumstances is terminal p.d. equal to e.m.f?
When current is zero or when there is negligible internal resistance.
Derive the equation for total resistance for resistors in parallel:
I = I1 + I2 + I3 +… (due to Kirchoff 1st Law) V/R = V/R1 + V/R2 + V/R3 +… (p.d. is the same due to Kirchoff’s 2nd Law)
How is internal resistance represented in a circuit diagram?
By a resistor in series with the power supply both enclosed with a dashed box.
Derive the equation E=I(r+r)
E = V + lost volts and V = IR,
E = V + Ir , E = IR + Ir. Since I is the same due to Kirchoff’s 1st Law then E = I(R+r)
What is the advantage of a potentiometer over a potential divider?
Output V has a wider range (can be varied from 0V to the maximum source p.d.)
Derive the equation for total resistance for resistors in series:
V = V1 + V2 + V3 + … (due to Kirchoff’s 2nd Law) IV = IR1 + IR2 +IR3 + … (current is the same at all points due to Kirchoff’s 1st Law) so R = R1 + R2 + R3 +…
How should a voltmeter be connected in a circuit?
In parallel with the component you’re measuring the p.d. through
Which conservation law leads to Kirchoff’s first law?
Conservation of charge
Define the coulomb
The charge that passes in 1 second when the current is 1 ampere (1C=1As)
Descrive the I-V graph for a diode or LED
Zero I for any V at all negative V and small positive V, gradient increases rapidly as V increases in positive direction.
A metal has a positive temperature coefficient for resistance, what does this mean?
Its resistance increases with temperature
Explain what is meant by charge being quantised:
The charge on an object is always a multiple of elementary charge, e
What are the similarities between emf and pd?
-> They are both work done per unit charge
->They are both measured in volts
What is the name for a conductor that obeys Ohm’s Law?
Ohmic conductor
Compare the direction of conventional current and electron flow in a circuit:
-> Conventional : Positive to negative
-> Electron flow: Negative to positive
What are charge carriers in a) metals b) electrolytes?
-> Metals: Delocalised electrons
-> Electrolytes: Ions
Describe the motion of electrons in a wire with a potential difference across it:
Random and high speed colliding with positive ions, with overall low average speed in one direction (mean drift velocity)
What’s the difference between emf and pd?
-Emf is work done per unit charge on the charge carriers
-Pd is work done per unit charge by the charge carriers
Emf is the energy transferred from other forms to electrical, whereas pd is energy transferred from electrical to other forms.
Derive I = Anev for a conductor length L and cross-sectional area A:
-> Number of charge carriers = ALn
-> Total charge = Q = ALne
-> v=s/t=L/t so t=L/v
-> I =▲Q/▲t
-> I = ALne/(L/v)
-> so I = Anev
For electrons accelerated through a potential difference give an expression for kinetic energy:
eV = 1/2 mv^2
What is the resistance of ideal ammeters and voltmeters?
-> Ideal ammeters have zero resistance
-> Ideal voltmeters have infinite resistance
Explain how resistance changes for a filament lamp as V increases:
Resistance increases because V increases, I increases and therefore temperature increases (heating effect of a current) so R increases
Explain how resistance changes for a diode or LED:
For negative V, R is infinite(or very high), for positive V, R is initially very high and then decreases rapidly because the number density of free electrons increases.
How does the average speed of a free electron compare to mean drift velocity?
Electron speed»_space; Mean drift velocity
Why is the average speed of an electron higher than the mean drift velocity?
- Average speed : This refers to the overall speed of electrons as they move randomly within a conductor. They move fast due to thermal energy.
-Mean drift velocity: This is the average velocity of the electrons in a specified direction when an electric field is applied. While the electrons do have a high average speed, they collide with atoms and other obstacles which slows their net movement down.
State the charge on a proton and an electron in terms of elementary charge:
Electron : -e (-1.6x10^-19)
Proton : +e (+1.6x10^-19)
