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GCSE Physics > P4 Electric Circuits > Flashcards

P4 Electric Circuits Flashcards

1
Q

What is electric current?

A
  • Flow of electrical charge
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2
Q

Why can a current only flow if there’s a source of potential difference?

A
  • Electrical charge will only flow round a complete (closed) circuit if there is a potential difference
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3
Q

What is the current in a single, closed loop?

A
  • Has the same value everywhere in the circuit
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4
Q

What is potential difference (voltage)?

A
  • The driving force that pushes the charge around
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5
Q

What is the the unit of potential difference?

A
  • Volt (V)
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6
Q

What is resistance?

A
  • Anything that slows the flow down
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7
Q

What is the unit of resistance?

A
  • Ohm (Ω)
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8
Q

What does the current flowing through a component depend on?

A
  • Potential difference across it
  • Resistance of the component
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9
Q

What is the relationship between resistance & current?

A
  • The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component)
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10
Q

What does total charge through a circuit depend on?

A
  • Current
  • Time
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11
Q

What is the size of the current?

A
  • The rate of flow of charge
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12
Q

When do we use the charge flow formula?

A
  • When current flows past a point in a circuit for a length of time
  • Formula is given by the charge passed
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13
Q

What is the unit for charge flow?

A
  • Coulombs (C)
  • Q in formula
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14
Q

What is the unit for current?

A
  • Ampere (A)
  • l in formula
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15
Q

What is the unit for time?

A
  • Seconds (s)
  • t in formula
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16
Q

What is the formula for charge flow?

A
  • Q = l x t
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17
Q

What is the relationship between charge & current?

A
  • More charge passes around the circuit when a larger current flows
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18
Q
A

Cell

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19
Q
A

Battery

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20
Q
A

Switch open

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21
Q
A

Switch closed

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22
Q
A

Filament lamp (or bulb)

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23
Q
A

Fuse

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24
Q
A

LED

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25
Q
A

Resistor

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26
Q
A

Variable resistor

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27
Q
A

Ammeter

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28
Q
A

Diode

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29
Q
A

LDR

30
Q
A

Thermistor

31
Q

What is the formula linking potential difference, current & resistance?

A

Potential difference (V) = Current (A) x Resistance (Ω)

32
Q

What is an ammeter?

A
  • Measures the current (in amps) flowing through the test wire
  • The ammeter must be placed in series with whatever you’re investigating
33
Q

What is a voltmeter?

A
  • Measures the potential difference across the test wire (in volts)
  • The voltmeter must always be placed in parallel around whatever you’re investigating - NOT around any other bit of the circuit, e.g. the battery
34
Q

What is the resistance of ohmic conductors (e.g. a wire or a resistor)?

A
  • Doesn’t change with the current
  • At a constant tempurature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it (R is constant in V = IR)
35
Q

What resistors & components can change resistance?

A

Some, e.g. diode or filament lamp

36
Q

What happens when an electrical charge flows through a filament lamp?

A
  • Transfers some energy to the thermal energy store of the filament, which is designed to heat up
  • Resistance increases with temperature, so as the current increases, the filament lamp heats up more & the resistance increases
37
Q

What is the resistance of diodes?

A
  • For diodes, the resistance depends on the direction of the current
  • They will happily let current flow in one direction, but have a very high resistance if it is reversed
38
Q

What is an ‘I-V characteristic’?

A
  • Graph that shows how the current (I) flowing through a component changes as the potential difference (V) across it is increased
39
Q

What is the I-V characteristic of linear components?

A
  • Straight line
  • e.g. fixed resistor
40
Q

What is the I-V characteristic of non-linear components?

A
  • Curved
  • E.g. filament lamp or a diode
41
Q

What I-V characteristic is this?

A
  • Ohmic conductor (e.g. resistor at a constant temperature)
  • The current through an ohmic conductor (at constant temperature) is directly proportional to potential difference so you get a straight line
42
Q

What I-V characteristic is this?

A
  • Filament lamp
  • As the current increases, the temperature of the filament increases, so the resistance increases
  • This means less current can flow per unit pd, so the graph gets shallower - hence the curve
43
Q

What I-V characteristic is this?

A
  • Diode
  • Current will only flow through a diode in one direction, as shown
  • The diode has very high resistance in the reverse direction
44
Q

What does LDR stand for?

A

Light Dependent Resistor

45
Q

What is an LDR?

A
  • A resistor that is dependent on the intensity of light
46
Q

What is the resistance of an LDR in bright light?

A
  • In bright light, the resistance falls
47
Q

What is the resistance of an LDR in the dark?

A
  • In darkness, the resistance is darkest
48
Q

What are some uses of LDRs?

