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Transcript

Electricity Electric Charge Can come from batteries and generators. Some materials can be charged when they are rubbed – this charge can be called electrostatic charge. Polythene Wool Transfer of electrons More electrons than normal: net negative charge. Perspex Wool Transfer of electrons More electrons than normal: net negative charge. When the polythene is rubbed with wool it becomes ___________ charged. This is because _________ are being transferred to the wool. When the perspex is rubbed with the wool the electrons move onto it and it becomes _________charged. Two pieces of polythene will __________ each other because like charges ___________. A piece of polythene and a piece of perspex will __________ each other because unlike charges _________. Where does charge come from? Can you name the parts of the atom? This is the NUCLEUS. ELECTRONS orbit the nucleus and have a negative charge. PROTONS (positive charge)and NEUTRONS (no charge) are found in the nucleus Usually atoms have an equal number of protons and electrons, therefore the net charge is zero. Rubbing, separates the charges by electron transfer, leaving one material positive and the other one negative. Conductors and insulators Conductors Insulators Metals are good conductors – such as copper, silver Plastic, glass rubber Have loosely held electrons Have tightly held electrons Electrons free moving Electrons not free moving Difficult to charge by rubbing Easy to charge by rubbing Charge by induction Charged objects will attract uncharged objects close to them by inducing a charge in them. Charged objects that are brought close to uncharged objects induce charge – as in the diagram – opposite charges are attracted to the rod. Touching the sphere with a finger will Earth the sphere. Earthing If enough charge builds up on something, electrons can be pulled through the air. This causes sparks. Objects must be Earthed. Unwanted charge flows to Earth through a conducting material. The Van De Graaf Generator i. carbon brushes remove electrons from rubber band ii. rubber band turned by motor iii. electrons pass to metal dome iv. dome becomes negatively charged Homework • Find out the units of charge • Research and find at least two uses of electrostatic charge Questions on electrostatic charge Give an example of where electrostatic charge might be a hazard. How can a build-up of charge be prevented? In the diagram a charged metal rod is held close to a metal can. Copy the diagram and draw any induced charge on the can Why is the can attracted to the rod even though the net charge is zero? In which direction is the electron flow when your finger touches the can? d) What type of charge is left on the can after it has been touched? 4. Explain why a balloon becomes negatively charged after it has been touched. 5. Why does the balloon stick to the ceiling? 1. 2. 3. a) b) c) Circuits •Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), lamps, ammeters, voltmeters, magnetising coils, transformers, bells, fuses, relays •Understand that the current at every point in a series circuit is the same Current The conventional current direction is from + to – and describes the flow f positive charge. This was before the electron was discovered and scientists realised that positive charge did not flow round circuits • Defined as a flow of charge (Q) • Measured in Amperes/Amps • Current measured using an ammeter • In a circuit the current is not affected by the ammeters in it • The current is the same at all points around the circuit There is a link between charge and current • Charge (Q) = current (I) x time (t) Q l t 1 Coulomb/sec = 1 Ampere 2 Coulomb/sec = 2 Amperes • If a current of 2A flows for 3 seconds the charge delivered is 6 Coulombs. • The direction of the current is from a high electron potential to a low electron potential (from – to +) Voltage Defined as the amount of energy given to electrons in a current. Measured in volts (V) Also known as potential difference (PD). Energy is wasted inside a cell when it is connected to a circuit so the maximum PD a cell can have is when it is not connected to a circuit. This maximum PD is called the electromotive force (EMF). Voltage is measured using a voltmeter In a circuit, voltmeters are always placed in parallel to the component over which they are measuring the voltage difference. The total voltage across the bulb (in the diagram) is equal to the voltage in the main circuit. No energy is lost in the main circuit but some is lost in the first bulb and more in the second. Measuring current and voltage 1. 2. 3. Write the title and draw the circuit symbols on page 176 Make a parallel and series circuit – draw a diagram. Compare two ways of wiring up two lamps and two switches in a circuit – one in a parallel and one in a series circuit. Record your findings in a table: Series Circuit diagram Appearance of lamps Behaviour of current Advantages Disadvantages Voltage across bulb A Voltage across bulb B Voltage across cell Current between cell and bulb A Current between cell and bulb B Parallel Resistance the degree to which current is reduced when flowing through a conductor Resistance (Ω) = voltage (V)/Current (A) V I R Factors affecting resistance – Investigation length, thickness, material, temperature Why do bulbs get hot? Plan an investigation of how resistance is affected by length. Apparatus • circuit board • bulb • voltmeter • ammeter • constantan wire • meter rule • power supply Think about what you will vary and what you will measure Make a prediction of how you think the resistance will vary when you change the length. Investigation of factors affecting resistance 1) Length of circuit 2) Size of material 3) Type of material 4) Temperature Resistance and Ohm’s Law Aim: to investigate Ohm’s Law In a metal conductor, current will vary with potential difference if the temperature of the conductor is kept the same. This is Ohm’s Law: o Current is proportional to the PD Method o Draw a circuit diagram and describe how you will