Electricty Flashcards
Electric Current
The rate of flow of charge
Coulomb
The unit of charge
Elementary charge e
1.6 × 10–19 C
Quantised
Net charge on a particle or an object is a multiple of e
Current in Metals
Movement of electrons
Current in Liquids
Movement of ions
Conventional Current
Moves from positive to negative
Electron flow
Moves from negative to positive.
Kirchhoff’s first law
The sum of currents flowing into a junction is always equal to the sum of currents flowing out of the same junction
I (symbol)
Current
A (symbol)
Area of cross section
n (symbol)
Number density
e (symbol)
Elementary charge
v (symbol)
Mean drift velocity
Conductors Number density
High
Insulators Number Density
Low
Potential Difference Definition
How much energy each unit per unit charge is being transferred at each component.
Electromotive force
E.M.F Definition
Is equal to the maximum (terminal) Pd when no current flows
How is resistance caused?
Resistance is cuased by collisions between electrons and the positive ions
R(symbol)
Resistance
ρ (symbol)
Resistivity
L (symbol)
Length
A (symbol)
Area
E.M.F vs Pd in terms of energy transfer
E.M.F is the quantity that each coulomb of charge receives, P.D is the quantity of energy that each coloumb of charge uses.
Kirchoffs Second Law
In any circuit, the sum of electromotive forces is equal to the sum of p.d.s around a closed loop ∑ε = ∑V$, where
∑ε is the sum of the e.m.f.s and ∑V is the sum of the p.d.s.
Current in a series circuit
The current is the same in every position, the charge is not used up, it just flows around the circuit
P.d in a series circuit
The total e.m.f is the sum of the p.d across the components which is split based on the ratio of the resistances
How is p.d split in a series circuit?
The component with the greater resistance will take the greater proportion of the e.m.f as p.d
Current in a parallel circuit
The greater the resistance of the branch the lower the current that passes through it, if one branch has half the resistance of the other, it will have twice the current through it
Resistors in series (equation)
R = R_1 +R_2 + …
Resistors in Parallel (equation)
1/R = 1/R_1 +1/R_2 +…
What are a lost volts?
When there is a current in a power source, work has to be done by the charges as they move through the power source so some energy is lost
What does increasing the current do to the lost volts?
Increasing the current means that more charge travels through the cell increasing the lost volts and decreasing the terminal p.d
Internal Resistance Equation
ε = V + Ir
What is the y intercept of a V against I graph mean?
It is the e.m.f
What is the gradient of a V against I graph?
The gradient is the negative current
