Electricity

Question 2

What is the definition of electric current?

Answer 2

The rate of flow of electric charge.

Question 3

What is the SI unit for electric charge?

Answer 3

The Coulomb (C).

Question 4

Formula: The relationship between charge (Q), current (I), and time (t).

Answer 4

Q = It

Question 5

Definition: One Coulomb (1 C)

Answer 5

The charge that passes a point in a circuit in 1 s when the current is 1 A.

Question 6

What are the primary charge carriers in a metallic conductor?

Answer 6

Free electrons.

Question 7

What are the charge carriers in an electrolyte?

Answer 7

Ions.

Question 8

What is the approximate value of the elementary charge e?

Answer 8

1.60 x 10^-19C

Question 9

How is the net charge Q on an object related to the elementary charge e?

Answer 9

Q = nxe, where n is an integer number of charge carriers.

Question 10

State Kirchhoff’s first law.

Answer 10

The sum of the currents entering a junction is equal to the sum of the currents leaving the junction.

Question 11

Kirchhoff’s first law is a consequence of the conservation of _____.

Answer 11

Charge

Question 12

Term: Mean drift velocity (v)

Answer 12

The average displacement of charge carriers along a conductor per unit time.

Question 13

Formula: The transport equation for current (I) in terms of mean drift velocity (v).

Answer 13

I = nAev

Question 14

In the equation I = nAev, what does the variable n represent?

Answer 14

The number density of free charge carriers
(number of carriers per unit volume)

Question 15

How do the number densities (n) of charge carriers in semiconductors compare to those in metals?

Answer 15

Semiconductors have significantly lower number densities than metals.

Question 16

Why is the mean drift velocity in a wire typically very small (e.g., 10^-3 ms^-1

Answer 16

Charge carriers constantly collide with the positive ions of the conductor’s lattice.

Question 17

Definition: Potential Difference (p.d.)

Answer 17

The energy transferred per unit charge from electrical energy into other forms.

Question 18

Definition: Electromotive Force (e.m.f.)

Answer 18

The energy transferred per unit charge from other forms into electrical energy.

Question 19

Which quantity describes energy being transferred TO the charge carriers: p.d. or e.m.f.?

Answer 19

e.m.f.

Question 20

Definition: One Volt (1 V)

Answer 20

One Joule of energy transferred per Coulomb of charge (1 J C^^-1).

Question 21

Formula: Work done (W) on a charge (Q) by a potential difference (V).

Answer 21

W = VQ

Question 22

Definition: Electronvolt (eV)

Answer 22

The energy gained by an electron when it is accelerated through a potential difference of 1 V.

Question 23

What is the conversion factor from electronvolts (eV) to Joules (J)?

Answer 23

$1.60 x 10^-19J eV^-1

Question 24

Term: Resistance (R)

Answer 24

The ratio of the potential difference across a component to the current through it.

Question 25

Formula: The definition of resistance.

Answer 25

R = V/I

Question 26

Definition: One Ohm (1 Ω)

Answer 26

The resistance of a component when a p.d. of 1 V produced a current of 1 A.

Question 27

State Ohm's Law.

Answer 27

The current through a conductor is directly proportional to the p.d. across it, provided physical conditions remain constant.

Question 28

Under what condition is a conductor considered 'Ohmic'?

Answer 28

When its resistance remains constant regardless of the p.d. applied.

Question 29

How does the resistance of a filament lamp change as the potential difference across it increases?

Answer 29

The resistance increases.

Question 30

Why does the resistance of a metal wire increase with temperature?

Answer 30

Lattice ions vibrate with greater amplitude, increasing the frequency of collisions with charge carriers.

Question 31

Term: Threshold Voltage

Answer 31

The minimum forward potential difference required for a diode to conduct significant current.

Question 32

Concept: Negative Temperature Coefficient (NTC) Thermistor

Answer 32

A component whose resistance decreases as its temperature increases.

Question 33

How does increasing light intensity affect the resistance of a Light Dependent Resistor (LDR)?

