Current, EMF, and Voltage

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Current, EMF, and Voltage [Copy link]

1. Current

The orderly movement of charged ions is electric current.

The amount of charge is called electricity, represented by the letter Q. The unit is coulomb, and the charge of 6.24×1,000,000,000,000,000,000 electrons is equal to 1 coulomb.

The amount of electricity passing through the cross-section of a conductor per unit time is called current intensity, represented by the symbol "I" or "i".

If the magnitude and direction of the current do not change with time, it is called a constant current, or direct current for short. (DC)

If the direction of the current changes over time, it is called alternating current, or AC for short.

Obviously, for a constant current, the current intensity I can be expressed as follows:

I =Q/tt is time, Q is the amount of electricity passing through the cross section of the conductor during this period of time.

The strength of the current is the ampere: (A)

1 milliampere (mA) = 0.001 ampere (A)

1 microampere (uA) = 0.001 milliampere (mA) = 0.000001 ampere (A)

1 nanoampere (nA) = 0.001 microampere (uA) = 0.000000001 ampere (A)

2. Voltage is represented by the symbol U or u

The voltage between points a and b in a circuit indicates the energy gained or lost per unit time when a positive charge is transferred from point a to point b, that is, U=A/Q, where Q is the amount of charge transferred from point a to point b, and coulomb A is the energy gained or lost by charge Q during the transfer process, measured in joules.

V is the unit of voltage, volt:

From the perspective of potential difference, if the positive charge transfers from point a to point b and gains electrical energy, point a is at low potential and point b is at high potential. If the positive charge transfers from point a to point b and loses electrical energy, point a is at high potential and point b is at low potential.

Electromotive force refers to the function of a power source to maintain a certain voltage. Symbol E, unit V

3. Ohm’s Law

In a closed circuit, the current intensity is proportional to the electromotive force of the power source and inversely proportional to the resistance in the circuit. I=E/R; I=U/R; (I=E/γ+R Ohm's law for the entire circuit)

4. The relationship between the electromotive force of a power supply and its terminal voltage

The greater the internal resistance of the power supply, the greater the voltage fluctuation caused by load changes. Therefore, the size of the power supply internal resistance determines the power supply's ability to carry loads.

Resistors in parallel: R = R1 × R2 ÷ (R1 + R2) or R = 1/R1 + 1/R2 + 1/R3

Series formula: R=R1+R2+R3

Notice:

When current passes through a power supply, if the direction of the current is consistent with the direction of the electromotive force of the power supply (that is, from the negative pole to the positive pole), the potential will increase; if the direction of the current is opposite to the direction of the electromotive force of the power supply, the potential will decrease.

Summarize:

1. The voltage across a resistor is numerically equal to the product of the resistor and the current flowing through the resistor. Its direction is consistent with the direction of the current flowing through the resistor. The potential of the end where the current flows into the resistor is higher, and the potential of the end where the current flows out is lower.

2. The voltage across the power supply is equal to the electromotive force E, and its direction is exactly opposite to the direction of the electromotive force.

3. Starting from a certain point in the circuit and going around the circuit, the value of the potential increase is equal to the value of the potential decrease. In other words, the algebraic sum of the electromotive forces in the circuit is equal to the algebraic sum of the voltage drops on the resistors in the circuit.

4. The zero point potential can be selected arbitrarily. Different zero potential points will result in different calculated potential values between different points, but the potential difference (voltage) between the two points is a certain value, which has nothing to do with the choice of reference point.