General notes Flashcards
What is electric current?
Rate of flow of charged particles
What is the equation for current?
I=ΔQ/Δt
What is potential difference?
Energy transferred per unit charge
What is the equation for p.d.?
V=E/Q
What is resistance
How difficult it is for charge carriers to pass through a component
What is the equation for resistance?
R=V/I
What is ohm’s law?
For an ohmic conductor, current is directly proportional to the p.d across it, given that other conditions are kept constant.
What is the principle of charge conservation?
The total electric charge in a closed circuit does not change, only if other conditions remain constant
What is Kirchoff’s first law?
- Uses principle of charge conservation
- The current flowing into a junction is equal to the current flowing out of a junction
How is current distributed in a series circuit?
The current is the same everywhere in the circuit
How is current distributed in a parallel circuit
The sum of the currents in each parallel branch is equal to the total current.
What is the principle of energy conservation?
Energy cannot be created or destroyed, but can be transferred from one form to another. Therefore the total energy of a closed system remains constant.
What is Kirchoff’s second law?
In any closed loop, the sum of p.ds in the loop is the same as the sum of pds around the loop.
How does pd work in a parallel circuit?
The pd accross each branch is the same
How does pd work in a series circuit?
The pd across each component is shared.H
Derive the equation for sum of resistances in series.
- V1=I1R1, V2=I2R2
- Using kirchoff’s second law, VT = V1+V2,
- We get ITRT = I1R1 + I2R2
- IT = I1 = I2, kirchoff’s first law. Divide equation by I
- We get RT = R1 + R2
Derive the equation for sum of resistances in parallel.
- I1 = I2 + I3
- VT/RT = V1/R1 + V2/R2
- 1/RT = 1/R1 +1/R2
What is power(elec)?
- Energy transferred over time
What is the electrical equation for power
P = IV
What are the electrical equations for power with I and V
P = I^2*R
P = V^2/R
Describe ohmic conductor graph
- Follows ohm’s law
- looks like straight line through origin
Describe diode graph
- Past the threshold voltage, current will flow easily.
- In direction of negative bias, little no current will flow as the resistance is very high
Describe filament bulb graph
- At low currents, ohm’s law is obeyed.
- At higher currents, the current increases at a lower rate than p.d. due to the resistance of the component increasing as the wire heats up
Describe a thermistor graph
- At low currents, ohm’s law is obeyed
- At higher currents, currents increase at a higher rate than p.d., this is because increasing the temperature of the thermistor causes electrons to be released from the atoms to the conduction band
What is resistivity
A measure of how easily a material conducts electricity
What is resistivity a measure of?
Resistance of 1 metre cube of a material
What effects resistivity?
Temperature
What is the formula for resistivity
R = ρl/A
How do you get to the transport equation for current?
I = ΔQ/Δt
I = Nq/Δt
I = nVq/(l/v)
- I = nVvq/l
I = nAqv
What is each part of the transport equation for current?
I: current
n: charge carrier density
A: area
q: charge of one electron
v: drift velocity
Describe the potential along a uniform current-carrying wire
Take a uniform wire with constant surface area and resistivity. As resistance length increases, resistance increases. Linking this to V=IR, we can see that V increases uniformly with the distance along it.
What is a potential divider circuit?
A circuit with several resistors in it which is used to produce a required fraction of the source potential difference, which remains constant.
What is internal resistance?
Caused by electrons colliding with each-other inside the battery. Causes some energy to be lost even before electrons leave the battery.
What is emf
energy transferred by a cell per coulomb of charge that passes through it.
What is the p.d. in a circuit minus the internal resistance called
terminal resistance
How can the emf of a battery be measured, not cp.
Using a voltmeter when there is no current in the circuit
How does temperature affect the resistance of a metal
As the temperature of the metal increases, the metal ions vibrate with a higher amplitude. Due to the higher amplitude of vibrations, electrons have more frequent collisions with the ions therefore leading to a lower drift velocity of the electrons. Due to I = naqv, as drift velocity decreases, the current will also decrease proportionally. Current and resistance are inversely proportional. So as current decreases, resistance increases.
What happens as the temperature of a semiconductor increases?
- Atoms gain energy
- Once they gain enough energy, electrons are released by thermionic emission
- Moves electrons to the conduction band
- Increases number of charge carriers, n, in the semiconductor
- Due to I = nAqv, as n increases, the current increases, outweighing the effect of lattice vibrations
- Due to Ohm’s law, as current increases, resistance decreases.
What happens as the light intensity on an LDR increases?
- Atoms gain energy from light
- Once the electrons have enough energy, they are released from the atom to join the conduction band
- Increases number of charge carriers, n, in the semiconductor
- Due to I = nAqv, as n increases, the current increases, outweighing the effect of lattice vibrations
- Due to Ohm’s law, as current increases, resistance decreases.