Answer 33

The resistance decreases.

Question 34

Formula: The relationship between resistance (R), resistivity (ρ), length (L), and cross-sectional area (A).

Answer 34

ρ = (RA)/L

Question 35

What is the SI unit for resistivity?

Answer 35

The Ohm-meter (\Omega m).

Question 36

Why does the resistivity of a semiconductor decrease as temperature increases?

Answer 36

The number density (n) of charge carriers increases significantly as more electrons are liberated.

Question 37

Formula: Electrical power (P) in terms of current (I) and potential difference (V).

Answer 37

P = VI

Question 38

Formula: Electrical power (P) in terms of current (I) and resistance (R).

Answer 38

$P = IR$

Question 39

Formula: Electrical power (P) in terms of potential difference (V) and resistance (R).

Answer 39

P = V²/R

Question 40

Formula: Total electrical energy ($W$) transferred in time ($t$) at potential difference ($V$) and current ($I$).

Answer 40

W = VIt

Question 41

Definition: Kilowatt-hour (kWh)

Answer 41

The energy transferred by a 1 kW device operating for 1 hour.

Question 42

How many Joules are in 1 kWh?

Answer 42

3.6 x 10⁶J

Question 43

State Kirchhoff's second law.

Answer 43

The sum of the e.m.f.s is equal to the sum of the p.d.s around any closed loop in a circuit.

Question 44

Kirchhoff's second law is a consequence of the conservation of _____.

Answer 44

Energy

Question 45

How is the total resistance calculated for resistors connected in series?

Answer 45

R_tot = R₁}+ R₂

Question 46

How is the total resistance calculated for resistors connected in parallel?

Answer 46

1/R_tot = 1/R₁ + 1/R₂

Question 47

In a series circuit, what can be said about the current at any point?

Answer 47

The current is the same at all points.

Question 48

In a parallel circuit, what can be said about the potential difference across each branch?

Answer 48

The potential difference is the same across each branch.

Question 49

Term: Internal Resistance ($r$)

Answer 49

The resistance to electric current within the power source itself.

Question 50

Term: Terminal Potential Difference ($V$)

Answer 50

The potential difference measured across the terminals of a power source when it is connected to a circuit.

Question 51

Term: Lost Volts

Answer 51

The potential difference across the internal resistance of a power source, calculated as $Ir$.

Question 52

Formula: The relationship between $e.m.f.$ ($\epsilon$), terminal $p.d.$ ($V$), current ($I$), and internal resistance ($r$).

Answer 52

epsilon = V + Ir

Question 53

Formula: e.m.f. (ϵ) in terms of total circuit resistance (R + r).

Answer 53

ϵ = I(R + r)

Question 54

As the current drawn from a power source increases, how does the terminal potential difference change?

Answer 54

It decreases.

Question 55

On a graph of terminal p.d. (V) against current (I), what does the y-intercept represent?

Answer 55

The e.m.f. (ε) of the source.

Question 56

On a graph of terminal p.d. (V) against current (I), what does the magnitude of the gradient represent?

Answer 56

The internal resistance (r) of the source.

Question 57

Concept: Potential Divider

Answer 57

A circuit designed to provide a specific output voltage by dividing the source e.m.f. between two or more resistors.

Question 58

Formula: Output voltage (V_{out}) for a potential divider with resistors $R_{1}$ and $R_{2}$ (where $V_{out}$ is across $R_{2}$).

Answer 58

$V_{out} = \frac{R_{2}}{R_{1} + R_{2}} \times V_{in}$

Question 59

How does the ratio of voltages across resistors in a potential divider compare to the ratio of their resistances?

Answer 59

The ratios are equal ($\frac{V_{1}}{V_{2}} = \frac{R_{1}}{R_{2}}$).

Question 60

What component can be added to a potential divider to create a temperature-sensitive circuit?

Answer 60

A thermistor.

Question 61

What component can be added to a potential divider to create a light-sensitive circuit?

Answer 61

A Light Dependent Resistor (LDR).

