Electrical basics - solutions - Huaqiang Electronic Network

First, the circuit

Circuit formation: Current is formed by the directional movement of electric charges. Any kind of charge movement can create current.

Current direction: From the positive terminal of the power supply to the negative terminal.

Power source: A device that provides continuous current or voltage. It converts other forms of energy into electrical energy. For example, a dry battery converts chemical energy into electricity, and a generator converts mechanical energy into electricity.

Conditions for continuous current: There must be a power source and a closed circuit.

Conductor: A material that allows electricity to flow easily. Examples include metals, human body, earth, and saltwater.

Insulator: A material that resists the flow of electricity. Examples are glass, ceramic, plastic, oil, and pure water.

Circuit components: A circuit consists of a power source, wires, switches, and electrical devices.

Circuit states: (1) Closed circuit – when the circuit is complete; (2) Open circuit – when the circuit is broken; (3) Short circuit – when the wire directly connects the two poles of the power source.

A diagram showing the connections in a circuit using symbols is called a circuit diagram.

Series connection: When components are connected one after another, it's called series. If any part is disconnected, the current stops.

Parallel connection: When components are connected side by side, it's called parallel. Each branch operates independently.

Second, current

Unit of current: Ampere (A). Other units include milliampere (mA) and microampere (μA). 1 A = 10³ mA = 10⁶ μA.

An ammeter measures current. It should be connected in series, with the current entering from the “+” terminal and exiting from the “-” terminal. The measured current must not exceed the ammeter’s range. Never connect an ammeter directly to the power supply without passing through a device.

Common laboratory ammeters have two ranges: 0–0.6 A (each small division is 0.02 A) and 0–3 A (each small division is 0.1 A).

Third, voltage

Voltage (U): Voltage is what causes current to flow in a circuit. A power source provides voltage.

Unit of voltage: Volt (V). Other units are kilovolt (kV) and millivolt (mV). 1 kV = 10³ V = 10⁶ mV.

A voltmeter measures voltage. It should be connected in parallel. The current should enter from the “+” terminal and exit from the “-” terminal. The measured voltage must not exceed the voltmeter’s range.

Common lab voltmeters have two ranges: 0–3 V (each small division is 0.1 V) and 0–15 V (each small division is 0.5 V).

Memorized voltages: A dry cell has 1.5 V, a lead-acid battery has 2 V, household lighting is 220 V, safety voltage is ≤36 V, and industrial voltage is 380 V.

Fourth, resistance

Resistance (R) indicates how much a conductor opposes the flow of current. Higher resistance means less current flows through the conductor.

Unit of resistance: Ohm (Ω). Common units are megaohm (MΩ) and kiloohm (kΩ). 1 MΩ = 10³ kΩ = 10⁶ Ω.

Factors affecting resistance: material, length, cross-sectional area, and temperature. Resistance is independent of voltage and current.

Sliding rheostat: It changes resistance by adjusting the length of the resistor wire in the circuit. It is used to control current and voltage.

Nameplate: A rheostat marked “50Ω 2A” means its maximum resistance is 50Ω, and it can handle up to 2A of current.

Correct usage: Connect in series, use “one up,” and set to maximum resistance before turning on the power.

Fifth, Ohm’s Law

Ohm’s Law: The current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.

Formula: I = U/R. Units: I (A), U (V), R (Ω).

Understanding the formula: I, U, and R must belong to the same circuit. With any two values, you can calculate the third. Units must be consistent during calculations.

Application of Ohm’s Law:

1. Resistance is constant, so as voltage increases, current increases (R = U/I).

2. At constant voltage, higher resistance results in lower current (I = U/R).

3. At constant current, higher resistance leads to higher voltage (U = IR).

Series resistors:

1. Current: I = I₁ = I₂ (same in all parts)

2. Voltage: U = U₁ + U₂ (total voltage equals sum of individual voltages)

3. Resistance: R = R₁ + R₂ (total resistance is sum of individual resistances)

4. Voltage division: U₁/U₂ = R₁/R₂

5. Proportional relationship: I₁/I₂ = 1:1 (heat Q is proportional to I²Rt)

Parallel resistors:

1. Current: I = I₁ + I₂ (main current is sum of branch currents)

2. Voltage: U = U₁ = U₂ (voltage is the same across all branches)

3. Resistance: 1/R_total = 1/R₁ + 1/R₂ (reciprocal of total resistance is sum of reciprocals)

4. Current division: I₁/I₂ = R₂/R₁

5. Proportional relationship: U₁/U₂ = 1:1 (heat Q is proportional to I²Rt)

Sixth, electric power and energy

1. Electric work (W): The amount of electrical energy converted into other forms of energy.

2. Unit of work: Joule (J). Common unit: kilowatt-hour (kWh). 1 kWh = 3.6 × 10⁶ J.

3. Measuring tool: Electric meter.

4. Formula: W = Pt = UIt. Units: W (J), U (V), I (A), t (s).

When using W = UIt, ensure that W, U, I, and t belong to the same circuit. Units must be consistent. Another formula: W = I²Rt.

