Year 1 Power and Energy: AC Circuits Flashcards
What does T&D stand for in “The conventional electrical T&D system”?
Transmission and Distribution
What are the four parts of the conventional electrical T&D system?
- Bulk generation
- Transmission system
- Distribution system
- The consumers
What happens during the bulk generation part of the conventional electrical T&D system?
Energy is provided to the system
What happens during the transmission system part of the conventional electrical T&D system?
Bulk energy is transferred using high voltage alternating currents which are very efficient
What happens during the distribution system part of the conventional electrical T&D system?
Energy is delivered to individual users from high voltage to low voltage for convenience and safety
What happens during the consumers part of the conventional electrical T&D system?
The energy supply is absorbed
What is the SI unit and symbol of length?
Meter, m
What is the SI unit and symbol of time?
Seconds, s
What is the SI unit and symbol of mass?
Kilogram, kg
What is the SI unit and symbol of temperature?
Kelvin, K
What is the SI unit and symbol of electric current?
Ampere, A
What are the units and alternative SI expression of Force?
- Newton(N)
- kg.m/s²
What are the units and alternative SI expression of Energy?
- Joule(J)
- N.m
What are the units and alternative SI expression of Electric charge?
- Coulomb(C)
- A.s
What are the units and alternative SI expression of Power?
- Watt(W)
- J/s = N.m/s
What are the units and alternative SI expression of Voltage?
- Volt
- W/A = N.m/s/A
What are the four parts of the basic electrical system?
- The source
- The load
- The transmission/distribution system
- The control apparatus
How does an electrical system transmit energy?
Due to the movement of electric charge
What is current?
The rate of flow of charge
What happens when 1C of charge crosses an area in 1 second?
A current of 1A flows
What are the conditions for continuous current flow?
- Complete circuit
- Presence of a driving influence
Define electromotive force(EMF)
The energy expended during the passing of a unit charge through a source
Define potential difference(PD)
The energy expending during the passing of a unit charge between two points of a circuit
How is current measured?
At one point and has a direction
How is voltage always measured?
Across two points and has a polarity
What is the link to electrons and conventional current?
Electrons move in the opposite direction to conventional current
Define power
The rate of change of energy with time
State Ohm’s Law
For a given material, voltage is directly proportional to current at a constant temperature
What is conductance?
The reciprocal of resistance
What is the symbol for conductance?
G
What is the units for conductance?
Siemens(S)
Define a short circuit
Direct connection between two points in a circuit, with zero resistance
Describe the types of short circuit
- Deliberate short circuit: Normally a connecting wire
- Accidental short circuit: Usually very bad as they cause very high currents and destruction of components
Define an open circuit
- No connection between two points in a circuit, i.e infinite resistance
- Sometimes caused by a bad solder joint
Define load
The power dissipated in a load resistor
What happens when there is no load?
- No load = No load power
- Current = 0
- Infinite resistance, i.e an open circuit
What is a node?
Any point where two or more circuit elements are connected together
What is a branch?
A circuit element between two nodes
What is a loop?
A collection of branches that form a closed path returning to the same node without going through any other nodes or branches twice
Define Kirchoff’s Current Law(KCL)
At any instant, the algebraic sum of all the currents at a node in a circuit is equal to zero
Define Kirchoff’s Voltage Law(KVL)
At any instant, the algebraic sum of all the voltages around a closed path in a circuit is equal to zero
What is a closed path?
A loop traced through connecting elements, starting and ending at the same node and encountering each intermediate node only once
State the equation that displays the experimental relation between current and capacitance
I ≈ C dv/dt
What is the role of the resistor?
To limit the flow of electrical current in a circuit
State 2 features of conventional generation
- Dispatchable
- Simple power flow in one direction
What is the role of a switchgear?
Connect and disconnect under normal and fault conditions
Define parasitic capacitance
Capacitance caused by the space between cables connecting conductors
Define 1 Farad
Capacitor is able to store one coulomb of charge when the voltage across it is one volt
What is an inductor?
Circuit element able to store electromagnetic energy in its magnetic field
What is the unit of inductance?
Henrys(H)
What is the equation for energy stored in an inductor?
W = ½L²
What is the equation for energy stored in a capacitor?
W = ½CV²
What is the equation for capacitors in series?
1/Ceff = 1/c1 + 1/c2
What is the equation for capacitors in parallel?
Ceff = c1 + c2
How does an ideal voltage source work?
- Always supplies the same voltage
- The current delivered by it, changes with the load
Define power
The rate of change of energy over time
What does a positive sign on a circuit element indicate?
Power is absorbed
What does a negative sign on a circuit element indicate?
Power is supplied
How does an ideal current source work?
- Always supplies the same current
- The voltage delivered by it, changes with the load
State the voltage divider rule equations
- v1 = V x r1/r1+r2
- v2 = V x r2/r1+r2
State the current divider rule equations
- i1 = i x r2/r1+r2
- i2 = i x r1/r1+r2
What is the equation for resistors in series?
Reff = r1 + r2 + r3
What is the equation for resistors in parallel?
1/Reff = 1/r1 + 1/r2 + 1/r3
What is the equation for TWO resistors in parallel?
Reff = r1 x r2/r1 + r2
What can an ideal voltage source be replaced with, and why?
- A short circuit
- As the internal resistance is zero
What can an ideal current source be replaced with, and why?
- An open circuit
- As the internal resistance is infinite
What is the relationship between voltage and inductance?
V = L di/dt
State superposition theorem
In any network containing more than one source, the current in or the voltage across any branch can be found by considering each source separately and adding their effects
