Electrical Systems Flashcards
what is electrical energy?
form of energy made available by flow of electric charges through an electric conductor
why are metals good conductors of electricity?
thye have free mobile electrons
what is the difference between an open and closed circuit?
closed circuit: electric charges can flow through an uninterrupted path
open circuit: electric charges do flow through circuit at all
what is a switch?
used to open and close a circuit safely and conveniently
what are electric cells?
source of electricity which produces energy for electric charges to move through circuit
what are the differences between a series and parallel circuit? (connection, current, 1 bulb removed/damaged)
series:
1) electrical components connected in single path, current same throughout circuit
2) same current flows through each component
3) when 1 bulb removed/damaged, open circuit, no electric charges flow through other bulbs
parallel:
1) electric circuit split into more than 1 path
2) current divides itself (not necessarily equally) and flows through all branches
3) when 1 bulb removed/damaged, other bulbs still light up as connected to electric cells by separate paths, electric charges can still flow through circuit
what happens when there is an alternative path of lower resistance (short circuit)?
current will flow through path of lowest resistance (no bulb lights up as bulb has resistance)
what are electric currents?
rate of flow of electric charges
what is the SI unit of electric currents?
ampere (A)
what is the equation for electric currents? (equation, representation, SI units)
I = Q / t
I = current flowing through circuit: ampere (A)
Q = amount of electric charges flowing in circuit: coulomb (C)
t = time taken: second (s)
what is the difference between the directions of conventional current flow and electron flow?
conventional current: positive to negative terminal of cell/battery
electron flow: negative to positive terminal of cell/battery
what is used to measure electric currents?
ammeter
how is an ammeter connected? (2 points)
connected in series with circuit
positive (red) and negative (black) terminals connected to positive and negative terminals of cell/battery respectively
what is the precision of an ammeter?
half of smallest interval, 0.01A (2d.p.)
what is the use of reflective stripes on ammeters and voltmeters?
prevents parallax error, observe that pointer and the image of pointer are aligned before taking reading
what is electromotive force?
work done by source in driving a unit charge around complete circuit
what is potential difference?
work done in driving a unit charge between 2 points in a circuit
how do positive electric charges flow (p.d)?
from point of higher potential to point of lower potential
what is used to measure potential difference?
voltmeter
how is a voltmeter connected? (2 points)
connected parallel to electric component
positive (red) and negative (black) terminals connected to positive and negative terminals of cell/battery respectively
what is the precision of a voltmeter?
half of smallest interval, 0.05V (2d.p.)
how to calculate the electromotive force of a series connection of electric cells?
add up the potential differences of each cell
what is electrical resistance?
ratio of potential difference across 2 points of a circuit to the current which flows through it
(higher the resistance, more difficult it is for electric charge to flow)
why does resistance happen (how does it arise)?
collision of electrons with particles in conductor
what does resistance usually result in?
change of electrical energy into other forms of energy (heat)
what is the purpose of resistors?
made to create some resistance in circuit to control current flow
what is the equation for resistance? (equation, representation, SI units)
R = V / I
R = resistance between the 2 points in circuit: ohm (Ω)
V = potential difference across the 2 points in circuit: volt (V)
I = current flowing through the 2 points in circuit: ampere (A)
what is effective resistance?
total resistance caused by a whole network of resistors
how to calculate effective resistance? (series/parallel, voltage/current, increase/decrease)
series:
Reff = R₁ + R₂ + R₃
current (I) remains the same
effective resistance increases with more resistors
parallel:
1 / Reff = 1/R₁ + 1/R₂ + 1/R₃
current flowing into resistors same as current flowing out of them
voltage (V) remains the same
effective resistance decreases with more resistors
how to find the resistance of a fixed resistor?
vary current (I) by changing resistance of variable resistor
obtain a series of p.d (V) readings for each change in current (I)
resistance of fixed resistor obtained by plotting graph of V against I and finding gradient of line (V / I)
what does Ohm’s Law state?
states that the current passing through a conductor is directly proportional to the potential difference across it, provided that physical conditions (temperature) remains constant
V = IR
what are ohmic conductors? (+ shape of I-V graph)
conductors that obey Ohm’s Law
graph: straight line that passes through the origin
what are non-ohmic conductors? (+ shape of I-V graph)
conductors that do not obey Ohm’s Law
graph: resistance can vary, not a straight line, V/I ratio not constant
what is the effect of temperature on resistance of a conductor?
