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A level Physics > Unit 2.3 - DC Circuits > Flashcards

Unit 2.3 - DC Circuits Flashcards

1
Q

What is current?

A

A measure of the flow of charge per second

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2
Q

How does the number of charges flowing into a wire compare to the charges coming out?

A

Equal

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3
Q

What is equal in terms of charges in a wire?

A

The number of charges flowing into a wire = the number of charges flowing out

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4
Q

What do charges all do which leads to the current into a wire being the same as the current out?

A

They all drift at the same velocity

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5
Q

What happens to the sum of current flowing into a junction?

A

It’s the same as the current flowing out

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6
Q

What is the same flowing in and out of a circuit?

A

The number of charges
So, the current

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7
Q

Can charges gather in one spot?

A

No, charge does not accumulate

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8
Q

What’s the principle that states that the number of charges flowing into a wire = the number of charges flowing out?

A

Conservation of charge

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9
Q

Which energy do charges carry?

A

The energy which is gained in the cell/battery and lost in circuit components

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10
Q

Where is energy gained in a ciruit?

A

In the cell/battery

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11
Q

Where is energy lost in a circuit?

A

In circuit components

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12
Q

Explain the principle of conservation of energy in a circuit

A

We cannot lose more energy than gained in the battery, so the total energy gained in the battery is equal to the sum of energy lost in the circuit

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13
Q

Principle of conservation of energy in a circuit in electrical terms

A

The sum of the p.d.s in a circuit is equal to the p.d across the battery

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14
Q

What would count as a circuit component?

A

A bulb, for example

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15
Q

What happens if the current in a circuit splits into more than 1 wire in terms of potential difference?

A

The potential difference in each branch will be the same (potential is a measure of energy)

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16
Q

Why does the fact that the potential difference in each branch of a parallel circuit is the same not mean that we’ve got energy “for free”?

A

The current is spit between the branches

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17
Q

Do more charges pass per second is a series or parallel circuit?

A

Not as many charges pass per second in a parallel circuit as would in a series circuit

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18
Q

Are ammeters components that cause energy loss in circuits? Why?

A

No - a perfect one does no resistance and no work is done when passing one

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19
Q

Give an example of an addition to a circuit which isn’t a component

A

Ammeters

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20
Q

What’s equal between resisters in SERIES? Why?

A

The current through them is equal (conservation of charge)

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21
Q

In what type of circuit is the current through all of the resistors equal?

A

Series circuit

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22
Q

What is the current equal between all of in a circuit?

A

Resistors

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23
Q

How do we work out the total resistance in a series circuit?

A

It’s just the sum of each individual resistance

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24
Q

In what type of circuit is the resistance the sum of each individual resistance?

A

Series circuits

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25
Q

What’s the calculation that shows that the total resistance in a series circuit is the sum of each individual resistance?

A

Power = I^2R
Total power dissipated in circuit = Ptotal = I^2Rtotal
Ptotal = I^2R1 + I^2R2 + I^2r3
Dividing by I^2 gives
Rtotal = R1 + R2 + R3

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26
Q

What is equal in a parallel circuit?

A

The potential difference across resistors (even if they aren’t of the same value)

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27
Q

In what type of circuit is the potential difference the same across the resistors?

A

Parallel circuits

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28
Q

What’s the potential difference across the circuit the same for when resistors are in parallel?

A

The resistors

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29
Q

What’s the calculation that shows how to work out the resistance in a parallel circuit?

A

Power = P = V^2/R
Ptotal = V^2/Rtotal = V^2/R1 + V2^2/R2 + V^2/R3
Divided by V^2 = 1/Rtotal = 1/R1 + 1/R2 + 1/R3

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30
Q

What would you type into the calculator to work out the total resistance of a parallel circuit with the following values for resistance…?
5, 17, 150

A

(1/5 + 1/17 + 1/150)^-1

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31
Q

Which values do we put into our calculations for calculating total resistance?

A

Resistance values

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32
Q

What do we do to resistance if they’re in series?

A

Just add them

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33
Q

e.m.f

A

Voltage across a battery

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34
Q

What does a variable resistor control in a circuit?

A

The current

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35
Q

How many terminals in a circuit does a rheostat have?

A

2

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36
Q

An electrical instrument used to control a current by varying its resistance

A

Rheostat

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37
Q

Rheostat

A

An electrical instrument used to control a current by varying its resistance

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38
Q

What does a variable resistor used as a potential divider control?

A

The potential difference (not the current) supplied to a circuit or component

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39
Q

When does a variable resistor control the potential difference and not the current supplied to a circuit or component?

A

When used as a potential divider

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40
Q

Potential divider

A

Resistances connected in series to “divide up” the potential difference (Vtotal) placed across the combination

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41
Q

What are the 2 ways of making a potential divider in a circuit?

