Electricity Flashcards
current, charge
I = Q/t
voltage, energy
V = E/Q
advantage of potential divider for IV circuits instead of variable resistor
can make current go to zero
ohms law
I ∝ V
provided physical conditions constant
IV graph for ohmic things such as resistor or wire
linear graph
resistance = 1/gradient
IV graph axis
I on y axis
V of x axis
Bulb IV graph
temp varies
S shaped
at higher voltage, temperature higher, so resistance higher and curve flattens
Diode IV graph
only increases when voltage higher than 0
like parabola just on positive side
thermistor behaviour and R,t graph shape
- as temp increases resistance decreases
- but not linear relationship, graph curved shape
Resistivity of wire equation
ρ = RL/A
resistivity = resistance x length / cross sectional area
at constant temperatures
Area of circle using diameter
A = π(d^2)/4
semiconductors
- some electrons still bounded to material
- when it gets hotter, electrons are liberated
- so more charge carriers freed
- so resistance decreases
thermistors (negative temp coefficient) work like this
temperature vs resistance relationship in most materials
as t increases, more vibrations and collisions
harder for electrons to pass through
resistance increases
superconductivity and uses
a property of materials where R drops to zero below a certain critical temperature
used in
- reducing energy loss in transmission of electrical power
- making strong magnetic fields
Kirchhoff’s laws
1) current flowing into a junction is equal to current flowing out of it
(conservation of charge)
2) the sum of the emfs is equal to the sum of the p.ds in a closed loop
(conservation of energy)
ε emf (electro motive force)
work done per unit charge transferring energy into electrical energy
V potential difference
work done per unit charge transferring energy from electrical energy
Resistors in series and prove
Rt = R1 + R2 + …
Vt = V1 + V2 + …
ItRt = I1R1 + I2R2 + …
I is constant in series
Rt = R1 + R2 + …
resistors in parallel
1/Rt = 1/R1 + 1/R2 + …
It = I1 + I2 + …
Vt/Rt = V1/R1 + V2/R2 + …
V is constant in parallel
1/Rt = 1/R1 + 1/R2 + …
power, current, voltage
P = IV
Power, current resistance
P = I^2R
power, voltage, resistance
P = v^2/R
energy, current, time, voltage
E = ItV
Variable resistor symbol
-
Battery symbol
-
Diode symbol
-
Thermistor symbol
-
LDR symbol
-
LED symbol
-
Internal resistance
resistance in the cell
emf equation with internal resistance
ε = I(R + r)
r is internal resistance
R is circuit resistance
i.e. ε = V + Ir
where Ir are the lost volts
therefore V<ε
plotting voltage, current on graph to find emf and resistance
ε = I(R + r)
V = -rI + ε
y = mx + c
therefore -r is gradient
and ε is y intercept
emf for cells in series
εT = ε1 + ε2 +…
emf for cells in parallel
εT = ε1 = ε2 = …
