Electricity (pg. 16 - 22)

• Current is the rate of flow of charge around the circuit. Unit: amp, A.
• Voltage/ potential difference is what drives the current round the circuit. Unit: volt, V.
• Resistance is anything in the circuit which slows the flow down. Unit: ohm Ω.
• Resistance acts opposite to voltage.
• An ammeter measures the current (in amps) flowing through the component. It must be placed in series anywhere in the main circuit, but never in parallel like the voltmeter.
• A voltmeter measures the voltage (in volts) across the component. It must be placed in parallel around the component: not under the variable resistor or the battery.
• The variable resistor alters the current flowing through the circuit.
• An alternating current supply means the current is constantly changing direction.
• Cells and batteries supply direct current, which means that the current keeps flowing in the same direction.
• Formula for voltage:

  $V =$$I *$$R$
• The gradient of an I-V graph shows you how the resistance of the component behaves. The steeper the graph, the lower the resistance.
• Current-Voltage graphs: wires and resistors. In both these cases, the current is proportional to voltage.

  A) Current (I)

  B) Voltage (V)

  
• Current-Voltage graphs: metal filament lamp. As the temperature of the metal filament increases, the resistance increase, creating an upwards curve.

  
• Current_Voltage graphs: diode. Current will only flow in one direction.

  
• Light-Emitting Diodes (LEDs) emit light when a current flows through them in the forward direction. They are used for the numbers on digital clocks, in traffic lights and in remote controls. Unlike a light bulb, they don't have a filament that can burn out.
• LED's, like lamps, indicate the presence of current in a circuit. They are often used in appliances to show that they are switched on.
• A light-dependent resistor (LDR) is a special type of resistor that changes its resistance depending on how much light falls on it. In bright light, the resistance falls and in darkness, the resistance is highest.
• LDRs are used for burglar detectors.
• A thermistor is a temperature-dependent resistor. In hot conditions, the resistance drops and in cool conditions, the resistance goes up.
• Thermistors make useful temperature detectors like for example, car engine temperature sensors, thermostats and fire alarms.
• Loudspeaker: converts an electrical audio signal into a corresponding sound.
• Microphone: to induce (create) a changing current from the pressure variations of sound waves.
• Electric bell: produces a repetitive buzzing, clanging or ringing sound.
• Motor: converts electrical energy into mechanical energy.
• Generator: converts mechanical or chemical energy into electrical energy.
• Relay: protects the electrical system from too high of a voltage or current.
• In series circuits, the different components are connected in a line, end to end.
• If you remove or disconnect one component in a series circuit, it is broken and they all stop working.
• For a series circuit:

  1. There's a bigger supply potential difference when more cells are in series if they're all connected the same way.
  2. The current is the same everywhere.
  3. The total p.d of the supply is shared between components.
  4. The p.d for each component depends on its resistance.
  5. The total resistance (which depends on the number of components and the type) is the sum of the resistance of each component in the circuit.
• In parallel circuits, each component is separately connected to the positive and negative of the supply.
• If you remove or disconnect one component in a parallel circuit, it will hardly affect the others at all.
• For a parallel circuit:

  1. The p.d is the same across all branches.
  2. Current is shared between the branches.
  3. The total current going into a junction equals the total current leaving it.
  4. If two identical components are connected in parallel then the same current will flow through each component.
  5. The total resistance of the circuit decreases if you add a second resistor in parallel.
• $Charge =$$Current *$$Time$
• Voltage is the energy transferred per unit charge passed.
• One volt is one joule per coulomb.
• Energy transferred = Charge x Voltage
• Energy transferred = Charge x Current X Resistance
• The live wire alternates between a positive and negative voltage of about 230 V.
• The neutral wire is always at 0 V.
• The earth wire and fuse (or circuit breaker) are just for safety and work together.