Electric power (P): Indicates the rate at which electrical work is done. Unit: Watt (W). Common unit: kW.

Formula: P = W/t = UI. Units: P (W), W (J), t (s), U (V), I (A).

Use consistent units: if W is in joules and t is in seconds, P is in watts. If W is in kWh and t is in hours, P is in kilowatts.

Right formula for calculating power: P = I²R or P = U²/R.

Rated voltage (Uâ‚€): The voltage at which a device operates normally.

Rated power (Pâ‚€): The power of the device under rated voltage.

Actual voltage (U): The actual voltage applied to the device.

Actual power (P): The power of the device under actual voltage.

If U > Uâ‚€, P > Pâ‚€, the lamp is bright and may burn out. If U < Uâ‚€, P < Pâ‚€, the lamp is dim. If U = Uâ‚€, the lamp works normally.

Example: A 220V 100W bulb connected to 110V will have an actual power of 25W.

Thermal power: The heat generated is proportional to the square of the current and the resistance of the conductor.

Formula: P = I²Rt. Units: P (W), I (A), R (Ω), t (s).

If all electrical work is converted to heat, then thermal power equals electric power. Use electric power formulas to calculate thermal power (e.g., electric heater).

Seventh, household electricity

Household circuits consist of: incoming lines (live and neutral) → electric meter → main switch → fuse box → appliances.

All appliances and sockets are connected in parallel. Each appliance is connected in series with its switch.

Fuse: Made of lead-bismuth alloy with high resistance and low melting point. It melts when current is too high, cutting off the circuit and protecting it.

Causes of excessive current: short circuit or high total power consumption.

Safe electricity use: Do not touch low-voltage charged objects; do not approach high-voltage charged objects.

Eighth, electricity and magnetism

Magnetism: The property of attracting iron, nickel, and cobalt.

Magnet: An object that exhibits magnetic properties. It has directionality and points toward the north.

Magnetic pole: The most magnetic part of a magnet. Every magnet has two poles: North (N) and South (S).

Like poles repel, opposite poles attract.

Magnetization: The process of making a non-magnetic object magnetic.

A magnetic field exists around a magnet, and interactions between poles occur through this field.

Properties of a magnetic field: It exerts force on magnets placed within it.

The direction of the magnetic field is the direction the north pole of a compass needle points when stationary.

Magnetic field lines: Imaginary lines used to represent the strength and direction of the field. They do not intersect and run from north to south.

The direction of the magnetic field line at a point is the same as the direction the north pole of a compass needle points.

The geomagnetic north pole is near the geographic south pole, and the geomagnetic south pole is near the geographic north pole. The angle between them is called magnetic declination. Shen Kuo was the first to describe this in China.

Oersted’s experiment proved that a current-carrying wire generates a magnetic field.

Ampère’s rule: Hold a solenoid with your right hand, fingers pointing in the direction of current, thumb points to the north pole of the solenoid.

Properties of a current-carrying solenoid: stronger current → stronger magnetism; more turns → stronger magnetism; inserting a soft core → stronger magnetism; changing current direction → reversing polarity.

Electromagnet: A solenoid with an iron core inside.

Features of electromagnets: Magnetism can be controlled by turning current on/off; strength can be adjusted by changing current or number of turns; polarity can be changed by reversing current direction.

Electromagnetic relay: A switch controlled by an electromagnet. It allows long-distance operation, uses low voltage/weak current to control high voltage/strong current, and enables automatic control.

Basic principle of telephone: vibration → varying current → vibration.

Electromagnetic induction: When a conductor moves through a magnetic field and cuts the magnetic field lines, current is induced in the conductor. This is the basis for generators.

Conditions for induced current: closed circuit, part of the conductor moving in the magnetic field, and cutting the field lines.

Direction of induced current: depends on the motion of the conductor and the direction of the magnetic field lines.

Generator principle: based on electromagnetic induction. Converts mechanical energy into electrical energy.

Effect of magnetic field on current: a current-carrying conductor experiences a force in a magnetic field. This is the principle behind electric motors.

Direction of force on a current-carrying conductor: depends on the direction of current and magnetic field.

Motor principle: based on the rotation of a current-carrying coil in a magnetic field.

Commutator: helps convert AC to DC.

Alternating current (AC): current that periodically reverses direction.

Direct current (DC): current that flows in one direction only.

Experiment

Volta-Ampere Resistance

Experimental principle: (equipment and circuit diagram shown on the right). Note: Adjust the sliding rheostat to maximum resistance before starting the experiment.

The role of the sliding rheostat in the experiment is to adjust the voltage across the measured resistance.

2. Measuring the power of a small bulb – experimental principle: P = UI.