Electromagnetism and Electricity

• Charge


  A property of matter that experiences a force when near other charges or in an electric field
• Charging objects


  1. Rubbing an insulator with another insulator, causing one to gain electrons and the other to lose electrons

  2. Charging a metal by induction - bringing a charged rod near the metal, causing the charges in the metal to redistribute, and then connecting the metal to an earth wire to remove some electrons, leaving the metal positively charged
• Current electricity


  Important in understanding voltage, current, and resistance
• Circuit


  A loop that delivers energy from a source (like a battery) to a device (like a light bulb) using metal wires
• Voltage (V)


  The energy per charge, measured in volts (J/C)
• Current (I)


  The rate of flow of charges, measured in amperes (C/s)
• Resistance (R)


  The property of a material that resists the flow of charges, measured in ohms (Ω)
• Ohm's Law


  V = IR, R = V/I, I = V/R
• Resistance of a resistor must be constant for Ohm's Law to work, and temperature affects resistance
• Current remains constant throughout a circuit, regardless of resistance
• Resistance affects the current from the moment it leaves the battery, not just within the resistor
• Resistance does not affect voltage initially, as both resistance and voltage are determined independently
• Resistance of a wire

  Directly proportional to its length and inversely proportional to its cross-sectional area
• Power


  Calculated as voltage (V) multiplied by current (I)
• Energy


  Calculated as power (P) multiplied by time (t)
• When calculating energy, time must always be converted to seconds to align with SI units
• Kilowatt-hour (kWh)


  A unit of energy consumption, representing the energy used by 1000 watts of power in one hour
• Calculating monthly cost of electricity

  Multiply the average daily kilowatt-hour consumption by the cost per kilowatt-hour and then by the number of days in a month
• Variable resistor (potentiometer)


  A resistor whose resistance can be adjusted by sliding a contact along a resistive element
• Thermistor


  A type of resistor whose resistance changes with temperature. As the temperature increases, the resistance of a thermistor decreases
• Light-dependent resistor (LDR)


  A type of resistor whose resistance changes with the amount of light hitting its surface. As the light intensity increases, the resistance of an LDR decreases
• Diode


  A one-way gate for current. It allows current to flow in one direction but not in the opposite direction
• Diodes can be used to convert alternating current (AC) to direct current (DC)
• IV graph


  Shows the relationship between the voltage and current in a circuit
• Resistors connected in series

  The total resistance increases, and the current remains the same
• Resistors connected in parallel

  The total resistance decreases, and the current increases
• In a series circuit, the voltage is split across the resistors according to the ratio of their resistances
• In a parallel circuit, the current is divided among the resistors according to the ratio of their resistances
• Formula for total resistance of resistors in parallel

  1/Rtotal = 1/R1 + 1/R2
• If the resistors are equal in a parallel circuit, the current splits equally
• Each component in a parallel circuit gets the full voltage from the supply
• Potential divider


  Used to control the voltage across a component
• Variable potential divider


  Used to control the voltage across a component by changing the position of a sliding contact
• Dimmer switches control the voltage, not the current
• Mains electricity


  Has two wires: the live wire, which is always at 240 volts, and the neutral wire, which is always at zero volts
• Fuse


  A safety feature that protects a circuit from an overload of current by melting
• Circuit breaker


  Protects a home from an overload of current by using an electromagnet to open the switch and stop the current from flowing
• Earth wire


  A safety feature that redirects current away from metal cases to protect people from getting electrically shocked
• Magnet


  Has a North Pole and a South Pole and attracts certain metals, such as iron and steel
• Magnetic field


  A region where magnets experience a force