Electrical Flashcards
What are the 3 main effects of current?
- Magnetic (relays,motors,generators)
- Heating (Cookers,water heater,electric fence)
- Chemical (Primary & secondary cells & electroplating)
Whats the difference between AC & DC?
- DC -current flow in 1 direction in a constant magnitude
- AC - current changes in 1 direction rising to maximum back to 0 then to maximum in other direction
Charge
What is current?
Flow rate of charge
What is Ohms law?
Current flowing through a conductor is directly proportional to the applied Voltage through it & inversely proportional to the Resistance providing temperature is constant
What does a fuse do?
Prevents overcurrent in a circuit,if the current exceeds the fuse melts breaking the circuit
Insulation resistance decreases due to…
- Dampness
- Brittleness with age
- Accidental damage
If volatge is the same across all resistors in parallel,what is the current?
Current is different
In a series circuit the current is the same through each component,what is the voltage?
The voltage is different
Name 2 types of resistors & their uses
- Surface mount technology (SMT) used in electronics
- Wire wound used in motor starters & power circuits
Name 2 ways to identify resistors
- Colour coding
- Alpha numerical coding
R=pL/A
What 4 things affect resistance
1. Type of material
2. Length of conductor
3. Cross-sectional area of conductor
4. Temp of material
Name hazards with Capacitors
A charged capacitior can cause electric shock, need to discharge across a resistive load to make safe.
What does a capacitor consist of?
2 plates seperated by insulating material known as dialectric.
What is Kirchoffs current law?
Current going into a junction is always the same a current out
What is Kirchoffs Voltage law?
In any closed loop the amount of voltage drop = the total voltage
Series circuit/variable resistor
Describe a Rheostat
- Variable resistor with 2 terminals,1 fixed &1 sliding
- Controls current flow by changing resistance
- Connected in series
- Slider to left=max current
- Slider to right=min current
Voltage
Describe a Potentiometer
- Variable resistor ,3 terminals
- Provides an adjustable voltage divider circuit,produces a variable p.d from fixed e.m.f
- Slider left=V increase
- Slider right=V decrease
dimmer switch,tv controls
What is Absolute potential?
Voltage with respect to earth
Name 3 things that determine capacitance
- Area of plate
- Thickness of dialectric
- Dialectric material
In paralell circuits what is the same over each capacitor?
Voltage
In series circuits what is the same over each capacitor
Q
Charge
In a capacitor how long does it take for voltage to reach steady state?
5T (5xtime)
Name 2 types of capacitors
- Fixed
- Variable
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Disadvantage of electrolytic capacitors?
Used in DC circuits
Advantage of electrolytic capacitors
Larger capacitance
How to charge a capacitor
- Turn on switch,emf applied to capacitor
- Electrons build up on 1 plate with neg charge,positive on other
- Surplus of positive ions on one plate
- No electron go through plate
- Flow continues till capacitor is charged
What is Time constant?
Time it takes for capacitor to reach max charge if inital charge is maintained
Describe a magnetic field
Area around a magnet where magnetic force can be detected
What is Flux density measured in?
Tesla (T)
Describe magnetic flux
Amount of magnetic field produced by a magnetic source
In relation to magnetism, what is Reluctance?
Magnetic resistance to flux
Magnetism
Name 4 things that affect Force
- Flux density
- Current strength
- Length of conductor
- Direction of field + current
What happens when a current carrying conductor is placed in a magnetic field?
Field strong on one side of conductor, weak on other, the conductor moves outward
Application DC Motor , used in generators
Explain Self-Inductance
When EMF is induced in the same circuit in which current is charging
Explain Mutual-inductance
When EMF is induced in a circuit by a change in flux due to current charging in an adjacent circuit
Explain the principle of a generator
- Conductors form a circuit that move through mag field
- EMF induced in conductor
- Generators convert mechanical energy to electrical energy
Define electromagnetic induction
When a conductor is moved across a field,cutting the lines of flux it produces EMF & current in a conductor
Back
What is Lenz’s law?
The changing current in a conductor induces a back EMF across it to oppose the motion which produces it
Name 3 factors that affect inductance
- Number of turns of wire
- Cross-sectional area of coil
- Presence of mag core
In safety, insulation uses
Peak value
In safety, fuses use
RMS value
State 2 advantages to AC
- Easier to distribute
- Cheaper to generate
What is the power rating of a transformer?
The Volt-Amperes that it can transform without overheating
What does Back EMF do?
Tries to stop changes in current, when current at max there are no changes so it drops to 0
What is the steady state current?
Max
What is the steady state Voltage?
0
What is inductive reactance?
Opposition to changing current flow
Name 2 conditions of resonance
- Power factor = 1
- Phase angle = 0
Nmae 2 practical applications of resonance
- Radar recievers
- Radio recievers
In resonance name 3 powers
- Apparent
- True
- Reactive
How does a capacitor discharge?
The negative electrons on one plate flow towards the positive plate,acting as a voltage source in the circuit.When the p.d between both plates is 0 the flow stops
What is periodic time?
Time it takes to complete one cycle
Define frequency of a waveform
Number of cycles completed in 1 second
CCL
In an AC inductive circuit current?
Lags V by 90°
In a capactive circuit current?
Leads V by 90°
