4A3 Electrical Current and Capacitance

Question 1

Define:

Electric current

Answer 1

The flow of electric charge through a conductor per unit time.

Measured in amperes (A); 1 A = 1 coulomb/second.

Question 2

How does potential difference affect current in a circuit?

Answer 2

The potential difference drives the current through a circuit.

Also called voltage, it is measured in volts (V).

Question 3

How is electrical current created?

Answer 3

It is caused by electrons moving through a conductor due to an applied voltage.

The movement is driven by the electric field established by the voltage source.

Question 4

What is the SI unit of electric current?

Answer 4

Ampere (A)

It is equivalent to Coulomb/second (C/s), where Coulomb (C) is the SI unit of charge.

Question 5

True or false:

Resistance is the opposition to the flow of electrical current.

Answer 5

True

Resistance is measured in ohms (Ω).

Question 6

Fill in the blank:

Electrical resistance is affected by the material’s _______ and its temperature.

Answer 6

resistivity

Resistivity quantifies how strongly a material opposes current flow.

Question 7

Why do metals like copper have low resistance?

Answer 7

Metals have a high number of free electrons that can move easily under an electric field.

This makes them excellent electrical conductors.

Question 8

What does resistivity measure?

Answer 8

It measures how strongly a material opposes the flow of electric current.

It is measured in ohm-meters (Ω·m).

Question 9

How does the length of a conductor affect its resistance?

Answer 9

The longer the conductor, the higher its resistance.

Longer conductors provide more material for electrons to collide with.

Question 10

How is the resistance of a circuit measured?

Answer 10

Using an ohmmeter.

To make the measure, the circuit must be de-energized to avoid inaccurate readings or damage to the device.

Question 11

Fill in the blank:

The instrument used to measure electrical current is called a(n) _______.

Answer 11

Ammeter

An ammeter is connected in series with the circuit to measure the current flowing through it.

Question 12

What is the difference between a galvanometer and an ammeter?

Answer 12

Both devices detect electric current, but an ammeter measures its magnitude, while a galvanometer determines the direction and strength of small currents.

Galvanometers are often converted into ammeters by adding a low-resistance shunt.

Question 13

Define:

Potential difference

Answer 13

Difference in the amount of energy that charge carriers have between two points in a circuit.

It is measured in volts (V)

Question 14

How is the voltage measured?

Answer 14

It is measured using a voltmeter connected in parallel with the component or circuit.

It is important to ensure that the range setting of the voltmeter matches the expected voltage for accurate measurements.

Question 15

What does Ohm’s Law state?

Answer 15

Current (I) is directly proportional to voltage (V) and inversely proportional to resistance (R).

Formula: V=I×R

Question 16

What is the formula to calculate current using Ohm’s Law?

Answer 16

I= V/R

Current is measured in amperes (A).

Question 17

True or false:

If resistance increases while voltage remains constant, current will increase.

Answer 17

False

Current decreases when resistance increases at constant voltage.

Question 18

Explain why Ohm’s Law is important in circuit design.

Answer 18

It helps determine the correct resistance values to ensure safe current flow and voltage distribution in circuits.

Essential for power regulation and safety.

Question 19

For which types of circuits does Ohm’s Law apply?

Answer 19

It applies to linear circuits where the resistance remains constant regardless of the voltage or current.

These circuits are typically made of ohmic materials, such as metals under normal conditions. Non-linear components, like diodes and transistors, do not follow Ohm’s Law as their resistance changes with voltage and current.

Question 20

Define:

Non-ohmic material

Answer 20

Material in which the resistance does not remain constant and does not follow Ohm’s Law.

In these materials, the current-voltage relationship is non-linear. Examples of non-ohmic materials include semiconductors, diodes, and transistors.

Question 21

If a circuit has a voltage of 12V and a resistance of 4Ω, what is the current?

Answer 21

3A

I = V / R = 12V / 4Ω = 3A

Question 22

What happens to current if both voltage and resistance double?