Ohm’s Law defined based on using metal conductors kept at a constant temperature
as the temperature of conductor increases, atoms vibrate more vigorously and hinder the flow of free electrons across the conductor, resulting in an increase in resistance
increase in temperature of metal conductor will increase resistance of it and vice-versa
what is the difference between series and parallel circuits? (current, voltage, effective resistance)
series:
current — same throughout
voltage — Vε = V₁ + V₂
effective resistance — Reff = R₁ + R₂ + R₃
parallel:
current — I = I₁+ I₂
voltage — Vε = V₁ = V₂ (only if circuit is solely parallel)
effective resistance — 1 / Reff = 1/R₁ + 1/R₂ + 1/R₃
what are 2 uses of electricity? (explain)
electric heating:
1) most household appliances that convert EE to TE have heating elements made of nichrome
2) nichrome used as it has high resistivity and can withstand high temperatures
3) when electric current passes through these heating elements, they heat up rapidly
filament lamp:
1) converts EE to LE, filament is tungsten wire
2) tungsten used as it has high resistivity and melting point
3) wire has small cross-sectional area and is coiled
4) when electric current flows through it, it is heated to ~2500°C, intense heat generating light
what is power?
rate of work done or rate of energy converted
what are the equations for power? (equations, representations, SI units)
1) P = W / t (power as rate of work done)
P = power: watt (W)
W = work done: joule (J)
t = time taken: second (s)
2) P = E / t (power as rate of energy converted)
P = power: watt (W)
E = energy converted: joule (J)
t = time taken: second (s)
what is the SI unit of power?
watt (W)
what is the unit for electrical energy used in a household?
kilowatt hour (kWh)
what is the equation for electrical energy? (equation, representation, SI units)
Electrical energy (kWh) = Power (kW) x Time (h)
what is the equation for the cost of electrical energy? (equation, representation, SI units)
Cost of electrical energy = Energy consumption (kWh) x Cost per kWh ($)
what are the 2 main dangers of electricity?
electric shock
electric fire
what are 3 hazards of electricity? (explain)
damaged insulation:
1) conducting wires may be exposed when insulating material covering them gets damaged
2) exposed conducting wires can cause electric shocks if touched
overheating of cables:
1) can cause electric fire
2) caused by overloaded power sockets (unusually large amount of current flows through wires)
3) also caused by use of inappropriate wires (appliances requiring high power require thicker wires)
damp environments:
1) water that comes into contact with exposed electrical wires provide conducting path for current
2) can lead to electric shock
3) electrical appliances should be kept in dry places and handled with dry hands
describe a circuit breaker (3 points)
safety device that can switch off electrical supply in circuits when large currents flow through them
without them, surge of current can damage home appliances/cause electric fire
connected to live wires
describe a fuse (5 points)
prevents excessive current flow
unlike circuit breaker, which can be reset after it trips, fuses have to be replaced once blown
consists of short, thin piece of wire, if current flow is too large, wire heats up and melts, causes open circuit, current no longer flows in circuit
should be rated slightly higher (~1A) than current flow
connected to live wires
describe a switch (2 points)
breaks/completes electric circuit
fitted to live wires
describe earthing/earth wire (3 points)
prevents electric shocks to user
when there is fault in circuit and live wire touches casing, earth wire provides conducting path (short circuit) for current to flow to ground instead of person touching appliance
earth wires can do so due to its very low resistance and zero potential
describe a 3-pin plug (4 parts)
live wire (brown):
connected to high voltage, delivers current to appliance
circuit breakers, fuses, switches connected to live wires
neutral wire (blue):
completes circuit by providing returning path to supply
at 0V
earth wire (yellow and green):
low resistance wire connected to casing of appliances
fused plug:
fuse protects appliance when there is electrical fault
earth wire protects user from electric shocks if electrical fault causes appliance to be live
describe double insulation (4 points)
2-pin plugs, no earth wire
protects user from electric shocks
electric cables insulated from internal components, internal components insulated from outer casing
these appliances typically have non-metallic outer casings