Question 62

Term: Potentiometer

Answer 62

A variable resistor with three terminals and a sliding contact used as a variable potential divider.

Question 63

What is the ideal internal resistance of an ammeter?

Answer 63

Zero Ohms ($0 \Omega$).

Question 64

What is the ideal internal resistance of a voltmeter?

Answer 64

Infinite Ohms ($\infty \Omega$).

Question 65

How should an ammeter be connected to measure the current through a component?

Answer 65

In series with the component.

Question 66

How should a voltmeter be connected to measure the $p.d.$ across a component?

Answer 66

In parallel with the component.

Question 67

In a parallel circuit with two resistors, if $R_{1} > R_{2}$, which resistor will have the greater current?

Answer 67

$R_{2}$

Question 68

What happens to the total resistance of a circuit if an additional resistor is added in parallel?

Answer 68

The total resistance decreases.

Question 69

Formula: The electrical energy ($W$) in terms of charge ($Q$) and $p.d.$ ($V$).

Answer 69

$W = VQ$

Question 70

The current in a semiconductor is caused by the movement of electrons and _____.

Answer 70

Holes

Question 71

How does the resistance of a diode in reverse bias compare to its resistance in forward bias?

Answer 71

The resistance in reverse bias is much higher (nearly infinite).

Question 72

In a series circuit with two resistors, if $R_{1}$ is increased, what happens to the $p.d.$ across $R_{1}$?

Answer 72

The $p.d.$ across $R_{1}$ increases.

Question 73

What is the physical meaning of a number density $n = 10^{28} m^{-3}$ for a material?

Answer 73

There are $10^{28}$ free charge carriers in every cubic meter of the material.

Question 74

Why is the $e.m.f.$ of a battery often higher than its terminal $p.d.$ when a current is flowing?

Answer 74

Energy is dissipated by the internal resistance of the battery.

Question 75

Formula: Resistance ($R$) in terms of resistivity ($\rho$), length ($L$), and diameter ($d$).

Answer 75

$R = \frac{4 \rho L}{\pi d^{2}}$

Question 76

What effect does increasing the length of a wire have on its resistivity?

Answer 76

No effect (resistivity is a property of the material).

Question 77

In a potential divider circuit, if $R_{1}$ is a fixed resistor and $R_{2}$ is an LDR, what happens to $V_{out}$ (across $R_{2}$) as light intensity increases?

Answer 77

$V_{out}$ decreases.

Question 78

In a potential divider circuit, if $R_{1}$ is a fixed resistor and $R_{2}$ is an NTC thermistor, what happens to $V_{out}$ (across $R_{2}$) as temperature increases?

Answer 78

$V_{out}$ decreases.

Question 79

What is the name of the rule that states 'at any junction in a circuit, the total current entering the junction equals the total current leaving it'?

Answer 79

Kirchhoff's first law.

Question 80

What is the name of the rule that states 'around any closed loop in a circuit, the sum of the $e.m.f.s$ is equal to the sum of the $p.d.s$'?

Answer 80

Kirchhoff's second law.

Question 81

If the current in a circuit is $2 A$, how much charge passes a point in $5$ minutes?

Answer 81

$600 C$

Question 82

A component has a resistance of $10 \Omega$ and a current of $0.5 A$. What is the power dissipated?

Answer 82

$2.5 W$

Question 83

What is the relationship between the number of charge carriers and current in the transport equation?

Answer 83

Current is directly proportional to the number density ($I \propto n$).

Question 84

How is the $p.d.$ across a component related to the work done on each Coulomb of charge?

Answer 84

They are numerically equal ($V = \frac{W}{Q}$).

Question 85

What is the total resistance of two $100 \Omega$ resistors connected in parallel?

Answer 85

$50 \Omega$

Question 86

What is the total resistance of two $100 \Omega$ resistors connected in series?

Answer 86

$200 \Omega$

Question 87

Which equation represents the conservation of energy for a charge moving around a complete circuit loop?

Answer 87

$\sum \epsilon = \sum V$ (Kirchhoff's second law).