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H2 Physics > Current of Electricity > Flashcards

Current of Electricity Flashcards

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1
Q
A
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2
Q

What is electric current?

A

The rate of flow of charge

Electric current is represented by the symbol I.

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3
Q

What is the formula for electric current?

A

I = dQ/dt

For steady current, I = Q/t.

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4
Q

What is the S.I. unit of electric current?

A

Ampere, A

The ampere is a base unit in the International System of Units.

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5
Q

What is the relationship between charge and current?

A

Q = I t

Q is the total charge that passes through a point over time t.

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6
Q

What does the equation I = nAvq represent?

A

The current flowing in a conductor

Where n is the number density of charge carriers, A is the cross-sectional area, v is the drift velocity, and q is the charge of each carrier.

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7
Q

What does the term ‘drift velocity’ refer to?

A

The average velocity of free electrons in a conductor

Drift velocity occurs when an electric field is applied.

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8
Q

What is the S.I. unit of electric charge?

A

Coulomb, C

One coulomb is the charge that flows when a steady current of one ampere flows for one second.

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9
Q

Define resistance in a circuit.

A

The ratio of potential difference to current

Resistance is defined using the equation V = I R.

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10
Q

What is the equation for potential difference?

A

V = W/Q

Where V is potential difference, W is work done, and Q is charge.

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11
Q

What is the equation for electrical power?

A

P = IV

Other forms include P = I^2 R and P = V^2/R.

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12
Q

What distinguishes electromotive force (e.m.f.) from potential difference (p.d.)?

A

E.m.f. refers to energy supplied per unit charge, while p.d. refers to energy converted per unit charge

Understanding this distinction involves energy considerations.

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13
Q

What happens to free electrons in a conductor when an electric field is applied?

A

They drift towards the positive terminal

The drift of electrons results in a net current in the conductor.

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14
Q

True or False: The direction of conventional current is the same as the flow of electrons.

A

False

Conventional current flows from positive to negative, opposite to electron flow.

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15
Q

What is the formula for resistance in terms of resistivity?

A

R = ρℓ/A

Where R is resistance, ρ is resistivity, ℓ is length, and A is cross-sectional area.

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16
Q

Fill in the blank: The unit of charge for an electron is _______.

A

-1.6 × 10^-19 C

This value is also referred to as -1e.

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17
Q

What characterizes an NTC thermistor?

A

Resistance decreases with an increase in temperature

NTC stands for negative temperature coefficient.

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18
Q

What is the role of a battery in a circuit?

A

It provides electromotive force (e.m.f.)

This causes the flow of electric current in the circuit.

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19
Q

What occurs when there is no potential difference in a circuit?

A

No current flows

This is analogous to water flow stopping when there is no pressure difference.

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20
Q

What is the relationship between the number of charge carriers, charge, and total charge?

A

Q = Nq

Where N is the number of charged particles and q is the charge of each particle.

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21
Q

How does the electric field affect the motion of electrons in a conductor?

A

It causes electrons to drift in the direction opposite to the field

The electric field applies a force on the electrons.

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22
Q

What is the formula for current I in a conductor?

A

I = n A q v

Where n is the number density of mobile charge carriers, A is the cross-sectional area, q is the charge on each carrier, and v is the drift velocity.

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23
Q

What is the drift velocity of electrons typically measured in?

A

A few millimeters per second

This is contrary to the common belief that electricity travels instantaneously.

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24
Q

True or False: Drift velocity is the same as the speed of electricity.

A

False

Drift velocity refers to the average velocity of electrons, while the speed of electricity relates to the electric field’s propagation.

