Electricity Flashcards
What is electric current?
- I = ΔQ/Δt where Q is charge and t is time.
- the rate of flow of positive charge
What is potential difference?
- The energy transferred per unit charge BETWEEN TWO POINTS IN A CIRCUIT.
- This can either be to the charge carriers OR to the component from the carriers
- V = W/Q where W is energy transferred and Q is charge.
What is resistance?
R = V/I in ohms
- measure of how difficult is for charge carriers to pass through a component
What is required for current to flow?
- a closed circuit with a source of emf (electromotive force) and a complete path of conducting material
What is the difference between charge carriers and electrons?
- Conduction electrons are the charge carries in metals which move while repeatedly colliding with positive ions
- in solutions, ions are the charge carriers
How does conventional current flow and how do electrons flow?
- Conventional current from + to -
- Electron flow from - to +
How many electrons are in a Coulomb?
- 6 x 10^18
How does electric current flow in a conductor?
- Electrons experience a force due to the electric field
- This causes them to accelerate
- However, they are slowed in contact with ions
- greater force from field (due to higher p.d.) leads to greater acceleration AND more frequent collisions
What is Ohm’s law?
- for an ohmic conductor, current is directly proportional to potential difference
- given constant physical conditions (e.g. temp)
What does the I-V graph of a diode look like?
- Extremely high resistance and therefore very low current in reverse bias
- For forward bias, current is able to flow with low resistance past the threshold voltage (the smallest voltage needed to allow current to flow
- For an ideal diode, once threshold voltage is reached, there is an no change in V regardless of I so causes an infinitely variable resistance.
What are the I-V characteristics of a filament lamp?
- initially obeys Ohm’s law
- heats up as current increases
- causes resistance to increase
- as current increases, the graph begins to curve due to increasing resistance
Explain the process of increasing resistance due to temperature.
- When temperature increases, the amplitude of the vibrations of the lattice ions increase
- This causes electrons to collide more frequently
- The rate of energy transfer to ions increases
- Therefore, to maintain current, more p.d. is needed
- Therefore R=V/I so R increases
What are ideal voltmeters and ammeters?
Ideal voltmeters have infinite resistance, ideal ammeters have zero resistance.
What is superconductivity?
- At a critical temperature, electrical resistivity drops to 0
- This change is abrupt and complete
- Used for generating very strong magnetic fields (medical applications) and transferring electrical energy without dissipation through heating
What is resistivity and how is it found?
- The resisitivity of a material is the resistance that 1 metre long of that material with a cross-sectional area of 1m^2 has.
- ρ = RA/l
- where R is the resistance, A is the cross-sectional area, and l is the length in m
- resistivity is measured in ohm metres
What control factors are needed when measuring resistivity?
- temperature
- material
- cross-sectional area (take repeats along diameter and average)
What are the characteristics of thermistors?
- NTC Thermistors decrease resistance as temperature increases
- temperature causes electrons to be emitted from atoms and increase charge carriers
- used in a temperature sensing circuit e.g. heating
What are LDR’s and what are their characteristics?
- LDR’s or Light-Dependent Resistors decrease resistance with increasing light intensity incident upon the resistors
- similar graph to thermistors
- used for automatic security lights
What is temperature coefficient?
Positive temperature coefficient in metals to increase resistance with temperature.
Negative temperature coefficient in thermistors to decrease resistance with temperature.
What is electrical power in circuits and how is it calculated?
- Power is the energy transferred over time
- Work done is found through W = IVt
- Therefore, power is found as P = IV
- Alternately, P = I^2 x R or P = V^2 / R
What is the total resistance in series and parallel circuits?
Series:
Rtotal = R1 + R2 +R3
Parallel:
1/Rtotal = 1/R1 + 1/R2 + 1/R3
What is Kirchoff’s first law?
- At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction
- proves that no charge is lost
What is the difference between a series and a parallel circuits?
- In series circuits, there is only one path for the current to flow, so the current is the same everywhere and the potential difference is divided between components
- In parallel circuits, the total current is the sum of the current in each branch, which all have the same potential difference.
What happens to the p.d. when connecting cells in series as compared to in parallel?
Series:
Vtotal = V1 + V2 + V3
Parallel: (if identical cells)
Vtotal = V1 = V2 = V3
- this is because current is split between branches so total p.d. same as if flowing through one cell
What is Kirchoff’s second law?
- The sum of all the voltages in a series circuit is equal to the sum of the emf.
- proves that no energy is lost
What is a potential divider circuit?
A circuit with multiple resistors connected to a p.d. source to give a required fraction of the source potential difference. This can also be used to supply a variable potential using a rheostat.
What is internal resistance?
Batteries have an internal resistance (r) is caused by electrons colliding with atoms inside, losing energy before leaving the battery
- Rtotal = R + r
where R is the load resistance
What is emf and how is it defined?
EMF or electromotive force is defined as the electrical energy produced per unit charge passing through the source
- measured in volts
- 𝜀 = E/Q
Why does EMF not equal total voltage in a circuit and how can it be found?
𝜀 = 𝐼(R + 𝑟)
where R is load resistance and r is internal resistance
What is the terminal p.d. and lost volts?
- terminal p.d. is the p.d. across a single resistor in a circuit with a battery
- 𝜀 = V + Ir
- where Ir is the lost volts that is the energy wasted per Coulomb of charge, represented by v
How do you calculate internal resistance with cells in series and parallel?
Internal resistance adds in series.
For identical cells in parallel, rT = r/n where n is the number of cells in parallel.
How do you find the ratio of p.d. across multiple resistors?
V1/V2 = R1/R2
- the ratio of the p.d.’s across each resistor is equal to the resistance ratio of two resistors
How do potential divider circuits work and where are they used?
Used in sensor circuits, increase temp with thermistor with another resistor in series causes higher fraction of p.d. around thermistor to allow more Vout to components in parallel to thermistor
Also used with LDRs to dim light with greater light in surroundings
What is a high temperature superconductor?
Any material with a critical temperature above 77K, the boiling point of liquid nitrogen
How can you use operating voltage and power to find resistance?
R = V^2 / P
