Electricity

1. Electricity • The movement of charged particles • Electrons – Negatively charged particles found in atoms • Ions – atoms which have gained or lost electrons to get a full outer shell and become stable • Flow of electrons around a circuit is called CURRENT (I) measured in amps (A) • Voltage (V) – Increase or decrease in the amount of electrical energy carried by the current.

2. Atoms to Ions • Na + Cl 2,8,1 2,8,7

3. Static Is stationary Brush your hair Wool socks in tumble drier Current Electrons are forced around a complete circuit by an electrical force field turn on light Ipod Electricity occurs in 2 different forms:

4. Static Electricity • Occurs when materials which are usually insulators become electrically charged. • This is because electrons have been transferred (rubbing adds or removes electrons) • Like charges repel, opposite charges attract Use this information to draw a diagram explaining how the paper sticks to the balloon (assume your hair removes electrons)

5. Conductors and Insulators • Conductors allow the electrons to transfer through them easily. Metals are good conductors. • Insulators do not allow the electrons to move easily through them. Plastic, wool, wood, rubber etc. are good insulators

6. Static electricity in conductors • Electrons can move freely through a conductor. • The charge can therefore be distributed uniformly over the surface of the conductor.

7. Static electricity in an insulator • Electrons are not free to move over the surface of an insulator. • A temporary polarisation of the individual atoms can occur. • The charge is not uniformly distributed over the surface of the object. • Polarisation is strongest where the surface is close to the charged object. • Which is the conductor, sphere A or B?

8. Charge distribution and shape Charge accumulates at pointed surfaces • Electric fields occur at 90 degrees to the surface of the object. • Electrons A and B repel maximally in a flat surface. • Electrons C and D can accumulate closer because the repulsive effect of surrounding electrons is not as great.

9. Excellence Questions Consider these when answering: • Define insulator and conductor • State the charge on electron • Rubbing adds/ removes electrons • Like charges repel, opposites attract • State direction of electron movement • Distribution of charge in conductor and insulator • Electrostatic FORCE is greatest in close proximity

10. Electroscopes • Electroscopes are devices that detect static electricity. • Explain what is happening. • Van De Graff generator shows the effects of static electricity.

11. Lightning • Caused by static • water/ ice particles rub past each other due to convection currents. (hotter is less dense therefore rises) • Electrons are transferred creating charged clouds • When the charge is very large electrons jump by ionising the air, they take the shortest path to ground

12. Grounding / Earthing • Grounding is a physical connection between an object and the earth. This will stop an object becoming charged. • Explain why she gets a shock from touching the car. Assume the car is negatively charged. • How has the car become charged? • Why is the charge evenly distributed on the car? • Why does the charge build up on the car? • Why does the area under the car become positive?

13. How has the car become charged? • Rubbing air gains electrons • Why is the charge evenly distributed on the car? • Metal is a good conductor • Why does the charge build up on the car? • Rubber tyres are a good insulator • Why does the area under the car become positive? • Like charges repel, electrons move away leaving +ve • Person (+) touches the car (-) and electrons jump to her = grounded fire

14. CURRENT ELECTRICITY To have an electric current flow we need 2 things… We should also have a power user, or else we have a… short circuit, which means that the electricity is getting back to the power source still with its energy. This will result in the power source getting very hot, and there may be an electrical fire. Complete Closed circuit (use wires to join parts) Power supply What is wrong with this diagram? The source provides energy to the electrons, which move away from the –ve end and towards the +ve end. As they pass power users their energy is used up, and they return to the power source with none.

15. ELECTRIC COMPONENTS Power sources… Single cell Battery (of cells) Power pack (variable) Power users: Bulb Resistor Variable resistor 6 Current Controllers.. Also useful… Connecting Wires Wires joined Diode Switch LED

16. Current Controllers • Diodes let electrons flow in one direction • Switches allow electrons to flow or not • Fuses protect…………….. • Resistor/ Rheostat control the movement of electrons As resistance is increased current flow… decreases

17. CIRCUIT TYPES The simplest type of circuit involves electricity moving down one route. (electrons don’t choose!). This is called a Series circuit. Draw the path the electrons travel. The other main type of circuit has two or more branches. This is called a Parallel circuit. Draw on the electron flow. What sort of circuit is this? A parallel… but, more importantly… A short circuit.

