Electricity Flashcards
Define Current and give its formula
Current is the rate of flow of charge (I = Q/t) In formula sheet
- aka flow of charge per unit time.
Define Potential Difference and give its equation.
The energy transferred per unit charge between two points in a circuit (V = W/Q) On formula sheet
Define resistance and its formula
The difficulty of charge carriers to flow within a component, R = V/I
Define ohms law? and an ohmic conductor
Current is directly proportional to potential difference across it given that physical conditions are kept constant e.g temperature (Straight V/I graph) - constant resistance and obeys ohms law.
State the characteristics of a semiconducting diode.
Forward bias - where current can flow easily past the threshold voltage (0.6V)
Reverse bias - heavy resistance where only a small amount of current can flow
Graph looks like I/V straight until threshold voltage is met where is spikes
State the characteristics of a filament lamp?
Contains a tungsten wire that heats up as current increases therefore resistance increases. At low current resistance is low so ohms law is somewhat obeyed. (I/V) graph is curved when current is too high or low so potential difference increases at a slower rate.
What is assumed about ammeters and voltmeters
Ammeters have zero resistance and voltmeters have infinite resistance meaning no current flows through them.
Define resistivity
It is the measure of how difficult it is how a material to conduct electricity.
Explain the equation p = RA/L (resistivity = Resistancecross sectional area/ Length)
Units (Ohmmeter = ohmsm^2/m)
Resistivity is dependent on environmental factors like temperature.
Explain why when the temperature of a conductor increases, the resistance increases.
This is due to atoms gaining kinetic energy where charge carriers would collide more frequently which slows them down therefore current decreases whilst resistance increases.
Explain why thermistors are negative temperature coefficient?
As temperature increases their resistance increases. This is because increasing the temperature would lead to electrons being emitted from atoms so the number of charge carriers increases leading to current increasing while resistance decreases.
What are superconductors
When a critical temperature is met, a material has zero resistivity:
Uses include, Power cables to reduce energy loss via heating and Strong magnetic fields used in maglev trains for zero friction.
Resistance formula for series and parallel circuits:
Series: R Total = R1 + R2 + R3
Parallel 1/R total = 1/R1 + 1/R2 + 1/R3
Sometimes you must use both equation in a circuit.
What is power
It is energy transferred over time (P = E/t)
What are the characteristics of a series circuit?
The current is the same everywhere in a circuit
The battery P.d is shared across all components of a circuit so the total sum of voltages across all component is equal to the supply P.d
What are the characteristics of a parallel circuit?
Sum of currents in each parallel set branches is equal to the total current
The potential difference across each branch is the same
In series and parallel circuit how are total voltages calculated?
Series: V total = V1 + V2 + V3
Parallel: V total = V1 = V2 = V3
If the cells are identical
What is Kirchhoff’s first law?
In DC currents where charge and energy are always conserved:
- the total current flowing into a junction is equal to the current flowing out of current (showing the charge is conserved)
What is Kirchoff’s Second law?
In DC circuits, charge and energy are always conserved:
- the sum of all voltages in a series current is equal to the battery voltage. This shows no energy is lost at any point in a circuit.
What is potential divider?
It is a circuit with resistors in series connect across a voltage source used to produce a required fractions of the source of potential difference, which remains constant.
The divider can provide a variable potential difference by using variable potential resistors.
Because V = IR if you increase the resistance of R1, the p.d will decrease as circuit current has decreased.
What happens in a circuit when an LDR is exposed to high light intensity
The resistance would decrease in a circuit
If R1 is a LDR and R2 is where Voltage output occurs what happens to Vout when LDR is exposed to lower light intensity
The resistance on the LDR would increase so the current flowing into R2 would decrease. Since V=IR then the potential difference out would decrease as the current has decreased.
What causes internal resistance the P.d supply?
Electrons colliding in the battery so energy is lost before electrons leave the battery.
Therefore R total = R + r (r = internal resistance)
Define Electromotive force (EMF)?
The energy transferred by a cell per coloumb of charge that passes through it. emf = E/Q
(the amount of energy transferred by a source to each unit of charge passing through it)
Emf is the product of the total resistance and current of the circuit as (V = IR)
emf = IR + Ir
Emf can be measured by a cell with a voltmeter on a supply with no current
When internal resistance is taken into consideration, what is the PD across resistor R and PD across resistor r known as
R - terminal voltage
r - lost volts (energy wasted by the cell)