A

Use variable resistors
Use 2 separate resistors

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42
Q

Label the separate resistors and the following on a circuit…
Vin, Vout, “lost” unwanted volts

A

(See notes)

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43
Q

What is Vout in a circuit and how do we work this out?

A

The required volts
Vout = R x Vin
———
R total

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44
Q

What does Vout depend on?

A

The values of R1 an R2

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45
Q

In what circumstance is Vout a bigger fraction of Vin?

A

If R2 is a bigger fraction of Rtotal

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46
Q

Which is always highest - Vout or Vin?

A

Always Vin

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47
Q

What’s the advantage of using variable resistors as opposed to separate ones?

A

Resistors do not come in a continuous range of values (e.g - you can get 22ohms, but not 23ohms) but it’s possible to get any value from zero to the maximum value with variable ones

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48
Q

What usually implies a potential divider circuit?

A

A circuit drawn to be on its side

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49
Q

How many terminals of the variable resistor are there when used as a potential divider?

A

3

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50
Q

What does a variable resistor change the value of and how?

A

Vout by changing the current, which changes the p.d across R2 (hence changing Vout)

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51
Q

Sketch and label a circuit with a variable resistor forming a potential divider

A

(See notes)

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52
Q

What can a variable resistor have any value between?

A

Zero and Rtotal

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53
Q

What can Vout have a value of with a variable resistor?

A

Any value between zero and Vin

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54
Q

How does the sum of the potential differences across a potential divider compare to that of the supply and why?

A

It’s equal as a potential divider consists of 2 components connected in series

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55
Q

Make it clear which part of a parallel circuit have the same current and which parts don’t

A

(See notes)

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56
Q

What happens to the total resistance when the resistors are added in series?

A

Increases

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57
Q

What happens to the total resistance when resistors are in added in parallel?

A

Decreases

58
Q

What are potential divider circuits particularly useful for?

A

Circuits where the power in pit is needed to supply several components with different p.d.s

59
Q

How can we use a circuit as a sensor?

A

Use circuit components that have changing resistance depending on physical factors

60
Q

circuit components that have changing resistance depending on physical factors

A

Thermistor
LDR (light dependant resistor)

61
Q

What do thermistors do?

A

Has a significant change in resistance as the temperature changes

62
Q

Draw a thermistor symbol

A

(See notes)

63
Q

What’s the resistance caused by a thermistor at high temperatures?

A

Low

64
Q

What’s the resistance caused by a thermistor at low temperatures?

A

High

65
Q

What does the low resistance caused by thermistors in high temperatures lead to?

A

Current increase
P.d across the second resistor increases (Vout)

66
Q

What can thermistors be used in?

A

To trigger bells in fire alarms

67
Q

LDR

A

Light dependant resistor

68
Q

LDR symbol

A

(See notes)

69
Q

What does an LDR have?

A

Has a change in resistance depending on the light striking it

70
Q

What happens to the resistance of LDR as the light intensity increases?

A

Decreases

71
Q

What happens to positive charges in a battery?

A

“Pumped” from the negative to the positive end

72
Q

How are the positive charges in a battery concentrated in one end?

A

The battery must do work against the electrostatic forces - work done transfers energy to the charges

73
Q

What transfers energy to positive charges in a battery in order to move to the positive side?

A

Work done by the battery

74
Q

What’s the ideal energy transfer when transferring charges in a battery to the positive end?

A

Chemical potential energy —> electrical energy

75
Q

What is emf in terms of positive charges in a battery?

A

The energy that the battery gives the charges to concentrate themselves at the positive end

76
Q

What’s the energy that a battery gives charges to concentrate themselves at the positive end of the battery known as?

A

Electromotive force (emf)

77
Q

Emf symbol

A

ε

78
Q

Unit of electromotive force

A

Volt

79
Q

Why does emf have volt as its unit?

A

W/Q
Jc-1
Same as a volt

80
Q

Difference between emf and pd

A

Emf - involves the transfer of other types of energy into electrical energies
Pd - involved the change from electrical energy into other types

81
Q

Emf definition

A

The energy converted from chemical energy to electrical potential energy per unit charge that flows through the cell

82
Q

What do we use to measure a cell’s emf?

A

A digital voltmeter

83
Q

Current in an open circuit

A

Zero

84
Q

What is a voltmeter measuring in an open circuit, with a current of zero, and what is this equivalent to?

A

Open circuit terminal potential difference
The same as the emf of the circuit

85
Q

What do cell supply as they work?

A

A current

86
Q

If a voltmeter is in parallel with a resistor and a battery, what is it measuring?

A

The potential difference across both the battery and the load

87
Q

If a voltmeter is connected in parallel in a circuit, is the radio higher to lower than a reading for the battery in an open circuit?

A

Lower

88
Q

What does emf tell us about a circuit?

A

How much energy is given to a circuit

89
Q

What does pd tell us that emf doesn’t?