Answer 22

The current remains unchanged.

Doubling both voltage and resistance keeps their ratio constant.

Question 23

How does temperature affect the applicability of Ohm’s Law?

Answer 23

Rising temperatures can make materials non-ohmic by altering their resistance.

Filament lamps deviate from Ohm’s Law when heated.

Question 24

A resistor has 0.5 A of current and 5 V across it. What is its resistance?

Answer 24

10Ω

R = V / I = 5 V / 0.5 A = 10 Ω

Question 25

Explain why **high-resistance** materials are used in heating elements.

Answer 25

High resistance converts **electrical energy** into **heat** efficiently. ## Footnote Example: Nichrome in electric heaters.

Question 26

# Define: Electrical power

Answer 26

**Rate** at which electrical energy is transferred or converted. ## Footnote Measured in watts (W).

Question 27

What is the formula for calculating **electrical power**?

Answer 27

P=V×I ## Footnote Where P is power, V is voltage, and I is current.

Question 28

# True or false: Reducing the **resistance** in a circuit decreases the **power** output.

Answer 28

False ## Footnote Lower resistance at constant voltage typically increases current, thus increasing power.

Question 29

# Fill in the blank: The **unit of energy** used in **household** electricity billing is the \_\_\_\_\_\_\_.

Answer 29

kilowatt-hour (kWh) ## Footnote 1 kWh equals 1,000 watts used for one hour.

Question 30

What is the power dissipated by a **5Ω** resistor when a current of **2A** flows through it?

Answer 30

20W ## Footnote P=I² ×R=(2A)² × (5Ω)=20W

Question 31

Explain why **energy-efficient** appliances are preferred.

Answer 31

They **consume less electrical energy for the same output**, reducing costs and environmental impact. ## Footnote Lower power consumption translates to lower energy bills.

Question 32

What is the relationship between **power**, **energy**, and **time**?

Answer 32

Electrical energy is **proportional to power and inversely proportional to time.** ## Footnote It is given by the formula E=P×t. Energy is measured in joules (J), power in watts (W), and time in seconds (s).

Question 33

# True or false: The **power consumption** of an electrical device depends only on the **voltage** applied.

Answer 33

False ## Footnote Power depends on both voltage and current.

Question 34

If a bulb operates at **240V** and draws a current of **0.5A**, what is its power rating?

Answer 34

120W ## Footnote P=V×I=240V×0.5A.

Question 35

How does **resistance** affect the **power dissipated** in a circuit?

Answer 35

**Higher resistance reduces current flow**, which can decrease power dissipation if voltage remains constant. ## Footnote Power can be expressed as P= V²R

Question 36

What is the **power factor** in electrical systems?

Answer 36

The **ratio of real power to apparent power**, measuring how efficiently power is used.' ## Footnote A power factor of 1 indicates maximum efficiency.

Question 37

What are the two types of **electric current**?

Answer 37

1. Direct current (DC) 2. Alternating current (AC) ## Footnote Each type has unique applications and characteristics.

Question 38

# Define: Direct current (DC)

Answer 38

A type of current with a **fixed magnitude and direction**, flowing from the negative to the positive terminal. ## Footnote The current remains constant over time.

Question 39

# Define: Alternating current (AC)

Answer 39

A type of current with **time-varying magnitude and direction**, changing several times per second. ## Footnote The direction of the net flow of electrons changes periodically.

Question 40

# True or false: **DC power** transmission is more efficient than **AC** for long distances.

Answer 40

False ## Footnote AC is better for long distances due to easier voltage transformation.

Question 41

# Fill in the blank: In most homes, **electrical outlets** supply \_\_\_\_\_\_ current.

Answer 41

alternating ## Footnote Typically at 60 Hz in the U.S. and 50 Hz in other regions.

Question 42

What device is used to **convert DC to AC**?

Answer 42

Inverter ## Footnote Inverters are commonly used in renewable energy systems.