18. Explain what you would observe if a bulb breaks in each of the circuits? • The lights in the series circuit will go out but the other light in the parallel circuit will still be lit as there is an alternative pathway for current to flow which means there is still a complete circuit. A M E

19. Current (I) Current is the amount of charge past a point over time (flow of electrons around a circuit) I=q/t I= current, q = charge, t = time • To measure current we need to “count electrons” using an ammeter • 6.25 x1018 electrons = 1 coulomb • 1 coulomb per second = 1 Amp • Connect in series where you need to measure the current Red terminals to Red and Black to Black. • DO THE TOUCH TEST! If the needle goes backwards or off the scale – Recheck

20. Current (I) Current is the amount of charge past a point over time • In series circuits the current is the same everywhere • In parallel circuits the current is split between the branches and adds up to the total drawn from the supply

21. Voltage (V) Change in energy per charge between two points (energy gained or lost as it passes through a component) V = ΔE/q V = voltage (V), E = energy (J), q = charge (c) • Connect a voltmeter in parallel around the component Red terminals to Red and Black to Black. • DO THE TOUCH TEST! If the needle goes backwards or off the scale – Recheck

22. Voltage (V) Change in energy per charge between two points • In series circuits the voltage is shared between components • Therefore voltage loss = voltage gain • In parallel circuits the voltage is the same across all branches

23. SUMMARY Split between branches The branches add to the total Always the same Voltage from source = voltage used Voltage is shared between power users Voltage is the same in all branches

24. V I × R Resistance (R) • The amount that a component slows the current • Insulators have high resistance, whereas conductors have low resistance. • As the electrons are slowed by a resistor, energy is lost in the form of heat. • This means that current, resistance and voltage must be linked. • This is Ohms law V=I x R • The unit of resistance is the ohm, symbol 

25. ELECTRICITY PROBLEMS 1. What will the voltage be in V1 and V2? 2. If the current in the lower branch is 3A, what is the resistance in the bulb? 3. If the current at point Y is 4A, what is the total current in the circuit? 4. What is the current through the resistor below? Y

26. Resistance question • Find the current • If an extra 5Ω is inserted in series what is the voltage and current through each? • If the 5Ω is added in parallel what happens to the voltage and current in each? I=V/R I=12/5 I=2.4A 12V 5Ω

27. P I × V POWER In an electrical circuit energy may be used up by components like lamps and resistors. The rate at which this occurs is called It is measured in One watt means that 1 joule of electrical energy is being used up per second. Power is often given in kW (kilowatts) = The power in a circuit depends on… Power (P). Watts (W). 1000 W Current (I) and Voltage (V).

28. Power • Which bulb is brightest? Why? 12v 12v 0.4A 0.4A 0.5V 3 Describe the effect on battery life and bulb brightness when you replace a 100W bulb with a 60W one. 1.4A 2 1 4

29. Resistance in Series Circuits • If we add resistances (or any component with resistance ) the overall resistance of a circuit will increase, this implies that the current will decrease. • If R1 = 2 Ohm, R2 = 3 Ohm and R3 = 7 Ohm and 24v are supplied find: • The total resistance • Current in the circuit • Voltage across each resistor 12 Ohm I=V/R I= 24/12 I = 2Amp V=IR V =2 x 2 V1=4volts What happens to P1 if R2 increases? V2 = 6v, V3 = 14v

30. What effect does adding more bulbs have? Series: • The current in a series circuit is always the same. • The more energy users (bulbs), the higher the resistance and the lower the current. So the bulbs appear dimmer. Parallel: • The current in the branches of a parallel circuit add to equal the current from the power supply. • Morebranches, means less resistance so….. • As bulbs are added they drawmore current with the same voltage and so appear the same brightness.

31. rise rise V = = gradient = R Voltage Voltage run I run Current Current Effect of heat on Resistance If you change the voltage across a resistor the current will change. In this case the resistance is constant. If the resistor gets hot, the particles start to vibrate, and the electrons find it harder to get through, so the current drops more than expected. Some components like LED, thermistors, diodes make use of this resistance change. Eg LDR is on when the resistance is low (gradient is not steep) Why do bulbs blow when they are cold?