A

How much of the energy given to a circuit doesn’t come out as current

90
Q

Why does not all energy given to a circuit come out in the current?

A

Lost voltage due to internal resistance

91
Q

When are frictional forces caused in a battery?

A

When a battery does work to concentrate the charges at the positive end of the battery

92
Q

What causes internal resistance?

A

When work is done against frictional forces in a battery

93
Q

Why does doing work against friction in a battery lead to internal resistance?

A

As work is done, heat is released
(As happens when charges flow through any resistance)

94
Q

Draw and label a circuit to demonstrate internal resistance

A

(See notes)

95
Q

How many components are there that dissipate energy in a circuit with a resistor?

A

2
The resistor
Internal resistance of the battery

96
Q

What’s the relationship between emf and pd?

A

Sum of emf = sum of pd

97
Q

Compare the current flowing through internal and external resistances

A

The same

98
Q

ε in an equation

A

ε = Ir + IR

99
Q

In ε = Ir + IR, which are variables and which are constant?

A

I and V = variables (can measure)
ε and r = constants

100
Q

Is r constant?

A

Very nearly

101
Q

How do we re-arrange the equation ε = Ir + IR to be in y = mx + c form?

A

V = -Ir + ε

102
Q

What’s y = mx + c?

A

Equation for a straight line graph

103
Q

Symbol for gradient in y = mx + c?

A

m

104
Q

Symbol for y-intercept in y = mx + c

A

c

105
Q

What is the intercept on the y-axis on a current-voltage graph?

A

ε

106
Q

Why is the y - intercept ε on a current-voltage graph?

A

The potential difference when the current = zero

107
Q

What’s the gradient on a current-voltage graph?

A

-internal resistance
-r

108
Q

Which point of a current-voltage graph shows the maximum current that the battery can supply?

A

X-intercept

109
Q

Draw and label a current-voltage graph and label 3 key parts using the equation
V = -Ir + ε

A

(See notes)

110
Q

Resistance in an open circuit

A

Infinite

111
Q

Resistance in a short circuit

A

Zero

112
Q

What does the fact that an open circuit has infinite resistance mean?

A

Zero current can flow

113
Q

Resistance of a short circuit

A

Zero

114
Q

What does the fact that a short circuit has zero existence mean for it?

A

There’s zero voltage difference for any current

115
Q

Describe the resistance of a voltmeter and explain why

A

More or less infinite
Draws no energy or charge from the circuit in order to measure the actual potential difference across 2 points

116
Q

Equation for determining internal resistance

A

V = E - Ir

117
Q

E in V = E - Ir

A

Emf

118
Q

V in V = E - Ir

A

Terminal pd of a cell

119
Q

r in V =E - Ir

A

Internal resistance

120
Q

I in V = E - Ir

A

Current

121
Q

When is the power supply of a source as maximum?

A

When internal resistance = external resistance
R = r

122
Q

What does it mean when internal resistance = external resistance?

A

Maximum power supply

123
Q

What is brightness directly proportional to?

A

Power

124
Q

If the current and resistance for 2 bulbs stays the same, compare their brightnesses and explain why this happens

A

Brightness is directly proportional to POWER
P = I^2R
so, they have the same brightness

125
Q

What happens if the resistance decreases in a parallel circuit?

A

The potential difference across resistors decreases

126
Q

In which situation would the current coming up to a junction in a parallel circuit split evenly?

A

If the resistance values at both junctions were even

127
Q

Describe the difference in terms of energy between emf and potential difference

A

Emf - energy supplied by the source
Pd - energy used in external circuit

128
Q

What happens to the emf and internal resistance of a cell if another identical cell were added in series?

A

Both would double

129
Q

What is 1 volt equivalent to?

A

1 joule per coulomb

130
Q

What does a battery do?

A

Gives charge electrical potential energy

131
Q

Describe the voltage in a series circuit

A

Shared amongst components

132
Q

Describe the voltage in parallel circuits

A

All components get the full voltage

133
Q

Describe the current in a series circuit

A

The current is constant in all parts

134
Q

Describe the current in a parallel circuit

A

The current is split between branches based on resistance

135
Q

How do we work out the number of electrons passing through a point per minute?

A

Charge
———— x 60
e

136
Q

What would happen to the power of a circuit if the internal resistance decreased and why?

A

Increase
Total resistance of circuit is less for all values of R

137
Q

Which path does current always go through?

A

the one of least resistance

138
Q

Potential difference across parallel circuits

A

The same (even if they aren’t of the same value of resistance

139
Q

What does Ir represent in V = E - Ir?

A

Energy per unit charge lost in source

140
Q

What do we not write when using a graph to calculate internal resistance with the y-intercept?

A

The negative sign, even if it’s a negative gradient

141
Q

Compare conventional current and electron flow

A

Convention current —> + to -
Electrons —> - to +

A level Physics (29 decks)

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