investigate Ohm’s Law. o What are you going to vary (independent variable)? o What are you going to measure(dependent variable? o What are you going to keep the same in order to make this a fair test? Results: Copy and complete the table and plot a graph of your results Conclusion: what does the graph tell us? PD (V) Current (A) Resistance (Ω) Resistance components resistors are specially made to provide resistance – useful in complicated circuits for limiting currents and PDs Variable resistors (rheostats) vary resistance and therefore current Thermistors high resistance when cold and low when hot Light-dependent resistors (LDRs) high resistance in the dark and low in the light Diodes extremely high resistance in one direction and low in the other Current – PD graphs The tungsten filament As the current increases the temperature rises – increasing the resistance The semi-conductor diode A diode allows the current to flow in one direction only. Current is not proportional to pd and if pd is reversed the current is zero Questions 1) 240V, 3A. Calculate the resistance 2) 70Ω, 12A. Calculate the voltage 3) 12000V 36000Ω. Calculate the current. More questions on page 181. Series and parallel circuits •State that, for a parallel circuit, the current from the source is larger than the current in each branch •Recall and use the fact that the current from the source is the sum of the currents in the separate branches of a parallel circuit •Recall and use the fact that the sum of the p.d.’s across the components in a series circuit is equal to the total p.d. across the supply •State the advantages of parallel circuits Pages 186 and 187 out of books Questions page 187 Resistors in series and parallel •Draw and interpret circuit diagrams containing diodes as rectifiers •Give the combined resistance of two or more resistors in series •State that the combined resistance of two resistors in parallel is less than that of either resistor by itself •Calculate the effective resistance of two resistors in parallel Investigations for each investigation Investigation 7 - Resistors Resistors in a circuit In series In parallel If two or more resistors are connected in series their combined resistance is added together – this is like lengthening the wire in the circuit. R = R1 + R2 Eg: If two or more resistors are connected in parallel they give a lower resistance than any of the resistors by themselves – this is like using a thicker piece of wire in your circuit: 1/R = 1/R1 + 1/R2 Eg: R1 = 2Ω R2=2Ω R2=2Ω R1 = 2Ω Total resistance R = R1 + R2 = 4Ω Total resistance 1/R = 1/R1 + 1/R2 = 1Ω Electrical Energy •Energy is measured in Joules. •Electrical equipment always has a power rating. This tells us how much energy (Joules per second) is required by the appliance. •1 Joule per second = 1 Watt. Electrical Energy Usual Power ratings; Filament light bulbs are normally 60 or 100W. An electric heater can be 3 KW. This means the heater can convert 3000 Joules per second into heat (+ any wastage). Electrical Energy The formula for finding power of electrical equipment is; P = IV Where; P = Power (W) I = Current (A) V = Volts (V) Electrical Energy What is the power of an electric iron, if a current of 7A is obtained from a 230V supply? Electrical Energy P = IV P = 7 x 230 P = 1610 W Electrical Energy An electric oven has a power rating of 2KW What is the current measured when the oven is used with a 230V supply? Electrical Energy Calculating energy transferred E = Pt Where; E = energy is Joules P = Power t = time Electrical Energy We can also rewrite the equation. E = VIt Where; E = Energy V = Voltage I = Current t = time Review You will need the following equations; Q = It Q = Charge I = Current t = time GPE = mgh GPE = Gravitational potential Energy m = mass g = gravity h = height E = Pt E = energy is Joules P = Power t = time Electrical power in resistors Electrical power dissipated in a resistor P = I2 R Where P = Power (W) I = Current (A) R = Resistance () Electrical power in resistors Although we are discussing resistors, this also applies to heating elements and all other components that have resistance. When current flows through a resistor it has a heating effect. Electrons lose potential energy when is changed to thermal energy. Electrical power in resistors What power is dissipated in a 5 resistor when the current through it is a) 2A b) 4A? Electrical power in resistors 20 W b) 80 W a) Electrical safety Includes circuit breakers and fused plugs Circuit Breakers – automatic safety switch switches off with excess current 2. Fuses and fused plugs: Fuse circuit symbol 1. Deliberate weak link in a circuit for safety Fuse wire melts if too much current flows Prevents electrical fires Fused plugs have several safety features 1. 2. 3. 4. 5. 6. 7. 8. Live wire – colour coded – brings live voltage to plug Earth wire – safety wire – connects plug to Earth – excess current to Earth Neutral wire – completes the circuit – kept at zero voltage Plastic casing – electrical insulator Fuse – placed in the live wire – wire in fuse melts with excess current Cable grips hold cable firmly in place Connecting screws – holds wire in place Double insulation – has two plastic casings around main cable to plug Hazards Hazard Danger Damaged insulation Possibility of electric shock Cable overheating Excess current in the cable causes plastic to catch fire Damp conditions Water is an excellent electrical conductor Short-circuit Electrical current travels along path of least resistance Switches Live current present is accidentally switched on Fuse Ratings P I V All fuses have a fuse rating Use the power, current, The maximum current a fuse voltage triangle to calculate the fuse ratings Example A vacuum cleaner has a rating of 460W on the 230V mains. Calculate the fuse rating. Power = current x voltage 460W = current x 230V Current = 2A Here a 3A fuse should be fitted can carry without melting Only certain values are available: 1A, 3A, 5A, 10A, 13A. The fuse rating should always be slightly higher than the current in the wire Questions on pages 193 and 195