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Transcript
3rd/4th form – Electric circuits Conductors and insulators • • • • An electric current is a flow of charge. These charges are often electrons. Electrons carry a negative charge. Electrons are usually bound to atoms, but some more strongly than others • In a conductor some charges are able to flow so we can use them in an electric circuit. • In an insulator the electrons are held tightly in position and so are unable to move. These materials do not conduct electricity. Conductors and insulators • Generally, most metals are good conductors and most non-metals are not. • An electric cable makes use of both types of material. Metal wires to carry electricity Plastic insulation to isolate metal wires • There is a third category, semi-conductors, which are also very useful (not GCSE) Conduction electrons • Metals have lots of ‘free’ electrons – weakly bound outer electrons • so they are good conductors. • Normally they move around randomly • When an electric field is applied they move in a common direction Counting charges • The charge on a single electron is tiny • We define a “useful” quantity of charge which allows sensible measurements, called the Coulomb. • 1C is equal to the charge on 6.24151×1018 electrons – That’s 6,241,510,000,000,000,000 electrons! – Ammeters need to go in the circuit so the current can flow in and out of them. Electric current • An electric current is a flow of electric charge (pushed along by a voltage). • Where do these charges come from? • In a circuit, all the wires and components are full of electrons – so as soon as a power supply is connected a current starts flowing – No time delay with long wires! – (Electrons move quite slowly) Electric Current • Current is a flow of charge • We can define current as the number of coulombs flowing past a point in 1 second Current (A) Q I t Charge (C) time (s) • When 1C of charge flows through a wire in 1s, the current is 1A. Examples 1. What is the current when 4 C of charge flows for 2 seconds? 2. What is the charge flowing through a wire in 5 s if the current is 3 A? Measuring current AMMETER An ideal ammeter does not affect the circuit, no energy is transferred to it: it has zero resistance • Ammeters need to go in the circuit so the current can flow in and out of them. • To just indicate the flow of current without measuring it we can use an indicator light. Conventional Current • Charges come in two flavours, + and – • Conventional current is defined as the direction in which + charges flow • so in a wire, the conventional current is in the opposite direction to the electron flow Circuits: a reminder • Circuit diagrams are simplified drawings • Circuit diagrams are drawn with a ruler! – Connections must connect – There are symbols for each component Circuit symbols • You’ve just got to learn them! • Full list is on p. 242 Electric current • When a circuit with a battery is competed, the battery “pushes” the charges around. • Electric current is never “used up” as electrons flow around a circuit. – The current is the same through all components in series circuit – All ammeters read the same: – (note A connect in series) Electrical Energy • Electricity is useful because it can be easily converted into other types of energy. • A battery or power supply gives electrical energy to the electrons in a circuit. • Other circuit components then convert this to different forms of energy. Potential Difference (“Voltage”) • The voltage between two points in a circuit is a measure of how much energy is transferred to or from the charges as they pass between those points. • The voltage of a power supply is a measure of how hard it “pushes” charge – So a larger voltage supply means a larger current will flow in a circuit Voltage • Voltage is a measure of the energy change experienced by charges • We define voltage as the number of joules transferred per coulomb of charge voltage (V) E V Q energy (J) charge (C) • When charge passes through a p.d. of 1V, 1J of energy is transferred per coulomb. Examples 1. When 2C of charge pass through a battery they acquire 24J of energy. What is the voltage of the battery? 2. The p.d. across a lamp is 3V. How much energy is transferred when 5C of charge pass through it? 3. (harder) How much energy is transferred when a current of 2A flows through a lamp with a p.d of 3V across it for 1 minute? Combining cells • For batteries in series, the supply voltage is the sum of the individual batteries – Adding more batteries in series increases voltage and therefore increases current 7.5 V V • For batteries in parallel, the supply voltage is the same as a single branch, but the battery capacity is increased (battery lasts longer) – Adding more batteries in parallel does not affect voltage or current V 3V Measuring voltage • To measure the voltage between two points we connect a voltmeter in parallel across those two points Here we are measuring the voltage across the resistor R An ideal voltmeter does not affect the circuit, no energy passes through it: it has infinite resistance Resistance • Resistance is the opposition to current flow displayed by components – for a fixed voltage, the larger the resistance, the smaller the current • Resistance of connecting wires is usually so small it is ignored • Resistors dissipating energy get hot! – e.g. lamp filament Resistance • Resistance is caused by collisions between the free charges and the lattice of atoms which makes up the conductor • Each collision transfers energy to the atoms of material – material heats up • A high current means more collisions – resistor gets hotter Resistance • Resistance is a measure of the opposition to current flow • We define the resistance of a device as the voltage needed to push a given current V through it voltage (V) R resistance (W) (Ohms) I current (A) • When a p.d. of 1V causes a current of 1A to flow through a device, its resistance is 1 W Examples • If a lamp has a current of 3A when there is a p.d. of 12V across it, what is the resistance of the lamp? • What is the current through a 100W resistor with a p.d. of 5V across it? • A real ammeter has a resistance of 0.5W. What will the p.d drop across it be when a current of 5A is flowing? Ohm’s Law • “The current through a conductor is proportional to the current across it, provided the temperature remains Voltage (V) 6 constant” – Generally true for metals – Not true for all components 5 4 3 2 1 0 0 5 10 15 20 25 30 35 40 Current (mA) V Voltage, Resistance & Current • We have resistance (W) V R I voltage (V) current (A) • So for a given circuit: – What happens to the current if we increase the voltage of the power supply? – What happens to the current if we increase the resistance of the components? Resistors limit current • They can be used to protect vulnerable circuit components in case of a fault • Variable resistors can be used to control things Series and Parallel Circuits • A series circuit has only one path for the current to flow, all the components are joined together in a continuous line. • A parallel circuit contains branches, the current splits and recombines. – Each branch is unaffected by the other branches A A Current in series circuits • Remember, current is never “used up” as electrons flow around a circuit. • The current is the same through all components in series circuit – All ammeters read the same A A A A A A Voltage in a series circuit • The energy transferred to the charge by the battery = the energy dissipated by all the components in the circuit Vbattery V1 V2 V3 • The largest resistance has the largest voltage across it (most energy transferred) Vbattery V1 V2 – If all resistances are equal, the battery voltage is divided equally V3 Think about this circuit… • What happens to V1 as the resistance is increased? • What happens to V2? • What happens to the current? • What effect will this have on the lamp? Vbattery V1 A V2 Voltage in a parallel circuit • All components connected to a battery in parallel have the same voltage across them. Vbattery V1 V2 V3 • The current through each component is the same as if the other components weren’t there. – We can use V=IR on each branch in turn Vbattery A Current in a parallel circuit • The total current through the battery is equal to the sum of the currents through each parallel branch I battery I1 I 2 I 3 • The smallest current flows through the branch with the highest resistance. Ibattery I1 I2 I3 Characteristics of circuits • Series: – simple – one switch affects all components – one broken component affects all components – voltage is shared between components • Parallel: – components can be switched individually – one broken component only affects its own branch – all branches receive the full supply voltage