4A3 Electrical Current and Capacitance Flashcards
Describe the properties and types of electrical current, and explain the role of capacitance.
Define:
Electric current
The flow of electric charge through a conductor per unit time.
Measured in amperes (A); 1 A = 1 coulomb/second.
How does potential difference affect current in a circuit?
The potential difference drives the current through a circuit.
Also called voltage, it is measured in volts (V).
How is electrical current created?
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.
What is the SI unit of electric current?
Ampere (A)
It is equivalent to Coulomb/second (C/s), where Coulomb (C) is the SI unit of charge.
True or false:
Resistance is the opposition to the flow of electrical current.
True
Resistance is measured in ohms (Ω).
Fill in the blank:
Electrical resistance is affected by the material’s _______ and its temperature.
resistivity
Resistivity quantifies how strongly a material opposes current flow.
Why do metals like copper have low resistance?
Metals have a high number of free electrons that can move easily under an electric field.
This makes them excellent electrical conductors.
What does resistivity measure?
It measures how strongly a material opposes the flow of electric current.
It is measured in ohm-meters (Ω·m).
How does the length of a conductor affect its resistance?
The longer the conductor, the higher its resistance.
Longer conductors provide more material for electrons to collide with.
How is the resistance of a circuit measured?
Using an ohmmeter.
To make the measure, the circuit must be de-energized to avoid inaccurate readings or damage to the device.
Fill in the blank:
The instrument used to measure electrical current is called a(n) _______.
Ammeter
An ammeter is connected in series with the circuit to measure the current flowing through it.
What is the difference between a galvanometer and an ammeter?
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.
Define:
Potential difference
Difference in the amount of energy that charge carriers have between two points in a circuit.
It is measured in volts (V)
How is the voltage measured?
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.
What does Ohm’s Law state?
Current (I) is directly proportional to voltage (V) and inversely proportional to resistance (R).
Formula: V=I×R
What is the formula to calculate current using Ohm’s Law?
I= V/R
Current is measured in amperes (A).
True or false:
If resistance increases while voltage remains constant, current will increase.
False
Current decreases when resistance increases at constant voltage.
Explain why Ohm’s Law is important in circuit design.
It helps determine the correct resistance values to ensure safe current flow and voltage distribution in circuits.
Essential for power regulation and safety.
For which types of circuits does Ohm’s Law apply?
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.
Define:
Non-ohmic material
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.
If a circuit has a voltage of 12V and a resistance of 4Ω, what is the current?
3A
I = V / R = 12V / 4Ω = 3A
What happens to current if both voltage and resistance double?
The current remains unchanged.
Doubling both voltage and resistance keeps their ratio constant.
How does temperature affect the applicability of Ohm’s Law?
Rising temperatures can make materials non-ohmic by altering their resistance.
Filament lamps deviate from Ohm’s Law when heated.
A resistor has 0.5 A of current and 5 V across it. What is its resistance?
10Ω
R = V / I = 5 V / 0.5 A = 10 Ω
Explain why high-resistance materials are used in heating elements.
High resistance converts electrical energy into heat efficiently.
Example: Nichrome in electric heaters.
Define:
Electrical power
Rate at which electrical energy is transferred or converted.
Measured in watts (W).
What is the formula for calculating electrical power?
P=V×I
Where P is power, V is voltage, and I is current.
True or false:
Reducing the resistance in a circuit decreases the power output.
False
Lower resistance at constant voltage typically increases current, thus increasing power.
Fill in the blank:
The unit of energy used in household electricity billing is the _______.
kilowatt-hour (kWh)
1 kWh equals 1,000 watts used for one hour.
What is the power dissipated by a 5Ω resistor when a current of 2A flows through it?
20W
P=I² ×R=(2A)² × (5Ω)=20W
Explain why energy-efficient appliances are preferred.
They consume less electrical energy for the same output, reducing costs and environmental impact.
Lower power consumption translates to lower energy bills.
What is the relationship between power, energy, and time?
Electrical energy is proportional to power and inversely proportional to time.
It is given by the formula E=P×t. Energy is measured in joules (J), power in watts (W), and time in seconds (s).
True or false:
The power consumption of an electrical device depends only on the voltage applied.
False
Power depends on both voltage and current.