32. - + A Flow allowed B Flow Blocked DIODES Diodes only allow current to flow in one direction. And are drawn like this in a circuit diagram: - end + end Electron flow If you set up this circuit what would you notice? Lamp A goes, B does not. Explain what is going on. Diode allows current to flow in only one direction.

33. Practise question • Mr D. wants to wire some lights on a trailer. He has 2 lamps available 12v 6W and 12V 12W • He connects in parallel first with a 12V battery • Draw the circuit with a switch that controls both lamps. • Find the current and resistance in each lamp. • He then connects in series to see if anything changes. He measures the current to be 0.33A. • In series which is brighter and why?

34. Test Yourself • A series and parallel circuit are built containing a 4 ohm and 8 ohm bulb and a 12v battery. • Draw the circuits. • Explain using the terms resistance, current and power why the 4 ohm bulb in parallel will be brighter. • Calculate the power of each bulb. • The bulbs in parallel have less resistance and draw more current. Voltage in parallel is the same as the source which is also higher than it will be in the series circuit. As P=IV the bulb in parallel will have more power and be brighter.

35. S T E P × t ENERGY Work done by a component The total energy used in a circuit has to do with the powerand… the amount of time that it is running for. Energy is measured in Joules (J), Power (W), time (s) Questions: 1. What is the power in bulb S? 2. What is the voltage across bulb T if the power is 60W? 3. Both circuits are turned on for 30 seconds. How much energy is used?

36. Power question • A vacuum cleaner has 2200W of power. It is plugged in at 240V. If it runs for 2 mins find the current and energy it uses. • I= P/V • I= 2200/240 • I= 9.2A • Time = 120s • E= Pxt • E= 2200 x 120 • E= 264000J or 264KJ

37. Overview

38. PROBLEMS 1. Fill in the gaps in the table. a b c d e f g h j i k l

39. Magnetism • Objects that contain Iron are magnetic. • A magnetic field is a region where a magnetic object experiences a force. • The magnetic field moves from North to South pole. • The closer the lines the stronger the force.

40. Compass • A compass is a magnetic object. • They line up with the magnetic field lines. • The Earth has an Iron core which allows it to form a magnetic field. • Where is the north? • What we call the North Pole is actually the magnetic South pole!

41. Effect of Magnets • Magnetic fields can interact. • Like poles experience a repulsive force so they repel. • Opposite poles experience an attractive force so they attract.

42. Question • Explain how and why the magnetic fields interact to enable this • In your answer consider the strength of the magnetic force • First nail is attracted, N at top because N and S attract (opposite poles attract) • Therefore S at position B. This means the pin becomes a temporary magnet. • The force is weaker as you get further away. • Repeat for pin 2

43. Magnetic Field around a current carrying wire • When a current passes through a wire it creates a magnetic field around it. • It is circular in shape • It is stronger closer to the wire. • X represents current going into the page. • .represents current coming towards us.

44. Magnetic Field Direction • Grasp the wire with your right hand so your thumb points in the direction of the conventional current. • Your fingers are now pointing in the direction of the magnetic field. • Right Hand Grip Rule • What is the direction of the magnetic field?

45. Solenoid • A solenoid is a coil of wire. • This concentrates the magnetic field so it becomes stronger. • Grip the solenoid so your fingers curl in the direction of the current, your thumb points North. • Use the right hand grip rule to find the North pole on the iron bar in this solenoid. N

46. Electromagnets • Placing an iron core in the solenoid creates an electromagnet. • Electromagnets can be turned on or off. • The strength can be increased by: • Increasing the number of turns in the wire • Increasing the thickness of the iron core • Increasing the current

47. Explain how the bell works • When the switch is closed the circuit is completed and current flows through the electromagnetic coil. • The iron striker is attractedto the electromagnet and strikes the bell. • This means the circuit is broken so the coil loses its magnetism.The spring returns the striker to its original position which makes a new contact and the cycle repeats itself. • The bell will continue to ring as long as the switch is held closed.

48. Magnetic Field Strength • B=kI/d • B= strength of magnetic field (Tesla) • K = 2.0 x 10-7 TmA-1 • I= current • D = distance (metres) • Calculate the strength of the magnetic field created by a 12v, 30 Ω wire at distance of 1cm and 3cm. • 8 x 10-6 T and 2.7 x 10-6T