# 4.01 Introduction and summary of previous sections.

• Electricity is caused by the movement of charged particles (usually electrons) in an electrical circuit.
• Current (I) is a measure of the rate at which charge (Q) flows through the circuit.
• I = Q/t .
• Electrical current is measured in units called Amperes or Amps (A).
• Quantity of charge is measured in units called Coulombs (C).
• One coulomb is equal to the total charge carried by 6.25 x 1018 electrons.
• Power supplies contain electrical cells that produce an electro motive force (e.m.f.).
• An e.m.f. is what causes current to flow in an electrical circuit.
• The e.m.f. supplies electrical potential to the charge carriers (usually electrons).
• Electrical potential is measured in volts (V).
• Electrons flow from the negative terminal of a power supply, to its positive terminal, (this is called electron flow).
• Conventional current flow assumes (incorrectly), that electric current flows from the positive to the negative terminal. (This is due to a historical misconception about the true nature of electricity).
• Conventional current flow is still used to show the direction of current flow, in electrical circuit diagrams.
• Electrical circuits also possess a property called resistance (R). Resistance limits the flow of current through a circuit and converts electrical energy into heat.
• Resistance is measured in Ohms (Ω).
• A resistor is a component which has been designed to possess a specific amount of resistance.
• The electrical potential of charged particles is reduced, when they flow through a resistor. This causes a potential drop (or voltage drop) across the resistor.
• The relationship between the current, voltage and resistance in a circuit is described by Ohm's law, R=V/I.

After completing this section you should be able to:

• Recall the three basic circuit laws, - Ohm's law and Kirchhoff's current and voltage laws.
• Use these circuit laws, to find unknown values of current, voltage and resistance, in simple electrical circuits.
• Add resistors in series and parallel, to find total resistance.
• Label currents, e.m.f.s, voltage drops and resistors on a circuit diagram.