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25
What happens to free electrons in a conductor when a circuit is closed?
They drift due to the electric field ## Footnote This causes the current to appear almost instantaneously.
26
What is the S.I. unit of potential difference?
Volt (V) ## Footnote The potential difference is defined as the electrical energy converted per unit charge.
27
What is the formula for potential difference V?
V = W/Q ## Footnote Where W is the electrical energy converted and Q is the total charge that moves.
28
What is the formula for resistance R in terms of potential difference and current?
R = V/I ## Footnote The S.I. unit of resistance is the ohm (Ω).
29
Explain how to determine if resistance R is increasing, decreasing, or constant.
Compare the slopes of lines drawn from the origin to points on the I-V graph ## Footnote If the slope decreases, R increases; if it increases, R decreases; if the slopes are equal, R is constant.
30
What is Joule heating?
The heating effect caused by the current flowing through a resistor ## Footnote It occurs due to collisions between charge carriers and ions in the conductor.
31
What is the equation for electrical power P?
P = W/t ## Footnote Where W is the electrical energy converted and t is the time interval.
32
List the three equations for electrical power derived from Ohm's law.
* P = IV * P = I²R * P = V²/R ## Footnote These equations can be used to calculate electrical power in different scenarios.
33
What are the characteristics of an ohmic conductor?
The I-V graph is a straight line passing through the origin ## Footnote This indicates that current is directly proportional to the potential difference.
34
What does a non-ohmic conductor's I-V graph look like?
It does not pass through the origin and may curve ## Footnote This indicates that the resistance changes with voltage.
35
What is the effect of temperature on a thermistor?
Resistance decreases with increasing temperature ## Footnote This is because the I-V graph becomes steeper at higher temperatures.
36
How does an immersion heater's resistance relate to its power rating?
Resistance can be calculated using the formula P = I²R ## Footnote The power rating and current can help determine the resistance.
37
What is the relationship between the current through two bulbs operating at the same potential difference when one has four times the power output?
The bulb with greater power has the greater current ## Footnote The ratio of currents can be found using the power equations.
38
What does Ohm's Law state about current and potential difference?
The current in a conductor at constant temperature is directly proportional to the potential difference across it. ## Footnote This is a fundamental principle in electronics describing the relationship between voltage, current, and resistance.
39
How does the resistance of a filament lamp change with increasing potential difference?
The resistance increases as the potential difference increases. ## Footnote This is because the heat generated causes the temperature to rise, which increases the resistance.
40
What is the characteristic of a filament lamp's I-V graph?
The I-V characteristic of a filament lamp is non-linear. ## Footnote This reflects the changing resistance with temperature.
41
What is the behavior of a diode in forward-biased and reverse-biased conditions?
In forward-biased: low resistance, large current; in reverse-biased: high resistance, very little current. ## Footnote Diodes are used to convert alternating current (AC) to direct current (DC).
42
What is a Negative Temperature Coefficient (NTC) thermistor?
A thermistor where the resistance decreases as temperature increases. ## Footnote NTC thermistors are commonly used in digital thermometers and thermostats.
43
What factors does the resistance of a conductor depend on?
The resistance depends on its length, cross-sectional area, and type of material. ## Footnote The formula is R = ρ(ℓ/A) where ρ is resistivity.
44
What is the definition of resistivity?
The resistivity of a material is its resistance per unit length and unit cross-sectional area. ## Footnote The SI unit of resistivity is ohm-meter (Ω·m).
45
List some materials and their resistivity at 20 °C.
* Quartz: 7.5 × 10^17 Ω·m * Rubber: 1 to 100 × 10^13 Ω·m * Silicon: 0.10 to 60 Ω·m * Copper: 1.72 × 10^-8 Ω·m * Silver: 1.59 × 10^-8 Ω·m ## Footnote This list includes insulators, semiconductors, and conductors.
46
What is the definition of electromotive force (e.m.f)?
The e.m.f. of a source is the energy converted from other forms into electrical energy per unit charge driven through the source. ## Footnote E.m.f. is not a force despite its name.
47
What is the difference between e.m.f. and potential difference (p.d.)?
E.m.f. is energy converted into electrical energy from other forms, while p.d. is energy converted from electrical energy into other forms per unit charge between two points. ## Footnote This distinction is important in understanding how energy is transferred in electrical circuits.
48
What causes internal resistance in a source of e.m.f.?
Internal resistance is caused by the materials within the source that have some resistance. ## Footnote This includes coils in generators or chemicals in batteries.
49
What happens to power delivered by a battery as external resistance varies?
Power is zero when R = 0 (large current) or R → ∞ (very small current); maximum power occurs at an intermediate value. ## Footnote This is known as the Maximum Power Theorem.
50
Fill in the blank: The resistance of a conductor is _______ proportional to its length and _______ proportional to its cross-sectional area.
directly; inversely ## Footnote This relationship is fundamental in understanding how resistance works in different materials.
51
What is the significance of terminal potential difference?
It is the p.d. across the external circuit connected to the source. ## Footnote It is equal to the p.d. across the external resistance.
52
How does the resistance of an NTC thermistor change with applied potential difference?
The resistance decreases with increasing potential difference due to an increase in the number of charge carriers. ## Footnote This is a unique property of NTC thermistors.
53
What are the mobile charge carriers in metallic conductors?
Free electrons ## Footnote Free electrons are loosely bound to metallic ions and can move in any direction.
54
What characterizes semiconductors compared to metals and insulators?
Fewer free electrons than metals but more than insulators ## Footnote Majority of electrons are bounded to the atom; thermal energy can free some electrons, creating holes.
55
What are the charge carriers in ionic solutions?
Positive and negative ions ## Footnote Electrolytes are ionic compounds dissolved in solution or in molten state.
56
Under what conditions can gases conduct electricity?
When subjected to high potential difference or temperature ## Footnote Gas molecules may become ionised, producing electrons and positive ions.
57
Define conductance and its units.
Inverse of resistance; units: Ω⁻¹
58
What is conductivity and its formula?
σ = 1/ρ; units: Ω⁻¹ m⁻¹
59
How is current density defined?
J = I/A; units: A m⁻²
60
What is the formula for charge in relation to current?
Q = ∫ I dt
61
Calculate the total charge from given intervals: 6000 C, 4000 C, 9000 C.
19000 C
62
How do you calculate current due to positive ions?
Q+ = N q+ ## Footnote Example: (1.5 × 10¹⁵)(1.6 × 10⁻¹⁹) = 2.4 × 10⁻⁴ C
63
What is the total current when both positive ions and electrons contribute?
9.4 × 10⁻⁴ A
64
What happens when I = 0 in terms of emf?
E = p.d across the terminal of the battery
65
What is the relationship between emf, internal resistance, and external resistance?
E = I r + V
66
How do you calculate power dissipated in a resistor?
P = I²R
67
What is the common error when calculating power with non-constant current?
Using P=VI
68
State the formula for power loss in internal resistance.
P_loss = I²r
69
According to the Maximum Power Theorem, what is the condition for maximum power?
R = r
70
What is the efficiency formula related to power output and supplied power?
Efficiency = (Power output in R / Power supplied by battery) × 100%
71
As resistance increases, what happens to efficiency?
Efficiency increases ## Footnote Because PT decreases at a higher rate than PR.
72
Fill in the blank: Conductance is the _______ of resistance.
inverse
73
True or False: The current flow of positive ions and electrons is in the same direction.
True

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