Electricity Flashcards
What is alternative current a.c.?
changes its direction and instantaneous value with time
What is direct current d.c.?
The electrons flow in one direction only, and its value is constant
What is peak voltage equal to?
number of divisions x volts/divisions
(uses y gain ⬆️⬇️)
What is period (T) of the wave equal to?
number of divisions x time per division
(uses time base⬅️➡️)
-> f=1/T
What are a.c. voltages stated in terms of?
Their r.m.s (root mean square) values-these are a kind of ‘average’ of the alternating ac supply
-ac readings on meters measure rms
What is the rms value of an ac supply equal to?
a dc supply which gives the same power output
What is current?
The dow of electric charge per second
What is potential difference?
the work energy required to move 1C of charge through an electric field
what is supply voltage?
a measure of the energy given to electrons to make them move around the circuit
What is power
the energy transformed per second
The energy given to the electrons by the voltage supply is then transformed into other forms of energy by components
Current rules
series
same at all points
parallel
add up to total
voltage rules
series
add up to supply
parallel
same at all points
resistance rules
series
add up to total
parallel
1/Rt = 1/R1 + 1/R2 + 1/R3
What is the e.m.f. (electromotive force) of a source?
The electrical energy supplied to each coulomb of charge which passes through the source
What is internal resistance? r
The resistance of the battery
What is lost volts?
the energy (per coulomb) that will be used up in overcoming the internal resistance of the supply
What is the t.p.d. (terminal potential difference)?
The energy (per coulomb) left for the circuit (after energy is used for overcoming r).
The rod is the voltage obtained from a source of electrical energy when a current is being drawn from it.
Situation: open circuit
switch is open (oof)/current is 0, voltmeter reading across supply will be emf
no current—>lost volts=0
situation: under load (with an external resistance R)
The circuit is working normally->will be lost volts
emf=V+Ir
voltmeter reading across supply is tpd
situation: short circuit (no external resistance)
All the energy from the battery is lost because of internal resistance
R=0
E=I(0+r)
E=Ir
The maximum current is the short-circuit current large enough to blow a fuse or circuit breaker
Why does Vtpd decrease when current increases?
As current increases, the list colts increases (lost volts=Ir)
This means more energy is lost (per coulomb) as the electrons travel through the supply
If more voltage is lost across the supply, there will be less for the external circuit
Therefore, the rod will decrease (E=V+Ir
What is a capacitor?
a component which stores electrical charge
consists of 2 conducting layers separated by an insulator
(simplest:2 metal plates with an air gap between them)
What is capacitance measured in?
Farads F
The amount of charge that a capacitor can store depends on its capacitance
What is a farad?
One farad of capacitance means that the capacitor can store one coulomb of charge per unit
What happens as a capacitor is charging?
-pd across capacitor increases from 0 to the maximum value (supply V)
-the charging current decreases from an initial value of E/R to zero
-at all times the sum of the potential difference across the capacitor and the potential difference across the resistor equals the EMF of the supply
-the potential difference across the resistor decreases from an initial value of E to zero when the capacitor is fully charged
What happens as a capacitor is discharging?
-The voltage decreases from the max (Vs) to 0
-the discharging current decreases from an initial value of -E/R to zero
-the potential difference across the capacitor is always equal to the potential difference across the resistor
-the potential difference across the resistor (given by ) decreases from an initial value of to zero when the capacitor is fully discharged
Factors affecting the rate of charge and discharge?
-The resistance of the resistor (controls size of current)
-The capacitance of the capacitor (larger capacitances take longer to fill/empty)
SUPPLY VOLTAGE WILL NOT HAVE AN EFFECT
How can you get a constant current while charging a capacitor?
Use a variable resistor
Current is constant —> Q=It