A
  • Automatic night lights
  • Outdoor lighting
  • Burglar detectors
49
Q

What type of resistor is a thermistor?

A
  • Temperature dependent
50
Q

What is the resistance of a thermistor in hot conditions?

A
  • In hot conditions, the resistance drops
51
Q

What is the resistance of a thermistor in cool conditions?

A
  • In cool conditions, the resistance goes up
52
Q

Why are thermistors useful?

A
  • They make useful temperature detectors
  • e.g. car engine temperature sensors & electronic thermostats
53
Q

What are the uses of sensing circuits?

A
  • To turn on or increase the power to components depending on the conditions that they are in
54
Q

What is this circuit?

A
  • A sensing circuit used to operate a fan in a room
  • The fixed resistor & fan will always have the same potential difference across them (because they’re connected in parallel)
55
Q

How is potential difference shared in a sensing circuit?

A
  • The pd of the power supply is shared out between the thermistor & the loop made up of the fixed resistor & the fan according to their resistances - the bigger a component’s resistance, the more of the pd it takes
  • As the room gets hotter, the resistance of the thermistor decreases & it takes a smaller share of pd from the power supply
  • So the pd across the fixed resistor & the fan rises, making the fan go faster
56
Q

How could you connect the component across the variable resistor instead?

A
  • E.g. if you connect a bulb in parallel to an LDR, the pd across both the LDR & the bulb will be high when it’s dark & and the LDR’s resistance is high
  • The greater the pd across a component, the more energy it gets
  • So a bulb connected across an LDR would get brighter as the room got darker
57
Q

Why aren’t series circuits useful?

A
  • If you remove or disconnect one component, the circuit is broken and they all stop
  • Therefore in practice very few things are connected in series
58
Q

What is potential difference in series circuits?

A
  • In series circuits the total pd of the supply is shared between the various components
  • So the potential differences round a series circuit always add up to equal the source pd
  • V(total) = V(1) + V(2) + …
59
Q

What is the current in a series circuit?

A
  • In series circuits the same current flows through all components, i.e. I(1) = I(2) = …
  • The size of the current is determined by the total pd of the cells & the total resistance of the circuit: i.e. I = V / R
60
Q

What is the resistance of a series circuit?

A
  • In series circuits the total resistance of two components is just the sum of their resistances: R(total) = R(1) + R(2)
61
Q

Why are resistances in series circuits the sum of their resistances?

A
  • By adding a resistor in series, the two resistors have to share the total pd
62
Q

Why does the total resistance of a series circuit increase?

A
  • The potential difference across each resistor is lower, so the current through each resistor is also lower
  • In a series circuit, the current is the same everywhere so the total current in the circuit is reduced when a resistor is added
  • This means the total resistance of the circuit increases
63
Q

What is the relationship between a component’s resistance & potential difference in a series circuit?

A
  • The bigger a component’s resistance, the bigger its share of the total potential difference
64
Q

How do you increase the potential difference in a series circuit?

A
  • There is a bigger pd when more cells are in series, if they’re all connected the same way
  • E.g. when 2 cells with a potential difference of 1.5V are connected in series they supply 3V between them
65
Q

Why are parallel circuits useful?

A
  • If you remove or disconnect one of them, it will hardly affect the others at all
  • This is obviously how most things must be connected, for example in cars & in household electrics
  • You have to be able to switch everything on & off separately
66
Q

What parts do everyday circuits include?

A
  • Mixture of series & parallel parts
67
Q

What is the potential difference of a parallel circuit?

A
  • In parallel circuits all components get the full source pd, so the potential difference is the same across all components: V(1) = V(2) = …
  • This means that identical bulbs connected in parallel will be at the same brightness
68
Q

What is the current in a parallel circuit?

A
  • In parallel circuits the total current flowing around the circuit is equal to the total of the currents through the separate components
69
Q

Why does total current in parallel circuit have to equal the total current leaving?

A
  • In a parallel circuit, there are junctions where the current either splits or rejoins
  • Therefore, the total current going into a junction has to equal the total current leaving
70
Q

What happens if two identical components are connected in parallel?

A
  • The same current will flow through each component
71
Q

What will reduce the resistance of a parallel circuit?

A
  • If you have two resistors in parallel, their total resistance is less than the resistance of the smallest of two resistors
72
Q

Why is the resistance of two resistors in parallel less than the resistance of the smallest of two resistors?

A
  • In parallel, both resistor have the same potential difference across them as the source
  • This means the ‘pushing force’ making the current flow is the same as the source pd for each resistor that you add
  • But by adding another loop, the current has more than one direction to go in
  • This increases the total current that can flow around the circuit. Using V = IR, an increase in circuit means a decrease in the total resistance of the circuit

GCSE Physics (7 decks)

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