Direct Current

is an electric current that does not change in direction and magnitude over time. Charged particles move in one direction, creating a stable flow of electricity.

1. Basic Concepts and Ohm's Law

Current Strength:

I = \frac{Δ q}{Δ t}

Explanation

I — current strength; Δq — amount of charge passed through the conductor; Δt — time. Characterizes the intensity of charge movement in a circuit.

Voltage:

U = φ_{1} - φ_{2}

Comment

Potential difference between two points in a circuit. Determines the work done to move charge between them.

Conductor Resistance:

R = ρ \cdot l / S

Explanation

R — resistance; ρ — material resistivity; l — conductor length; S — cross-sectional area. Affects current: the higher R, the weaker the current at fixed voltage.

Ohm's Law for a Circuit Section:

I = \frac{U}{R}

Comment

Basic relationship between current, voltage, and resistance. Used in calculations for a linear circuit.

Full Circuit with EMF:

I = \frac{ε}{R + r}

Explanation

ε — electromotive force of the source; R — external circuit resistance; r — internal resistance of the source. The formula accounts for losses within the source, important for efficiency calculations.

2. Current Source and Voltage

Voltage across External Circuit:

U = ε \cdot I \cdot r

Explanation

ε — source EMF; I — current in the circuit; r — internal resistance. When a load is connected, current causes a voltage drop within the source. The larger r, the more U deviates from ε.

Comment:

A current source is a device that converts energy (chemical, mechanical, etc.) into electrical energy. EMF ε characterizes the maximum voltage available without a load. When a load is connected, current I appears, and part of the energy is lost due to internal resistance r. The actual voltage U at the terminals is always less than ε.

3. Work, Power, and Thermal Effect

Work of Current:

A = I \cdot U \cdot Δ t = Δ q \cdot U

Explanation

A — work done by electric current when charge passes through a conductor. Can be expressed as the product of current strength, voltage, and time, or as charge multiplied by voltage.

Equivalent Formulas for Work:

A = \frac{U^{2} \cdot Δ t}{R} = I^{2} \cdot R \cdot Δ t

Comment

Used when solving problems where not all parameters are known. With constant resistance and voltage, they allow quickly finding the dissipated energy.

Electric Current Power:

P = \frac{A}{Δ t} = I \cdot U = \frac{U^{2}}{R} = I^{2} \cdot R

Comment

P — power, characterizes the rate of energy conversion. In problems, it's important to choose the correct form: by voltage, by current, or by resistance.

Quantity of Heat:

Q = I \cdot U \cdot Δ t = I^{2} \cdot R \cdot Δ t

Comment

Heat released in a conductor when current passes through it — as a thermal effect. The Joule-Lenz formula explains how electrical energy is converted into heat.

4. Efficiency of Current Source

Source Efficiency:

η = \frac{U}{ε} = \frac{R}{R + r}

Explanation

η — efficiency; ε — EMF of the source; U — voltage across the external circuit; R — external resistance; r — internal resistance. Shows what fraction of the source's energy goes to the load, and what is lost internally.

Concise Physics Handbook

Formulas for Key Sections

Direct Current

1. Basic Concepts and Ohm's Law

Current Strength:

Voltage:

Conductor Resistance:

Ohm's Law for a Circuit Section:

Full Circuit with EMF:

2. Current Source and Voltage

Voltage across External Circuit:

Comment:

3. Work, Power, and Thermal Effect

Work of Current:

Equivalent Formulas for Work:

Electric Current Power:

Quantity of Heat:

4. Efficiency of Current Source

Source Efficiency: