Electricity Flashcards
DC
Direct current supply has one size and one direction.
AC
An alternating current supply is a current which changes direction and instantaneous value with time. This is shown as a sine wave on an oscilloscope screen. Time is measured using x axis and voltage on y axis.
Frequency of ac supply
Found from xaxis (timebase) using these steps:
1. find no boxes to complete one wave
2. find period by x the boxes by timebase (e.g. 4x(2x10^-3)ms
3. use equation f=1/T (Hz)
Peak voltage of AC supply
Found by counting no boxes which = amplutude (centre up)
Multiply by y-gain or voltage gain value. (e.g 2 x 3V=6V)
True value of ac voltage
Average voltagw for one whole wave. This is called r.m.s volatge (Vrms). Mains r.m.s is 230v while peak v is 325V.
rms and peak voltage equation
vrms = Vpeak/squareroot 2
rms current and peak current equation
Ipeak = squareroot2 x Irms
IMPORTANT info on calculating rms values
When doing a calculation ALWAYS use r.m.s. values. If peak values are used in a
calculation the answer will be wrong! For example;
If Ipeak = 2 A and Power = 8 W, calculate Vrms.
You must find Irms first then use the equation P = IV or the answer will be wrong.
(Answer is Vrms = 5.7 V)
Current
The rate of flow of charge or number of coulombs of charge passing a point each second.
relationship between charge and current
Q=It
Voltage
Energy given to each coulomb of charge
Potential difference
Same thing as voltage
Word done (energy) in moving one coulomb of charge between 2 points.
Relationship between p.d, current and resistance
V=IR
When describing p.d is ‘across’ component and current is ‘through’ component.
As resistance increases
Current decreases
Power related to ohm’s law
P=IV
P=I^2R
P=v^2/R
In a series circuit
Current is same at all points
Supply V is shared across components
Total resistance is found by adding resistors together
Parallel Circuit
Current split up through each branch
Vsupply is same as p.d across each branch.
Total resistance 1/Rt = 1/R1 +1/R2 +1/R3
When resistor in parallel is added, total resistance decreases.
If 2 resistors in parallel have same value, then total r will be half one of resistors.
Potential dividers
v1/v2 = r1/r2
v2 = (r2/(r1+r2)) x Vs
Bridge voltage
Voltage across V2 for both potential dividers then difference between 2 values
current and internal resistance
same as rest of circuit
e.m.f
electromotive force is the voltage across the battery when no current is in the supply
definition of e.m.f
Energy given to each coulomb of charge that passes through the supply. You could also say the voltage across the battery when there is an open circuit.
T.p.d
terminal potential difference is the voltage across the supply where there is a current in the supply (switch closed). The voltage across the battery is the same as what would be across the load resistor (resistors outside the battery) in the circuit.
‘lost volts’
Difference between e.m.f and t.p.d values. This occurs as the battery has a small internal resistance and when a current passes through battery a potential difference is generated across internal resistance.