Grade 9 Electricity Notes PDF

Electricity -- Symbols and Current ================================== Circuit diagrams are an important part in our understanding of electricity. We will start with the symbols used in circuit diagrams and only move onto drawing the diagrams once other work on series and parallel connections has be...

Electricity -- Symbols and Current ================================== Circuit diagrams are an important part in our understanding of electricity. We will start with the symbols used in circuit diagrams and only move onto drawing the diagrams once other work on series and parallel connections has been completed. Ammeter Voltmeter Lamp cell Switch Resistor **Ammeters and Voltmeters** An ammeter measures the strength of an electric current. It has a low resistance and is connected in series in a circuit as shown alongside. A voltmeter measures the potential difference or voltage between two points in a circuit. It has a high resistance and is connected in parallel across the two points as shown alongside. **Current** In order for current to flow, electrical energy is required. This is provided in the form of a cell, a battery (which is two or more cells connected together), mains supply or a dynamo. An electric current is the flow of charge (positive or negative) from one point to another in an electrical circuit. There are two ways in which the direction of current can be represented: 1. Conventional flow of current is the flow of positive charge and its direction is from the positive terminal through the circuit, to the negative terminal of the power source (usually a cell). 2. Electron flow of current is the flow of negative charge and its direction is from the negative terminal, through the circuit to the positive terminal of a power source (usually a cell). Either method can be used, but it is more common for us to use the conventional flow of current, despite us knowing that in metals the current is conducted by electrons which flow from negative to positive. It is therefore by **convention** that it has been agreed we use the flow of current from positive to negative. The strength of the current is measured in amperes (A) -- not amps. It is the rate at which charge passes a given point in a conductor. Quantity Symbol of quantity Unit of measurement Symbol of unit of measurement ------------------ -------------------- --------------------- ------------------------------- Current strength I ampere A Charge Q coulombs C time t seconds s (not sec or secs) The triangle alongside can be used as a guide in changing the subject of the formula. You must remember that writing this down is not the same as having written the formula down. This merely helps you to remember what to do. The idea is to point to the unknown, e.g: Q. What is left is l x t and that means that Q = It. If you point to I then Q over t is left, i.e.. Note that the formula Q = It can be used to define a coulomb: **Definition** A coulomb is the quantity of charge that passes through a conductor when a current of one ampere flows for one second. **Potential Difference (Voltage)** In the previous section we mentioned that in order for electric current to flow, energy is required. This energy is called the potential difference or voltage. **Definition** The potential difference between two points in an electric circuit is the energy required to move one coulomb of charge between the points. Note that when the potential difference is measured across a cell or battery, the measurement gives the amount of energy **given** to each coulomb of charge for the circuit and when across a component that uses up energy, the measurement represents the energy per coulomb of charge **used** by that component. **Series** Just as resistors can be connected in series and parallel, so too can cells. Cells are connected in series when their terminals are connected in a positive to negative sequence. The total potential difference of a battery, with the cells connected in series, is equal to the sum of the potential differences of the individual cells. The advantage of connecting cells in series is that the total potential difference is increased, therefore producing a stronger current. **Parallel** Cells are connected in parallel when their positive terminals are joined to each other separately from the negative terminals. The total potential difference of a battery, with the cells connected in parallel, equals the potential difference of a single cell. This is provided that all the cells have the same potential difference. The advantage of connecting cells in parallel is that the cells last longer, but unlike series connections, delivers the current of just one cell. Electrical Resistance ===================== The word 'resistance' in terms of a conductor is a measure of how much difficulty charge experiences in passing through the conductor. A substance with a low resistance is a good conductor and a substance with a high resistance is a poor conductor. Resistance is caused by collisions between electrons and metal atoms that interfere with the flow of charge. **Factors affecting the resistance of a conductor:** 1. 2. 3. 4. Resistance is defined as the ratio of potential difference across the ends of a conductor and current strength. Quantity Symbol of quantity Unit of measurement Symbol of unit of measurement --------------------------------- -------------------- --------------------- ------------------------------- Resistance R ohm Ω Potential Difference or Voltage V volt V Current I ampere A **Definition** An ohm is defined as the resistance of a conductor when a potential difference of one volt causes a current of one ampere to flow through it. Resistors in Series and Parallel ================================ We've considered simple calculations involving resistors. We now need to apply this knowledge to calculations involving resistors connected in series and parallel. **Resistors in Series:** - Voltage is divided or split,\ i.e. V~1~ = V~2~ + V~3~ - Current stays the same, i.e. A~1~ = A~2~ Note that if r~1~ = r~2~, then V~2~ = V~3~. **Resistors in Parallel:** - Voltage is the same, i.e. V~1~ = V~2~ = V~3~ - Current is divided or split, i.e. A~1~ = A~2~ + A~3~ If r~1~ = r~2~= r~3~ =..., then R~TOT~ = the value of one of the resistors divided by the number of resistors in parallel. If they are not equal, the formula shown alongside is used, but this won't be dealt\ with in any detail as it is done in Grade 10. Note that if r~1~ = r~2~, then A~2~ = A~3~ ------------------------------------------ The kilowatt-hour ================= - - - **Cost of electricity consumption** The cost of electricity consumption per year, month or week is calculated according to a tariff (Rands or cents) per kilowatt-hour used. ::: {.section.footnotes} ------------------------------------------------------------------------ 1. ::: {#fn1} ::: :::