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Level 2 Science > Electrical Systems > Flashcards

Electrical Systems Flashcards

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1
Q

what is electrical energy?

A

form of energy made available by flow of electric charges through an electric conductor

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2
Q

why are metals good conductors of electricity?

A

thye have free mobile electrons

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3
Q

what is the difference between an open and closed circuit?

A

closed circuit: electric charges can flow through an uninterrupted path

open circuit: electric charges do flow through circuit at all

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4
Q

what is a switch?

A

used to open and close a circuit safely and conveniently

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5
Q

what are electric cells?

A

source of electricity which produces energy for electric charges to move through circuit

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6
Q

what are the differences between a series and parallel circuit? (connection, current, 1 bulb removed/damaged)

A

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

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7
Q

what happens when there is an alternative path of lower resistance (short circuit)?

A

current will flow through path of lowest resistance (no bulb lights up as bulb has resistance)

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8
Q

what are electric currents?

A

rate of flow of electric charges

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9
Q

what is the SI unit of electric currents?

A

ampere (A)

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10
Q

what is the equation for electric currents? (equation, representation, SI units)

A

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)

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11
Q

what is the difference between the directions of conventional current flow and electron flow?

A

conventional current: positive to negative terminal of cell/battery

electron flow: negative to positive terminal of cell/battery

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12
Q

what is used to measure electric currents?

A

ammeter

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13
Q

how is an ammeter connected? (2 points)

A

connected in series with circuit

positive (red) and negative (black) terminals connected to positive and negative terminals of cell/battery respectively

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14
Q

what is the precision of an ammeter?

A

half of smallest interval, 0.01A (2d.p.)

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15
Q

what is the use of reflective stripes on ammeters and voltmeters?

A

prevents parallax error, observe that pointer and the image of pointer are aligned before taking reading

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16
Q

what is electromotive force?

A

work done by source in driving a unit charge around complete circuit

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17
Q

what is potential difference?

A

work done in driving a unit charge between 2 points in a circuit

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18
Q

how do positive electric charges flow (p.d)?

A

from point of higher potential to point of lower potential

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19
Q

what is used to measure potential difference?

A

voltmeter

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20
Q

how is a voltmeter connected? (2 points)

A

connected parallel to electric component

positive (red) and negative (black) terminals connected to positive and negative terminals of cell/battery respectively

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21
Q

what is the precision of a voltmeter?

A

half of smallest interval, 0.05V (2d.p.)

22
Q

how to calculate the electromotive force of a series connection of electric cells?

A

add up the potential differences of each cell

23
Q

what is electrical resistance?

A

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)

24
Q

why does resistance happen (how does it arise)?

A

collision of electrons with particles in conductor

25
Q

what does resistance usually result in?

A

change of electrical energy into other forms of energy (heat)

26
Q

what is the purpose of resistors?

A

made to create some resistance in circuit to control current flow

27
Q

what is the equation for resistance? (equation, representation, SI units)

A

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)

28
Q

what is effective resistance?

A

total resistance caused by a whole network of resistors

29
Q

how to calculate effective resistance? (series/parallel, voltage/current, increase/decrease)

A

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

30
Q

how to find the resistance of a fixed resistor?

A

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)

31
Q

what does Ohm’s Law state?

A

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

32
Q

what are ohmic conductors? (+ shape of I-V graph)

A

conductors that obey Ohm’s Law

graph: straight line that passes through the origin

33
Q

what are non-ohmic conductors? (+ shape of I-V graph)

A

conductors that do not obey Ohm’s Law

graph: resistance can vary, not a straight line, V/I ratio not constant

34
Q

what is the effect of temperature on resistance of a conductor?

A

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

35
Q

what is the difference between series and parallel circuits? (current, voltage, effective resistance)

A

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

36
Q

what are 2 uses of electricity? (explain)

A

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

37
Q

what is power?

A

rate of work done or rate of energy converted

38
Q

what are the equations for power? (equations, representations, SI units)

A

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)

39
Q

what is the SI unit of power?

A

watt (W)

40
Q

what is the unit for electrical energy used in a household?

A

kilowatt hour (kWh)

41
Q

what is the equation for electrical energy? (equation, representation, SI units)

A

Electrical energy (kWh) = Power (kW) x Time (h)

42
Q

what is the equation for the cost of electrical energy? (equation, representation, SI units)

A

Cost of electrical energy = Energy consumption (kWh) x Cost per kWh ($)

43
Q

what are the 2 main dangers of electricity?

A

electric shock

electric fire

44
Q

what are 3 hazards of electricity? (explain)

A

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

45
Q

describe a circuit breaker (3 points)

A

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

46
Q

describe a fuse (5 points)

A

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

47
Q

describe a switch (2 points)

A

breaks/completes electric circuit

fitted to live wires

48
Q

describe earthing/earth wire (3 points)

A

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

49
Q

describe a 3-pin plug (4 parts)

A

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

50
Q

describe double insulation (4 points)

A

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

Level 2 Science (15 decks)

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