Electricity Flashcards
What is charge
Where particles have positive / negative / neutral charge
Like charges repel, opposite charges attract
What are electric forces
Non-contact
Vector quantity (Has magnitude and direction)
Strength of force depends on distance between charges
- Greater unbalanced charge = Greater force
What are electrical conductors
Electrons free to move, charge can flow easily
What are electrical insulators
Electrons can’t move, charge can’t flow
What are electrical semi-conductors
Conducts electricity under certain circumstances
What is current
The flow of electric charge, net amount of charge passing certain point in given time interval
What do you need for current
An electric field, supply energy to move charges
Charges being free to move
How to measure current in circuits
Current is conserved quantity, charge carriers always somewhere
Current that flows in = Current that flows out
Measured using ammeter, connected anywhere
What is formula for current
Current (A) = Charge (C) / Time (s)
What is Direct-Current (DC)
Net charge flows in 1 direction, power from batteries
What is Alternating-Current (AC)
Charge flow alternates, moves back and forth, power from wall socket
What is electric potential
Potential energy per unit of charge (Coulomb)
What is potential energy
“Stored” energy with capacity to do work
- Gravitational Potential Energy
- Elastic Potential Energy
- Electric Potential Energy
What is potential difference / voltage
Change in potential energy per unit charge between two defined points in circuit
Why do electrons require energy to move
Charged particles naturally move from high to low potential
Requires energy to move from low to high potential = Electrons require energy to move
What is 1 volt
1 J/ Coulomb
How do you measure potential difference using voltmeter
Measured by comparing two different locations on circuit
Exists if charged particles gain/lose potential energy
Measured using voltmeter
What is formula for potential difference
Voltage = Change in Potential Energy / Work (J) / Electric Charge (C)
How do you create a current
Potential difference applied to conductor
- Creates electric field
- Exorts force on electric charges
- Electric charges move = Current
What is power
Rate of energy transfer
Power usage of element determined from voltage & current of element
What are the formulas of power
Power = Work Done (J) / Time (s)
Power = Voltage (V) * Current (A)
What is a circuit
Path through where electricity flows
What are the requirements for a circuit
Power source, produces EMF
Complete path of conducting material
What is a series circuit
All elements arranged in chain, only one path for current
What is the resistance in a series circuit
Total Resistance = Sum of individual resistances
Rt = R1 + R2 + R3 + …
What is voltage in a series circuit
Total Voltage = Sum of individual voltages
Vt = V1 + V2 + V3 + …
What is current in a series circuit
Current is the same in every part of circuit
Current in = Current out
What is a parallel circuit
Each component directly connected to power source
Atleast two pathways for current to travel
What is the resistance in a parallel circuit
Total Resistance = Amount of resistance a ‘single’ resistor needs to equal overall effect
1 / Rt = (1/R1) + (1/R2) + (1/R3) + …
What is the voltage in a parallel circuit
Same voltage applied to each pathway of circuit
What is the current in a parallel circuit
Current in = Current out
Current split between each branch
Split equally if resistance of each branch is the same
What is electrical resistance
Measure of how hard it is for current to flow through material
How does resistance work in wires
Charge moves through wire in zig-zags, as it collides with atoms
In collisions, charges transfer some energy to atoms = Being resisted
How does resistivity affect resistance
How much a material opposes flow of charge
Some materials resist movement of charge more than others
How does temperature affect resistance
Increase in temperature = Increase in resistance
Atoms vibrate faster, charge through conductor has more collisions
How does length of wire affect resistance
Longer wire = Higher resistance
Charges experience more collisions before reaching end of wire
How does area of wire affect resistance
Higher area = Lower resistance
More area for charges to move = Less collisions
What is Ohm’s Law
Current flowing through substance directly proportional to voltage
Resistance affects current & voltage
What is the formula for resistance
Resistance (Ohms) = Voltage (V) / Current (A)
What are ohmic devices
Resistance constant for wide range of voltages & currents
Obeys Ohm’s Law
Most wires & resistors
What are non-ohmic devices
Resistance not constant, varies depending on current & voltage
Doesn’t obey Ohm’s Law
Light diodes, digital devices
What are thermal hazards
Excessive electric power, undesired thermal effect
Eg. Starting a fire in the wall of a house
What are short circuits
Low resistance path between positive & negative terminals of voltage source
- Resistance small, power large = Melt / Cause Fire
What are shock hazards
Current passes through body (Severely painful to lethal)
- Voltage related to flow, energy available for flow
Amount of current depends on bodies resistance
- Dry skin = High resistance (Good)
- Wet skin = Low resistance, can be lethal (Bad)
What are electric shock fatalities
Current puts heart into fibrillation (Irregular heartbeat)
Effect of shock depends on:
- Amount of current
- Path of current
- Duration of shock
- Frequency of current
How do you prevent electrical shocks)
Unplug appliances when working on / cleaning them
Keep appliances dry, use dry hands
Qualified professionals do maintenance
Touch something ‘live’ with back of hand, if muscles contract, will pull away from object instead of grabbing it
What is a fuse and circuit breaker, and what is the difference
Wire overload
Innner wires overheat & melt when current too high (Breaks circuit)
Limits excessive currents
Works for small voltages / currents (Eg. Household)
Won’t save from electrocution
Circuit breakers can be reset, fuses need to be replaced
What are the types of wires in a household
Active Wire: Carries current from power station
Neutral Wire: Closes circuit so current can flow
Earth Wire: Safety device, connected to conductive parts of appliances
What does an earth wire do
If active wire in metal case touches the metal casing, whole case becomes electrically live
To prevent this, a short circuit is created by earth wire, current immediately flows to the Earth
What is double insulation
Device never exposes operator to active wire even if it becomes disconnected / loose
- Done by ensuring outer casing insulated from inner workings
- Earth wire not needed
What are residual current devices (RCDs)
Cuts the circuit when Active Current != Neutral Current
- Should be the same as current in = current out
- If not same, leakage of current (Eg. To Earth or To Person)
Cuts at as little different as 30mA
