OCR Year 9 Science Electricity 2024 PDF

Electricity Year 9 science 2024  Part 1 – Static Electricity Atoms are the basic building blocks of matter. Within atoms, we find even smaller particles (subatomic particles). Positive protons , neutral neutrons, and negative electrons Most matter is neutral and has no overall charge....

Electricity Year 9 science 2024  Part 1 – Static Electricity Atoms are the basic building blocks of matter. Within atoms, we find even smaller particles (subatomic particles). Positive protons , neutral neutrons, and negative electrons Most matter is neutral and has no overall charge. This is because there is one proton for every electron and the positive and negative charges balance each other. If we change the number of electrons or protons we can make something become charged. It is difficult to change the number of protons but much easier to move the electrons as they are just on the outside of the atom.  Electrostatic Charge Objects are normally uncharged (neutral) – their atoms usually have equal numbers of positive protons and negative electrons. When two objects are rubbed together, we may be able to transfer electrons from one object to the other. This causes the object with fewer electrons to become positively charged and the one with extra electrons to become negatively charged. Static Charge  Attraction or repulsion? Opposite charges attract each other and like charges repel each other. Predict what will happen in the following situations. Both objects are positively charged Both objects are negatively charged One object is positive and the other is negative  Mr Bean and the Van de Graaff generator Why does the paper stick to Mr Bean? Because they have opposite charges.  Lightning strikes Lightning occurs because of a build up of static charge in clouds. The particles in the cloud rub against each other as they are moved by changes in air pressure. The particles which end up negative as they gain electrons sit at the base of the cloud while the lighter positive particles, (which have lost electrons) sit at the top of the cloud. If the cloud’s charge becomes too great it must DISCHARGE to get rid of the excess electrons. It does this by a lightning strike to the earth, it will be attracted to any tall objects as the electricity can move easier through them than the air.  Part 2 - Conductors and Insulators Conductors are materials that allow heat and electricity to pass through them easily. Insulators are materials which try to stop heat and electricity passing through them. All metals are examples of good conductors. Non-metals are usually insulators and stop current from flowing. Many materials we see in our daily lives are composed of both metals and non-metals and so this means we have to test them. Pure water is a poor conductor (insulator), but if we add salt (sea water and sweat) it becomes a good conductor. Which of these are conductors and which are insulators? Aluminium Plastic Wood Gold Paper clip Paper Glass Fabric Nail Conductors The word "conductor" means something that can move from one place to another. Electrical conductors are able to move electricity. Metals are the most common conductors, Silver, Copper and Gold are the best conductors. Aluminium is used for high voltage conductors because it is cheaper and lighter than copper. Graphite (pencil lead), Silicon and water with dissolved salts are also conductors. Insulators Slow down or resist the flow of electricity. Plastic coatings on electrical wires prevent electricity from going where we do not want it. Glass, ceramic and air are also insulators.  Part 3 Electricity in circuits ‘Electricity’ is a general term related to the presence and flow of electric charge. It is produced when negative electrons and positive protons are separated and reunite through a closed circuit. If the charges are unable to move, then it is called an electrostatic charge. A conductor allows the charges to flow easily. An insulator restricts the movement of the charges – it provides resistance. Electric circuits and Circuit diagrams The pathway travelled by electrical energy is called an electric circuit. Electric circuits must have an energy source, wires to carry the charges, and a load that converts the electrical energy into heat, light or kinetic energy. Many devices have ‘gaps’ called switches to control the flow of electricity in a circuit. Circuits are represented by circuit diagrams. Each component of a circuit is represented by a symbol.  Circuit electricity Electricity needs a path to flow. Usually this is wiring made of metal. In a simple circuit we need: a power source called a cell (battery), connecting wires, a switch (optional), and a load (something to use up the power eg. a light globe). The circuit needs to have a circular shape that begins and ends at the cell. If there is not a complete circuit we say it is an open circuit, current cannot flow.  Symbols used in circuit diagrams Note that the symbols for resistor and lamp have alternative symbols, both of which are commonly used. Circuit diagram example A torch is an example of a simple circuit. – draw its circuit diagram in your book RECAP – open/closed circuit Would this work? RECAP – open/closed circuit Would this work? RECAP – open/closed circuit Would this work? Closed circuit  Types of circuits – Series circuits When two or more globes are connected in a circuit, two different types of connection are possible. In a series circuit, the globes are connected side by side so that the current goes through one globe and then through the second. The current is the same everywhere in a series circuit. If there is a break anywhere in a series circuit, then all globes in the circuit are affected.  Types of circuits – Parallel circuits In a parallel circuit, the circuit has two or more branches and the current splits between the branches and comes back together afterwards. This means the currents going through each globe must be added together to determine the total amount of current coming from the battery. A break in one of the branches of the circuit only affects the current and globe in that branch.  