Electrical basics - solutions - Huaqiang Electronic Network

First, the circuit

Circuits are the pathways through which electric current flows. Current is formed when charges move in a specific direction. This movement can be of any type of charge, and it results in an electric current.

The direction of current is from the positive terminal to the negative terminal of a power source. A power supply is a device that provides continuous current or voltage. It converts other forms of energy into electrical energy, such as chemical energy in a battery or mechanical energy in a generator.

A continuous current requires two conditions: a power source and a closed circuit. Conductors are materials that allow electricity to flow easily, like metals, human bodies, and saltwater. Insulators, on the other hand, resist the flow of electricity, such as glass, plastic, and pure water.

A circuit typically consists of a power source, wires, switches, and electrical devices. There are three main states of a circuit: a closed circuit (where current flows), an open circuit (where the path is broken), and a short circuit (where the current bypasses the load, potentially causing damage).

A circuit diagram is a visual representation of how components are connected using symbols. Components can be connected in series, where they are linked one after another, or in parallel, where they share the same voltage but have separate paths.

Series connections mean that if one component fails, the entire circuit stops working. In parallel circuits, each branch operates independently, so the failure of one doesn't affect the others.

Second, current

The standard unit for measuring current is the ampere (A). Commonly used units include milliampere (mA) and microampere (µA). The relationships are: 1 A = 1000 mA = 1,000,000 µA.

An ammeter is used to measure current. It must be connected in series with the circuit. The current should enter through the “+” terminal and exit through the “-” terminal. The ammeter’s range must be appropriate for the current being measured, and it should never be directly connected across the power supply without a load.

Laboratory ammeters often have two ranges: 0–0.6 A and 0–3 A. Each small division represents 0.02 A and 0.1 A respectively.

Third, voltage

Voltage, denoted by U, is the driving force behind electric current. It is provided by a power source. The standard unit for voltage is the volt (V), with kilovolt (kV) and millivolt (mV) as common alternatives. The relationships are: 1 kV = 1000 V = 1,000,000 mV.

A voltmeter measures voltage and must be connected in parallel. It also has a “+” and “-” terminal, and its range must match the expected voltage. Laboratory voltmeters usually have ranges of 0–3 V and 0–15 V, with each small division representing 0.1 V and 0.5 V respectively.

Common voltages include: 1.5 V for a dry cell, 2 V for a lead-acid battery, 220 V for household lighting, and 36 V as a safe voltage limit. Industrial systems often use 380 V.

Fourth, resistance

Resistance (R) is a measure of how much a conductor opposes the flow of current. The unit is ohm (Ω), with megaohm (MΩ) and kiloohm (kΩ) as common alternatives. The relationships are: 1 MΩ = 1000 kΩ, and 1 kΩ = 1000 Ω.

Resistance depends on factors such as material, length, cross-sectional area, and temperature. A sliding rheostat changes resistance by adjusting the length of the resistor in the circuit. It is used to control current and voltage.

A rheostat labeled “50Ω 2A” means it has a maximum resistance of 50 Ω and can handle up to 2 A of current. It should be connected in series, with only one wire connected at a time, and set to maximum resistance before turning on the power.

Fifth, Ohm's Law

Ohm's Law states that the current (I) through a conductor is directly proportional to the voltage (U) across it and inversely proportional to the resistance (R). The formula is I = U/R.

This law applies when all values are in the same circuit, and units must be consistent. For example, if you know two of the values, you can calculate the third. Applications include understanding how resistance affects current and voltage.

Series resistors increase total resistance, while parallel resistors decrease it. In series, current is the same throughout, and voltage adds up. In parallel, voltage remains the same, but current divides among branches.

Sixth, electric power and work

Electric work (W) is the amount of energy converted from electrical to other forms, such as heat or light. The unit is joule (J), with kilowatt-hour (kWh) as a commonly used unit. 1 kWh = 3.6 × 10⁶ J.

An electric meter measures energy consumption. The formula for work is W = Pt = UIt. When using this, ensure all units are consistent. Another form is W = I²Rt.

Electric power (P) indicates the rate at which work is done. The unit is watt (W), with kilowatt (kW) as a common alternative. Power can be calculated using P = UI, P = I²R, or P = U²/R.

Rated voltage (Uâ‚€) is the normal operating voltage of a device. Rated power (Pâ‚€) is the power it consumes under this voltage. Actual voltage (U) and power (P) may vary depending on the applied voltage.

If the actual voltage is half the rated voltage, the actual power becomes one-quarter of the rated power. For example, a 220V 100W bulb will consume 25W at 110V.

Seventh, living electricity

Household circuits consist of incoming lines, an electric meter, a main switch, a fuse box, and appliances. All appliances and sockets are connected in parallel, while each appliance is connected in series with its switch.

Fuses are made of alloys with high resistance and low melting points. They protect the circuit by melting when the current becomes too high, breaking the circuit and preventing damage.

Excessive current can result from a short circuit or too many appliances drawing power simultaneously. Safe electricity use involves avoiding contact with live parts and staying away from high-voltage sources.

Eighth, electricity and magnetism

Magnetism is the property of attracting iron, nickel, and cobalt. Magnets have poles—north and south—and opposite poles attract while like poles repel. A magnetic field exists around a magnet and influences other magnets through interaction.

The direction of a magnetic field is defined by the north pole of a compass needle. Magnetic field lines represent the strength and direction of the field, running from north to south without crossing.

Oersted's experiment proved that an electric current creates a magnetic field around a wire. Ampère’s rule helps determine the polarity of a solenoid: grasp it with your right hand, fingers pointing in the direction of the current, and the thumb points to the north pole.

Electromagnets are solenoids with iron cores that can be controlled by turning the current on or off. They are used in relays, which allow low-voltage circuits to control high-voltage systems.

Generators work based on electromagnetic induction, converting mechanical energy into electrical energy. Motors operate on the principle that a current-carrying conductor experiences a force in a magnetic field, converting electrical energy into mechanical motion.

AC (alternating current) changes direction periodically, while DC (direct current) flows in one direction. These differences affect how devices are designed and used.

Experiment: Measuring Resistance

The principle of the experiment involves using a voltmeter and ammeter to measure the resistance of a component. A sliding rheostat is used to adjust the voltage across the resistor. Before starting, the rheostat should be set to maximum resistance to prevent damage.

Measuring the power of a small bulb follows the principle P = UI, where power is the product of voltage and current.