Electrical Energy

• Power (W) = Energy (J)/Time (s)

• Electric Power (kW) = Energy (kWh)/Time (h)

  • This is the rate at which electrical energy is changed into another form

  • Energy is measured in kiloWatt hours, which is more reasonable for large appliances

  • 1 Watt = 0.001 kiloWatt

  • 1 Hour = 3600s

Potential Difference

• Electric current - The movement of electrons in a wire

  • Electrons carry energy

• Electric Potential - The amount of potential energy carried by a coulomb

• The amount of electrons is determined by the total electric charge

  • Charge is measured in Coulombs (C), and represented by Q

• When electrons pass through an appliance, the energy is less after

• Electric Potential Difference - The change in energy of electrons

  • The change in electric potential when the current passes through a device that uses energy

  • Also known as voltage, which is measured by a Voltmeter

  • Voltage = Energy/Charge

  • V = E/Q

  • Voltage is measured in volts, or Joules/Coulomb

Electric Current

• Electric current has two types

  • Direct Current - The current flows in one direction

  • Alternating Current - The current direction changes regularly, and the current also changes

• To measure electricity, measure the Amperes (A) through an Ammeter

  • Current = Charge/Time

  • I = Q/t

  • Amperes = Coulombs/second

• Static electricity

  • Like charges repel, opposite charges attract

  • In a Van de Graaf Generator, the body becomes negatively charged, including the hair, and the hair repels itself

Resistance

• Resistance - The loss of energy experienced by electrons when they collide with atoms in a conductor

• Resistance = Voltage/Current

  • R = V/I

  • Measured in Ohms (Ω)

• Ohm’s Law - The potential difference between any two points in a conductor varies directly with the current between two points if the temperature remains constant

  • Voltage and Current are directly proportional

  • Series - Rseries = R1 + R2 + …

  • Parallel - 1/Rparallel = 1/R1 + 1/R2 + …

• Resistance can be measured in a Voltage/Current graph, where the slope of the line is resistance

• Resistance can be affected by the material

Circuit Analysis

• Voltmeters and Ammeters are used to measure the potential difference and current in a circuit

• Kirchhoff’s Laws

  • Voltage - In a single path for current in a circuit, the total electric potential increase at the sources is equal to the total electric potential decrease throughout the rest of the circuit

    • Series - Vt increase = Vt decrease

      • V intial = V2 + V3 …

    • Parallel - The initial voltage of a parallel circuit is equal to the voltage of each path in the circuit

  • Current - At a junction, the total current into the junction is equal to the total current leaving the junction

    • Series - I1 = I2 = I3 …

    • Parallel - The initial current going into the junction will split into each path, and then will equal the total once rejoined

Magnetic Fields

• Magnetic Field - A region of space around a magnet that causes a magnetic force on magnetic objects

  • Magnets have two poles

  • Opposite poles attract, like poles repel

  • Magnetism is a non-contact force, such as gravity or static forces

  • Forces in the magnetic field leave North and go to the South

  • There are permanent and induced magnets

• Electricity and magnetism are related

• When a current flows through a wire, or straight conductor, a field is produced

  • The magnetic field lines are circular (like a bullseye) with a wire at the centre

  • An open circle in a diagram means the current is flowing towards the observer

  • An x-ed circle in a diagram means the current is flowing away from the observer

  • In a conventional electric current, the positive charges are observed as moving

  • In an electron current, the negative charges are observed as moving

  • Right-hand rule for a straight conductor - The direction of the thumb is the conventional current, and curled fingers point in the direction of the magnetic field lines

• When a current flows through a solenoid, it behaves like a bar magnet

  • Solenoid - A coiled conductor

  • It becomes an electromagnet

  • Right-hand rule for a solenoid - The fingers wrap in the direction of the conventional current, and the thumb points towards the north magnetic pole of the coil

  • To increase the strength of the field, use more wire to make coils, use a larger battery to increase current, or use an iron core

Electric Motor

• Motor Principle - A current-carrying conductor that cuts across external magnetic field lines experiences a force perpendicular to both the magnetic field and the direction of the electric current

  • Right-hand rule for the motor principle - The fingers point in the direction of the external magnetic field, the thumb points in the direction of the conventional current, and the palm faces the direction of the force on the conductor

• Electric motors use the force on a loop of wire to cause it to rotate

  • The loop exists so that on one side, the current flows in one direction, and on the other side, it flows in the opposite direction

  • As the current direction is opposite, but the magnetic external field remains the same, one wire will move up, and the other will move down

  • This causes a rotating motion