P5 Electric Circuits OCR 21st Century Georg Simon Ohm 1789-1854 17/04/2017 17/04/2017 P5 Electric Circuits OCR 21st Century Georg Simon Ohm 1789-1854

P5.1 Electric Current – a Flow of What? 17/04/2017 P5.1 Electric Current – a Flow of What?

An introduction – click here Static Electricity 17/04/2017 17/04/2017 An introduction – click here

Static Electricity + + - - + - + + + - - - + + - + - - 17/04/2017 17/04/2017 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 17/04/2017 17/04/2017 + - - -

Short Static Experiments 17/04/2017 17/04/2017 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 17/04/2017 17/04/2017 Consider a gold-leaf electroscope… Now charge the top: - - - - - - - - - This effect was seen because charges can easily move through conductors

By definition, current is “the rate of flow of charge” Electric Current 17/04/2017 17/04/2017 Electric current is a flow of negatively charged particles (i.e. electrons). Note that electrons go from negative to positive - + e- By definition, current is “the rate of flow of charge” e-

Words: volts, amps, ohms, voltage, ammeter, voltmeter Basic ideas… 17/04/2017 17/04/2017 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 17/04/2017 17/04/2017 When a voltage is applied it basically causes the electrons in a conductor to move towards the positive end of the battery: Electrons Ions Negative Positive The main difference between conductors and insulators is that insulators have less of these free electrons.

17/04/2017 P5.2 Current and Energy

Circuit Symbols Variable resistor Diode Switch Bulb A V Ammeter 17/04/2017 17/04/2017 Variable resistor Diode Switch Bulb A V Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

More basic ideas… 17/04/2017 17/04/2017 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 17/04/2017 17/04/2017 Work is done by the battery to move the electrons around the circuit. Note that we usually assume the resistance of these leads is very small. - + e- When the electons arrive at the bulb energy is transferred to it. e-

Electrical Power revision 17/04/2017 17/04/2017 Power is defined as “the rate of transferring energy” and is measured in units called “Watts” (W). P I V 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 17/04/2017 17/04/2017 Recall our previous model of electric current but this time we’ll use it to explain resistance: Electrons Ions 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 17/04/2017 17/04/2017 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 V R I Resistance = Voltage (in V) (in ) Current (in A) Georg Simon Ohm 1789-1854 17/04/2017 17/04/2017 Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Ohm’s Law: V R I Resistance = Voltage (in V) (in ) Current (in A)

An example question: Ammeter reads 2A A V 17/04/2017 17/04/2017 Ammeter reads 2A V A What is the resistance across this bulb? Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V

What is the resistance of these bulbs? More examples… 17/04/2017 17/04/2017 3A 6V 12V 4V 2A 1A 2V 3A What is the resistance of these bulbs?

Resistance Resistance is anything that opposes an electric current. 17/04/2017 17/04/2017 Resistance is anything that opposes an electric current. Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) 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 17/04/2017 17/04/2017

Two simple components: 17/04/2017 17/04/2017 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES 2) Thermistor – resistance DECREASES when temperature INCREASES Resistance Amount of light Resistance Temperature

Using Thermistors and LDRs in circuits 17/04/2017 V A 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 17/04/2017 Consider the total resistance of these combinations: 10Ω 10Ω 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 17/04/2017 17/04/2017 I V Notice that a current-voltage graph for a resistor of fixed value shows that current increases in proportion to voltage. Resistor

P5.3 How do Parallel and Series Circuits work? 17/04/2017 P5.3 How do Parallel and Series Circuits work?

Current in a series circuit 17/04/2017 17/04/2017 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 17/04/2017 17/04/2017 A PARALLEL circuit is one where the current has a “choice of routes” 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…

Current in a parallel circuit 17/04/2017 17/04/2017 If the current here is 6 amps And the current here will be… The current here will be… The current here will be… The current here will be…

Some example questions… 17/04/2017 17/04/2017 3A 6A 4A 2A 1A each

Voltage and Work done What does “voltage” mean? 17/04/2017 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 17/04/2017 17/04/2017 If the voltage across the battery is 6V… V …and these bulbs are all identical… V V …what will the voltage across each bulb be? 2V

Voltage in a series circuit 17/04/2017 17/04/2017 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. V V

What if the Resistances are different? 17/04/2017 Q. What would each of these voltmeters read? 60V V1 V2 10Ω 20Ω 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 17/04/2017 17/04/2017 If the voltage across the battery is 6V… V V …what will the voltage across two bulbs be? 4V

Voltage in a parallel circuit 17/04/2017 17/04/2017 If the voltage across the batteries is 4V… What is the voltage here? V 4V And here? V 4V

Summary Current is THE SAME at any point 17/04/2017 17/04/2017 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: 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V 17/04/2017 17/04/2017 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V

Another example question: 17/04/2017 17/04/2017 3A 10V A1 3A 1.2A A2 V1 6.7V A3 1.8A V2 V3 5V 5V

P5.4 How is Mains Electricity Produced? 17/04/2017 P5.4 How is Mains Electricity Produced?

Electromagnetic Induction 17/04/2017 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 17/04/2017 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 Time Time Time Current through primary Magnetic field 17/04/2017 Current through primary Time Magnetic field Time Voltage induced in secondary Time

Words – alternating, magnetic field, induce, step up, voltage Transformers 17/04/2017 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: Voltage across primary (Vp) No. of turns on secondary (Ns) Voltage across secondary (Vs) No. of turns on primary (Np)

Some transformer questions 17/04/2017 Primary voltage Vp Secondary voltage Vs No. of turns on primary Np No. of turns on secondary Ns Step up or step down? 12V 24V 100 ? 400V 200V 20 25,000V 50,000V 1,000 23V 230V 150

Some example questions 17/04/2017 Primary voltage Vp Secondary voltage Vs No. of turns on primary Np No. of turns on secondary Ns Step up or step down? 6V 24V 100 ? 400,000V 200V 1,000 25,000V 20,000 20 230V 150 1,500 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 17/04/2017 Slip rings and brushes Magnetic Field

Generators (dynamos) 17/04/2017 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 17/04/2017 17/04/2017 17/04/2017 A generator at Drax power station in England

AC Generator N S N S Induced current can be increased in 4 ways: 17/04/2017 N S N S Current Induced current can be increased in 4 ways: Increasing the speed of movement Increasing the magnetic field strength Increasing the number of turns on the coil Increasing the area of the coil Time

DC and AC 17/04/2017 V DC stands for “Direct Current” – the current only flows in one direction and a common example is a battery: Time AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz). In the UK mains electricity is 230V AC, not DC, as AC is easier to generate and transmit over long distances. 1/50th s 230V T V

P5.5 How do Electric Motors work? 17/04/2017 P5.5 How do Electric Motors work?

The Motor Effect N S What will happen to this wire? 17/04/2017 S N What will happen to this wire? How can you make it move faster? How can you make it move in a different direction?

17/04/2017 Electric Motors

Using Motors 17/04/2017 The following devices are just some examples of devices that use electric motors: