Electricity Basics: DC vs AC Currents

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Questions and Answers

What happens when positive charge moves from the positive polarity toward the negative polarity in a circuit element?

The element supplies energy.

The current remains constant.

The voltage in the circuit decreases.

The element absorbs energy as heat. (correct)

How is power calculated in an electrical circuit?

By subtracting voltage from current.

By adding voltage and current.

By dividing voltage by current.

By multiplying voltage and current. (correct)

What is the difference between DC and AC voltages?

DC voltages are constant over time, while AC voltages alternate in magnitude and polarity. (correct)

DC voltages are found only in batteries, while AC voltages are found in circuits.

DC voltages vary with time, while AC voltages remain constant.

DC voltages are always zero, while AC voltages are always positive.

When an ideal switch is closed, what is the voltage across it?

Zero volts. (A) Signup and view all the answers

What does negative power indicate when associated with a battery in a circuit?

The battery is supplying energy to another element. (C) Signup and view all the answers

According to Kirchhoff’s current law, what can be stated about the net current entering a node?

It is equal to the sum of the currents leaving the node. (C) Signup and view all the answers

Which statement accurately reflects the behavior of charge in a closed loop circuit?

The net current at any junction must equal zero. (A) Signup and view all the answers

Which of the following describes stored energy within a battery experiencing positive power?

Energy is stored as chemical energy. (A) Signup and view all the answers

What defines a direct current (DC)?

It is constant over time. (B) Signup and view all the answers

In an alternating current (AC), what best describes the current flow?

It reverses direction periodically. (D) Signup and view all the answers

What does the voltage represent in an electrical circuit?

The energy transferred per unit of charge. (D) Signup and view all the answers

What units are equivalent to volts (V)?

Joules per coulomb. (B) Signup and view all the answers

When charge flows from a battery to a circuit, what happens to the energy?

It manifests as heat and light in the circuit. (B) Signup and view all the answers

How is the voltage measured in a circuit?

Across the terminals of a circuit element. (B) Signup and view all the answers

What occurs when charge moves through the battery in the reverse direction?

Energy is absorbed and stored as chemical energy. (D) Signup and view all the answers

What happens to energy when a battery supplies power to automobile headlights?

It appears as light and heat in the headlights. (C) Signup and view all the answers

Study Notes

Direct Current (DC) and Alternating Current (AC)

Direct current (DC) is constant over time, while alternating current (AC) varies periodically.
AC currents reverse direction, with their values changing sinusoidally over time.
When the AC current, denoted as ib(t), is negative, the direction of current movement is opposite to the reference direction.

Voltage and Energy Transfer

Voltage measures the energy transferred per unit charge as it moves between two points in a circuit.
Units of voltage are volts (V), equivalent to joules per coulomb (J/C).
Example: An automobile battery typically has a nominal voltage of 12 V, indicating 12 J are transferred per coulomb.

Current Flow and Energy Dynamics

Current signifies the flow of charge, while voltage represents the energy associated with that charge.
When charge flows from the battery to a load, energy is supplied as heat, light, or mechanical energy.
Charge flowing back to the battery represents energy being absorbed, converting energy to stored chemical form.

Voltage Polarity and Energy Flow

Voltage polarities indicate energy direction:
- Positive charge moving from positive to negative polarity indicates energy absorption.
- Positive charge moving from negative to positive polarity indicates energy supply.

Types of Voltages

Constant voltages are categorized as DC voltages.
Voltages that change over time, switching polarity, are termed AC voltages.

Switch Operation in Circuits

An open ideal switch has zero current, while the voltage across it is determined by the circuit's configuration.
A closed ideal switch results in zero voltage across it, determining the current based on the rest of the circuit.

Power in Circuits

Power is described as the product of current (i) and voltage (v), indicating the rate of energy transfer.
In an electrochemical battery:
- Positive power indicates charging (energy absorbed and stored).
- Negative power indicates discharging (energy delivered to other circuit elements).

Kirchhoff’s Current Law (KCL)

A node in an electrical circuit is formed by the intersection of two or more circuit elements.
KCL states that the net current entering a node equals zero, ensuring that current is conserved.
The law can also be articulated as the net current leaving a node is zero, achieved by balancing entering and leaving currents.

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Description

Test your knowledge on direct and alternating currents with this quiz on electricity fundamentals. Learn about the characteristics and behaviors of DC and AC currents and how they differ over time. Perfect for students studying basic electrical engineering concepts.