Introduction to Electricity
Static Electricity
Static electricity is the result of an imbalance between positively and negatively charged objects.
Types of Forces Between Electric Charges
- Like charges repel.
- Unlike charges attract.
How is Static Electricity Generated?
- The force of friction causes charging. When you rub two different materials together, friction transfers electrons from one material to another.
- Electrons have negative charges and are relatively loosely held to the atom, so they can be easily pulled away by friction.
- When an atom loses an electron, it becomes positively charged.
Electric Fields and Charges
Electrical field: A region in which an electric charge experiences a force.
Electromagnetic charge: A point of charge that creates an electric field around it.
Electrical charge (Q): A measure of how strongly something interacts with electromagnetic fields. Measured in Coulombs (C).
Electric Current
Electric current is the flow of charged particles through some sort of conductive material.
Requirements for Electric Current Flow
- A complete circuit.
- Something to push the current around the circuit (e.g., battery, cell).
Current flows from the positive terminal of a battery to the negative terminal.
Direct current (DC): Current flowing in the same direction.
Most switches work by bringing two pieces of metal into contact.
Conductors and Insulators
Good conductors of electricity: Copper, gold, silver, steel.
Good insulators of electricity: Plastic, minerals, glass.
Measuring Electric Current
Ammeter (AMPS) (A): Used to measure electrical current.
Analogue ammeter: Has a needle that moves across a scale.
Digital ammeter: Gives a direct readout in numbers.
1 milliamp (mA) = 0.001 Amps (A).
Voltage
Voltage (VOLTS) (V): The measure of energy given to charge flowing in a circuit.
- Connected in parallel circuits across a component.
- 1 millivolt (mV) = 0.001 Volts (V).
Electro-Motive Force (EMF): The voltage of a component that pushes a current around a circuit (e.g., a cell).
Resistance
Resistance is the opposition to the flow of electric current, producing heat. It is measured in Ohms (Ω).
- The greater the resistance, the smaller the current.
- The bigger the voltage, the bigger the current.
- In a series circuit, adding more resistors increases the resistance.
- In a parallel circuit, adding more resistors decreases the resistance.
Why Does Resistance Happen?
When electrons travel through a wire, they collide with the protons and neutrons that are unable to move.
Factors Affecting Resistance
- Length of the conductor: As length increases, resistance increases proportionally.
- Cross-section of the conductor: As area increases, resistance decreases proportionally.
- Resistivity of the conductor: As resistivity increases, resistance increases proportionally.
Ohm’s Law
The ratio of potential difference (voltage) to current is constant.
Energy and Power
Energy (Joules) (J): The energy transferred or transformed in a circuit.
Power (Watts) (W): The rate at which something transforms energy.
Resistors
Resistors control the amount of current in a circuit.
Light-Dependent Resistors (LDRs)
Light-dependent resistors (LDRs) have a resistance that depends on the amount of light falling on them. In the light, they have a low resistance, but in the dark, they have a high resistance.
How LDRs Work
When the intensity of light on the LDR increases, the resistance decreases. This reduces the overall resistance of the circuit, allowing more current to flow through the bulb (making it brighter).
Thermistors
Thermistors have a changing resistance with the temperature around them.
Negative temperature coefficient (NTC) thermistors have decreasing resistance with increasing temperature. When the temperature of the thermistor increases, the resistance decreases, creating a brighter bulb.
Positive temperature coefficient (PTC) thermistors have increasing resistance with increasing temperature. When the temperature of the thermistor increases, resistance increases, meaning the bulb will become dimmer.
Electrical Hazards and Safety
- Wires must be insulated from one another to avoid electric shock. You may also connect the metal case of an appliance to an earth wire.
- If an excessive current flows in the wires, they will heat up and may catch fire or release harmful fumes.
- Damp conditions or water should be avoided because water may provide a conductive path for electricity.
Fuses
Fuses are small wires that easily break when too much current passes through them, saving the rest of the circuit.
How Fuses Protect Circuits
If the current gets too high in a circuit, the wire will get hot. Fuses get hotter quicker than a normal wire because they are thinner and will break earlier. This protects the rest of the circuit, and the fuse can be replaced.
- Fuses are always placed in the live wire, so the appliances stay connected to a high potential point.
Choosing the Right Fuse
The fuse rating needs to be just a little bit bigger than the normal amount of current that passes through the circuit.