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Electric current is the flow of electric charge through a conductor. It is typically measured in amperes (A), which tells us how much charge passes through a point in the circuit per second.
In electrical circuits, current is often described in terms of direct current (DC) and alternating current (AC). DC is a constant flow of electric charge in one direction, while AC periodically reverses direction.

• Current direction is usually defined from positive to negative, opposite to the direction of electron flow.
• The amount of current in a circuit depends on the voltage and resistance present.

The motor problem illustrates how these factors interact. Without back emf, using Ohm's Law, we found the current to be equal to 44 A. However, with the back emf, the current significantly reduces to 4 A.

Resistance is a measure of the opposition to the flow of electric current. It is measured in ohms (\(\Omega\)) and depends on the material, length, cross-sectional area, and temperature of the conductor.
Each material offers different levels of resistance, influencing current and power within the circuit. In the exercise, the motor has a resistance of \(2.50 \Omega\), which is an essential factor in our calculations.

• Higher resistance means less current flows at a given voltage.
• Resistance can cause electrical energy to convert into heat, sometimes undesirably, leading to energy losses.

The calculation of current using the resistance value shows how resistance impacts the overall current flow in both scenarios: with and without the back emf.

Voltage refers to the electric potential difference between two points in a circuit. It is the force that pushes electric charges to move in a current and is measured in volts (V).
The voltage in a circuit is provided by a power source, like a battery or, in our example, a \(110 \mathrm{V}\) line. This initial voltage determines how much current could potentially flow through the circuit if connected directly across the motor.

• Ohm's Law connects voltage (V), current (I), and resistance (R) in the formula \(V = IR\).
• The difference in potential between two points causes current to flow in the circuit.

The exercise shows voltage as the driving factor for current, reduced by the back emf's effect, which is subtracted from the source voltage. Thus, back emf limits the current, shown by the reduced effective voltage of 10 V.

Back electromotive force (emf) is a voltage produced by the motor itself, opposing the applied voltage from the power source. It is a natural part of many electrical devices that have moving parts, like motors.
The back emf occurs due to the motor's operation; as the motor spins, it generates a voltage in opposition to the supply voltage. This effect reduces the effective voltage across the motor.

• Back emf depends on the motor's speed; as the speed rises, so does the back emf.
• It stabilizes motor operation, preventing it from over-speeding.

In the exercise, the back emf is \(100 \mathrm{V}\), significantly reducing the effective voltage from \(110 \mathrm{V}\) to \(10 \mathrm{V}\), which results in a much lower operating current of \(4 \mathrm{A}\). This control is essential for keeping the motor's performance stable and efficient.