Part 4 – Current Current is a measure of the flow of electrons through a circuit. Current is measured in amperes, or amps (e.g. a current of 1A) An ammeter measures the current passing a particular point in an electric circuit. Current is the flow of charge per second. If a wire has a current of 1A, there are 6,240,000,000,000,000,000 electrons passing through the wire every second.  Measuring current This is how we draw an ammeter in a circuit. A A SERIES PARALLEL CIRCUIT CIRCUIT  Measuring current SERIES CIRCUIT 2 2 A current is the same at A all points in the circuit. 2 A PARALLEL CIRCUIT 2 2 A A current is shared between 1 the components A 1 A  Electron flow V Conventional current An electric current (electron flow) results from the movement electrons in an electric circuit. The electrons move from the negative terminal of the energy source to the positive terminal. For historical reasons, the direction of the current is given as the flow of positive charge from the positive terminal to the negative terminal – this is referred to as conventional current.  An electrical current is moving charge Electrons moving in one direction (on average) is current In insulators the electrons can not move between atoms so current cannot flow Conventional current Electrical Current V Conventional current Even though it is electrons which are moving in a circuit, we use Conventional current to describe the flow of charge. Conventional current flows from Positive to Negative.  Converting units Sometimes units are not in the standard unit and need to be converted. The standard units of current is ampere or amps, A. If we know the current is 30mA and we need to change it to amps Since milli (m) means (10-3) we times the 30 by 10-3 So the current is now 30 x 10-3 = 0.03A Try to convert the following: 100mA 50µA 4.2kA 16mA 0.1 A 0.00005A 4200A 0.016A  Part 5 - Voltage (V) Voltage measured in volts (V), is a measure of how much energy is given to the moving electrons in a circuit. This likens voltage to a force: the higher the voltage supplied to a circuit, the bigger the "push" the electrons receive. Symbol : V Units : Volts (V) If current is the flow, then voltage is the push! Electrons need to be given energy to "push" them around a circuit. A battery stores chemical energy that is transferred to electrons when the battery's two metal contacts, or terminals, are connected by a circuit. Electrons use the energy to flow from the negative to the positive terminal. Along the way they transfer their energy to components such as light globes, motors and heating elements. Different batteries have different voltages  Measuring Voltage We use a voltmeter to measure voltage. V Where would you put a voltmeter in a circuit? Across the component (in parallel) Load (e.g. light globe, motor) Ammeter in series measures current Voltmeter in parallel measures voltage  Measuring Voltage This is how we draw a voltmeter in a circuit. V V SERIES PARALLEL CIRCUIT CIRCUIT  Series Circuit Voltage is shared between the components. 3 V 1 1.. 5 5 V V  Parallel Circuit Voltage is the same in all parts of the circuit. 3 V 3 V 3 V  Part 6 - Resistance Opposition to the flow of current is termed resistance. The fact that a wire can become hot from the flow of current is evidence of resistance. Conductors have very little resistance. Insulators have large amounts of resistance. Loads such as light globes and motors provide resistance. Resistance is measured in the special unit of ohms, symbolized by the Greek letter omega (Ω). The higher the resistance of a component, the harder it is for electrons to flow through it and the more energy they lose. The resistance of the connecting wires in typical electrical circuits is so low that we can assume it's zero.  Resistors Resistors are special components with high resistance designed to lower the current passing through a circuit.  Resistors Carbon resistors typically have four colour-coded bands on their case. These bands are part of a code that allows you to work out their approximate value (in ohms) and tolerance. The fourth band is the tolerance band, which indicates the accuracy of the resistor. Gold means a 5% accuracy, silver means a 10% accuracy and no fourth band means a 20% accuracy. The lower the percentage, the more accurate the resistor should be. Calculating resistance The tolerance band is gold, so the resistor has 5% accuracy. The first band is blue, so it has a value of 6. The second band is red, so it has a value of 2. The number so far is 62. The third band is also red, so this means 2 zeros need to be added to the number. The number is now 6200. Resistor values are always coded in ohms, so the value of this resistor is 6200 ohms or 6.2 kilo-ohms. We would write this as 6200Ω or 6.2kΩ Your turn 100Ω ± 5% 820,000,000Ω or 820MΩ ± 5%  Ohm’s Law The current at any point in a circuit can be calculated from the voltage and the resistance. I = Current (Amps) V = Voltage (Volts) Georg Simon Ohm (1787-1854) R = Resistance (ohms) VIR Triangle I = Current (Amps) V = Voltage (Volts) R = Resistance (ohms) Using the VIR Triangle Your turn 2 240 120 Your turn 1. Write the rule V = IR 2. Substitute the values V = 0.050 x 180 3. Write the answer with units V = 9V 1. Write the rule I = V/R 2. Substitute the values I = 12 /470 3. Write the answer with units I = 0.026A 1. Write the rule R = V/I 2. Substitute the values R = 12 / 0.004 3. Write the answer with units R = 3000Ω or 3kΩ  Part 7 - Power Power is the rate at which energy is being used. (Energy used per second) Power is measured in watts (W) and can be calculated P = Power (watts) with current and voltage. More energy-efficient devices use less power. V = Voltage (Volts) I = Current (amps) Your turn 1. Write the rule P = VI 2. Substitute the values P = 120 x 5 3. Write the answer with units P = 600W 1. Write the rule I = P/V 2. Substitute the values I = 60 /120 3. Write the answer with units I = 0.5A 1. Write the rule V = P/I 2. Substitute the values V = 60 / 30 3. Write the answer with units V = 2V  Summary Table Quantity Definition Symbol Unit of Unit measurement Abbreviation Current Flow of electrons around a I ampere (Amp) A circuit (how many charges pass a point per second) Voltage Driving force that pushes the V volt V current around (how much energy the electrons have) Resistance Anything in a circuit that slows R Ohm Ω the flow of electrons Power The rate of energy use P watt W