DC electricity Flashcards
Define current
The rate of transfer of charge
Define current quantitively
ΔQ/Δt
Define potential difference
The work done per unit between two points in a circuit
Define resistance
How difficult it is for charge carriers to pass through a component caused by the ions resisting the flow of charge
Define potential difference quantitively
ΔV = ΔE / Q
What is the gradient of a charge / time graph?
Current
What is the area under a Current / time graph?
The total charge transferred
How do you take the resistance from a non linear potential difference / current graph?
Take V at point at I / I
NOT GRADIENT as R is NOT ΔV/ΔI just V/I
Define power
The rate of energy transfer
What direction is conventional current?
Positive too negative , counter intuitively against the flow of electrons.
What end of a cell is the +ive / -ive end?
‘Long bit’ +ive
‘Short bit’ -ive
What direction is the potential difference across components?
- ive too + ive across a power supply
Against the flow of current across any other component
What is Kirchhoff’s voltage law?
The sum of directed voltages around any loop is zero
What is Kirchhoff’s current law?
The sum of currents into a branch = the sum of currents out
What is the emf?
The work done in moving 1 Coulomb of charge completely around the circuit.
The voltage across the source ε
What equation links the total resistance, emf and current of the cell?
Rt = ε / I cell
What is the resistance of an ideal ammeter? Why?
ZERO
So that resistance will not affect the measurement of current in a circuit
What is the resistance of an ideal voltmeter? Why?
INFINITE
As there should be no current flowing through the voltmeter
Filament lamp power rating assumptions
The resistance is constant
Brightness is proportional too power
What is ohms law?
The current through a component is directly proportional too the potential difference across it, provided the physical conditions do not change. (e.g. temperature)
Explain the current voltage graph of a filament lamp
As the pd increases, the current increases and so there are more collisions between the electrons every second. The vibration of the metal lattice increases as the temperature increases increasing the resistance.
Describe an ohmic graph
Linear graph through the origin
What is the relationship between temperature and resistance?
As the temperature increases, the particles vibrate more as their internal energy increases and there are more collisions per second increasing the resistance.
Describe an R/ T graph for a metal or conductor
Linear with a positive temperature gradient
Describe the R/T graph for semi conductors and thermistors
Negative temperature gradient (decaying exponential)
Explain the behaviour of a semi conductor as temperature increases
The electrons gain more energy so the number of charge carriers (electrons that can cross the valence/conduction band gap) increases so there is a large current per given voltage and the resistance decreases.
What is a super conductor?
A material who’s Resistance, R, drops to 0 ohms below critical temperature
Why are super conductors useful?
They do not loose any energy due too I²R heating so efficient anergy transfers (below the critical temperature)
Can easily get high currents so good at inducing strong magnetic fields
What is the resistivity of a material?
Material property describing how much the material opposes the flow of charge through it
What is resistivity measured in?
Ωm
What is a potential divider?
A circuit that returns an output voltage a fraction of its input voltage
What is the relationship between light intensity of an LDR and the resistance though it?
As light intensity increases, resistance decreases
What is the potential divider equation?
V out = (ε . R potential divider) / R total
What is internal resistance?
The resistance inside a source of emf which causes the source potential difference to drop when a current flows through the source.
Why does the reading across a source with internal resistance change when the switch is closed?
When the switch is open, no current flows through the source, and the voltmeter reads the value of the emf.
When the switch is closed, current flows through the source and there is a voltage drop to the voltage of the internal resistor and the voltmeter reads a lower value.
What is the emf equation when there is internal resistance?
ε = I . (R+r)
What is the maximum power theorem?
The maximum power transferred to the load of a circuit is when the load resistance is equal too the internal resistance
What is the external power dissipation of a circuit (with internal resistance)?
I = ε / Rt Rt = (R + r)
P = I² R
P = ε² .R / (R + r)²
