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## 02/01/2020

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**02/01/2020**Georg Simon Ohm 1789-1854 P5 Electric Circuits OCR 21st Century**Static Electricity**02/01/2020 An introduction – click here**Static Electricity**02/01/2020 + - - + Static electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used: + + - - + + + - - - + + - + - -**Static Electricity**02/01/2020 - - + - - + + - - - - -**Short Static Experiments**02/01/2020 Try the following quick static electricity experiments: • Rubbing a balloon on your jumper and “sticking” it to the wall • Charging a plastic rod by rubbing it with a cloth and then holding it near the water from a smooth-running tap • Charging a plastic rod and trying to pick up small pieces of paper (or someone else’s hair!) with it • Rubbing a balloon on someone else’s head – you might want to ask their permission first… Can you explain what you saw in each of these experiments?**Gold Leaf Electroscopes**02/01/2020 - - - - - - - - - - Consider a gold-leaf electroscope… Now charge the top: - This effect was seen because charges can easily move through conductors**Electric Current**02/01/2020 Note that electrons go from negative to positive + - e- e- Electric current is a flow of negatively charged particles (i.e. electrons). By definition, current is “the rate of flow of charge”**Basic ideas…**02/01/2020 Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____. Words: volts, amps, ohms, voltage, ammeter, voltmeter**Understanding Current**02/01/2020 Electrons Ions When a voltage is applied it basically causes the electrons in a conductor to move towards the positive end of the battery: Negative Positive The main difference between conductors and insulators is that insulators have less of these free electrons.**Circuit Symbols**02/01/2020 Variable resistor A V Diode Switch Bulb Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery**More basic ideas…**02/01/2020 If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit**Electric Current revisited**02/01/2020 Note that we usually assume the resistance of these leads is very small. + - e- e- Work is done by the battery to move the electrons around the circuit. When the electons arrive at the bulb energy is transferred to it.**Electrical Power revision**02/01/2020 P V I Power is defined as “the rate of transferring energy” and is measured in units called “Watts” (W). The amount of power being transferred in an electrical device is given by: Power = voltage x current in W in V in A • How much power is transferred by a 230V fire that runs on a current of 10A? • An electric motor has a power rating of 24W. If it runs on a 12V battery what current does it draw? • An average light bulb in a home has a power rating of 60W and works on 230V. What current does it draw?**Understanding Resistance**02/01/2020 Electrons Ions Recall our previous model of electric current but this time we’ll use it to explain resistance: Negative Positive Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. This effect causes the metal to heat up.**Using this heating effect**02/01/2020 This heating effect can have its advantages and its disadvantages. For example, consider an old-fashioned light bulb: This heating effect causes the filament to emit light… …but it also causes a lot of energy to be wasted to the environment**Resistance**02/01/2020 Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. Georg Simon Ohm 1789-1854 V Resistance = Voltage (in V) (in ) Current (in A) I R The resistance of a component can be calculated using Ohm’s Law:**An example question:**02/01/2020 Ammeter reads 2A A V Voltmeter reads 10V • What is the resistance across this bulb? • Assuming all the bulbs are the same what is the total resistance in this circuit?**More examples…**02/01/2020 3A 3A 2A 4V 2V 1A 6V 12V What is the resistance of these bulbs?**Resistance**02/01/2020 Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) Resistance is anything that opposes an electric current. What is the resistance of the following: • A bulb with a voltage of 3V and a current of 1A. • A resistor with a voltage of 12V and a current of 3A • A diode with a voltage of 240V and a current of 40A • A thermistor with a current of 0.5A and a voltage of 10V 3 4 6 20**LDRs and Thermistors**02/01/2020**Two simple components:**02/01/2020 Resistance Resistance Amount of light Temperature 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES 2) Thermistor – resistance DECREASES when temperature INCREASES**Using Thermistors and LDRs in circuits**A V • What will happen to the resistance of the thermistor when it gets hotter? • How will this affect the brightness of the bulb and the reading on the ammeter? • How will this affect the readings on the ammeter and voltmeter?**Resistors in Series and Parallel**10Ω 10Ω 10Ω 10Ω Consider the total resistance of these combinations: The total resistance of this combination is 20Ω as the battery has to move charges through both resistors The total resistance of this combination is only 5Ω as the charges have more paths to move through**Current-voltage graph for a Resistor**02/01/2020 I V Notice that a current-voltage graph for a resistor of fixed value shows that current increases in proportion to voltage. Resistor**Current in a series circuit**02/01/2020 If the current here is 2 amps… The current here will be… The current here will be… And the current here will be… In other words, the current in a series circuit is THE SAME at any point**Current in a parallel circuit**02/01/2020 Here comes the current… Half of the current will go down here (assuming the bulbs are the same)… And the rest will go down here… A PARALLEL circuit is one where the current has a “choice of routes”**Current in a parallel circuit**02/01/2020 And the current here will be… The current here will be… The current here will be… The current here will be… If the current here is 6 amps**Some example questions…**02/01/2020 3A 6A 4A 2A 1A each**Voltage and Work done**What does “voltage” mean? The voltage (or potential difference) between two points is a measure of the work done per “bit” of charge moving between these points.**Voltage in a series circuit**02/01/2020 V If the voltage across the battery is 6V… …and these bulbs are all identical… V V …what will the voltage across each bulb be? 2V**Voltage in a series circuit**02/01/2020 V V V Notice that the voltages add up to the voltage across the battery – this is because the work done on each unit of charge by the battery must equal the work done by it to the bulbs.**What if the Resistances are different?**60V 10Ω 20Ω V1 V2 Q. What would each of these voltmeters read? Total resistance = 30Ω Therefore current = 2A Therefore V1 = 2x10 = 20V and and V2 = 2x20 = 40V The voltage across the bigger resistance is higher as more work is done by the battery to get the current through it.**Voltage in a series circuit**02/01/2020 V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? V 4V**Voltage in a parallel circuit**02/01/2020 If the voltage across the batteries is 4V… What is the voltage here? V V And here? 4V 4V**Summary**02/01/2020 In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand”**An example question:**02/01/2020 3A 6V A1 6V 3A A2 2A V1 1A A3 V2 V3 3V 3V**Another example question:**02/01/2020 3A 10V A1 1.2A 3A A2 V1 6.7V A3 1.8A V2 V3 5V 5V**Electromagnetic Induction**N The direction of the induced current is reversed if… • The wire is moved in the opposite direction • The field is reversed The size of the induced current can be increased by: • Increasing the speed of movement • Increasing the magnet strength**Electromagnetic induction**The direction of the induced current is reversed if… • The magnet is moved in the opposite direction • The other pole is inserted first The size of the induced current can be increased by: • Increasing the speed of movement • Increasing the magnet strength • Increasing the number of turns on the coil**Transformers**Current through primary Time Magnetic field Time Voltage induced in secondary Time**Transformers**Voltage across primary (Vp) No. of turns on primary (Np) Voltage across secondary (Vs) No. of turns on secondary (Ns) Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil. Words – alternating, magnetic field, induce, step up, voltage We can work out how much a transformer will step up or step down a voltage:**Some example questions**• A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil? • A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V?**AC Generators**Slip rings and brushes Magnetic Field**Generators (dynamos)**The coil already has an iron core, but the induced current could still be increased by: • Increasing the speed of movement • Increasing the magnetic field strength • Increasing the number of turns on the coil**Large-scale production of Electricity**02/01/2020 02/01/2020 A generator at Drax power station in England