If a bulb operates at 240V and draws a current of 0.5A, what is its power rating?
120W
P=V×I=240V×0.5A.
How does resistance affect the power dissipated in a circuit?
Higher resistance reduces current flow, which can decrease power dissipation if voltage remains constant.
Power can be expressed as P= V²R
What is the power factor in electrical systems?
The ratio of real power to apparent power, measuring how efficiently power is used.’
## Footnote
A power factor of 1 indicates maximum efficiency.
What are the two types of electric current?
- Direct current (DC)
- Alternating current (AC)
Each type has unique applications and characteristics.
Define:
Direct current (DC)
A type of current with a fixed magnitude and direction, flowing from the negative to the positive terminal.
The current remains constant over time.
Define:
Alternating current (AC)
A type of current with time-varying magnitude and direction, changing several times per second.
The direction of the net flow of electrons changes periodically.
True or false:
DC power transmission is more efficient than AC for long distances.
False
AC is better for long distances due to easier voltage transformation.
Fill in the blank:
In most homes, electrical outlets supply ______ current.
alternating
Typically at 60 Hz in the U.S. and 50 Hz in other regions.
What device is used to convert DC to AC?
Inverter
Inverters are commonly used in renewable energy systems.
Explain why AC is preferred over DC for power distribution.
AC can be easily transformed to different voltages, making it more efficient for power transmission.
Transformers work only with AC.
What are some applications of direct current (DC)?
- Batteries in devices like watches and calculators.
- Operation of transistors in electronic devices.
- High-voltage direct current (HVDC) transmission for long distances.
DC is commonly used for energy storage and certain electronic applications.
What are some applications of alternating current (AC)?
- Power transmission from generation plants to households.
- Charging rechargeable batteries.
- Powering home appliances like microwaves and computers.
AC is preferred for its efficiency in power transmission.
How does electric current flow in liquids?
Through the movement of positively and negatively charged ions.
Liquids with free ions (like saltwater or acids) conduct electricity when voltage is applied.
What does the root mean square (RMS) value represent in AC systems?
The equivalent DC value that delivers the same power as the AC current.
RMS is crucial for measuring AC voltages.
Define:
Capacitance
Ability of a system to store electric charge per unit of potential difference.
The formula is C=Q/V, where C is capacitance, Q is electric charge, and V is voltage.
What is the SI unit of capacitance?
Farad (F)
One farad is the capacitance when one coulomb of charge is stored with one volt across its plates.
What is a capacitor?
A device that stores electrical energy using two conductors separated by a dielectric.
They are used in energy storage, power smoothing, and noise filtering in circuits.
True or false:
Capacitance depends only on the material between the capacitor plates.
False
Capacitance depends on plate size, separation distance, and dielectric material.
What is the formula for the capacitance of a parallel plate capacitor?
C = ϵA/d
Where: ϵ = permittivity, A = plate area, d = distance between plates.
What is the formula for energy stored in a capacitor?
U = 1/2 CV²
This formula can also be expressed using charge: U = Q²/(2C) or U = QV/2.
What are the key differences between capacitors and batteries?
- Capacitors store energy in the form of charge (electromechanical).
- Batteries store chemical energy transformed into electric energy.
Capacitors are faster in charging and discharging than batteries.
What is the process of charging a capacitor?
Charging involves connecting to a voltage source, creating a potential difference.
The charge builds up until the capacitor reaches its maximum capacity.
What happens during the discharge process of a capacitor?
The capacitor releases its stored charge, typically into a circuit.
This process occurs when the voltage source is removed.
List three common applications of capacitors.
- Radio turning circuits.
- Timer circuits like clocks and alarms.
- Powering electric and hybrid cars.
Capacitors preserve information during power failures.
What are the two main types of fixed capacitors?
- Polar capacitors (electrolytic)
- Non-polar capacitors (ceramic, mica, paper, plastic)
Polar capacitors have fixed positive and negative terminals, while non-polarized capacitors do not.
What is the key difference between polar and non-polar capacitors?
Polar capacitors have fixed polarity; non-polar capacitors can be connected either way.
## Footnote
Polar capacitors are used mainly in DC applications.
True or false:
Capacitors block direct current but allow alternating current to pass.
True
This property is used in AC signal coupling.