Question 43

Explain why **AC** is preferred over **DC** for **power distribution**.

Answer 43

**AC can be easily transformed to different voltages**, making it more efficient for power transmission. ## Footnote Transformers work only with AC.

Question 44

What are some **applications** of **direct current (DC)**?

Answer 44

* Batteries in devices like watches and calculators. * Operation of transistors in electronic devices. * High-voltage direct current (HVDC) transmission for long distances. ## Footnote DC is commonly used for energy storage and certain electronic applications.

Question 45

What are some **applications** of **alternating current (AC)**?

Answer 45

* Power transmission from generation plants to households. * Charging rechargeable batteries. * Powering home appliances like microwaves and computers. ## Footnote AC is preferred for its efficiency in power transmission.

Question 46

How does **electric current** flow in **liquids**?

Answer 46

Through the **movement of positively and negatively charged ions**. ## Footnote Liquids with free ions (like saltwater or acids) conduct electricity when voltage is applied.

Question 47

What does the **root mean square (RMS)** value represent in AC systems?

Answer 47

The **equivalent DC value that delivers the same power** as the AC current. ## Footnote RMS is crucial for measuring AC voltages.

Question 48

# Define: Capacitance

Answer 48

Ability of a system to **store electric charge** per unit of potential difference. ## Footnote The formula is C=Q/V, where C is capacitance, Q is electric charge, and V is voltage.

Question 49

What is the **SI unit** of **capacitance**?

Answer 49

Farad (F) ## Footnote One farad is the capacitance when one coulomb of charge is stored with one volt across its plates.

Question 50

What is a **capacitor**?

Answer 50

A **device that stores electrical energy using two conductors** separated by a dielectric. ## Footnote They are used in energy storage, power smoothing, and noise filtering in circuits.

Question 51

# True or false: **Capacitance** depends only on the **material** between the capacitor plates.

Answer 51

False ## Footnote Capacitance depends on plate size, separation distance, and dielectric material.

Question 52

What is the formula for the **capacitance** of a **parallel plate capacitor**?

Answer 52

C = ϵA/d ## Footnote Where: ϵ = permittivity, A = plate area, d = distance between plates.

Question 53

What is the formula for **energy stored in a capacitor**?

Answer 53

U = 1/2 CV² ## Footnote This formula can also be expressed using charge: U = Q²/(2C) or U = QV/2.

Question 54

What are the key differences between **capacitors and batteries**?

Answer 54

* Capacitors store energy in the form of charge (electromechanical). * Batteries store chemical energy transformed into electric energy. ## Footnote Capacitors are faster in charging and discharging than batteries.

Question 55

What is the process of **charging a capacitor**?

Answer 55

Charging involves **connecting to a voltage source**, creating a potential difference. ## Footnote The charge builds up until the capacitor reaches its maximum capacity.

Question 56

What happens during the **discharge process** of a capacitor?

Answer 56

The capacitor **releases its stored charge**, typically into a circuit. ## Footnote This process occurs when the voltage source is removed.

Question 57

List three common **applications of capacitors**.

Answer 57

* Radio turning circuits. * Timer circuits like clocks and alarms. * Powering electric and hybrid cars. ## Footnote Capacitors preserve information during power failures.

Question 58

What are the two main types of **fixed capacitors**?

Answer 58

* Polar capacitors (electrolytic) * Non-polar capacitors (ceramic, mica, paper, plastic) ## Footnote Polar capacitors have fixed positive and negative terminals, while non-polarized capacitors do not.

Question 59

What is the key difference between **polar and non-polar capacitors**?

Answer 59

**Polar capacitors have fixed polarity**; non-polar capacitors can be connected either way. ## Footnote Polar capacitors are used mainly in DC applications.

Question 60

# True or false: **Capacitors** block **direct current** but allow **alternating current** to pass.

Answer 60

True ## Footnote This property is used in AC signal coupling.