P2.5.Mains Electricity AC and DC Current Cables and Plugs Electrical Safety Calculating Power Calculating energy and charge (HGR ONLY)

Direct Current Direct current always flows in one direction It is produced by cells and batteries d.c. time voltage The voltage of a d.c. supply is steady and always in the same direction.

Alternating Current Alternating current is constantly changing direction. It is produced in most generators and is used in mains electricity. a.c. time voltage peak forward peak reverse The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on.

Why do plugs have three pins? Cables and Plugs Why do plugs have three pins? The three pins in an electrical plugs are connected to the three wires inside a cable. Two of the wires form the electrical circuit. The third is a safety feature.

What does each wire do? The live wire carries a current that alternates between a negative and positive voltage. The earth wire is a safety wire that is needed to earth appliances with a metal case. This makes it safer to touch the appliance if it develops a fault. The neutral wire completes the circuit. It is kept at a zero voltage by the electricity company.

What are the parts of a plug? earth terminal live terminal fuse neutral terminal cable grip cable

Boardworks GCSE Additional Science: Physics Electrical Safety How does earthing work? Teacher notes This five-stage animation shows how an earth wire and a fuse are used to increase the safety of an appliance with a metal case. Suitable prompts include: Stage 1: What is the role of the neutral wire? Stage 2: Where is the current from the loose live wire flowing into? Stage 3: Why did the person get an electric shock? Stage 4: Which wire takes the current from the faulty appliance? Stage 5: Why does the current increase when the current flows through the casing? See the ‘Resistors and Resistance’ presentation for more information about resistance.

Boardworks GCSE Additional Science: Physics Electrical Safety Double insulation Another way of improving the safety of electrical appliances is to make them double insulated. Double insulated appliances have plastic cases, without any wires connected to the case. This means that the case cannot become live, because plastic does not conduct electricity. So, if one layer of insulation is damaged the appliance is still safe for the user. symbol on double insulated appliance

Teacher notes This true-or-false activity could be used as a plenary or revision exercise on plugs, or at the start of a lesson to gauge students’ existing knowledge of the subject matter. Coloured traffic light cards (red = false, yellow = don’t know, green = true) could be used to make this a whole class exercise.

Identifying wrongly-wired plugs Boardworks GCSE Additional Science: Physics Electrical Safety Identifying wrongly-wired plugs Teacher notes This activity could be used a plenary exercise to check students ability to identify faults in the wiring of plugs.

Materials and properties of plug parts Teacher notes This identifying activity could be used a plenary or revision exercise on the useful properties of the materials used to make the parts of a plug.

Plug Plug

Plug Plug

Electrical Safety - Fuses A fuse contains a thin wire that heats up and melts if too much current flows. The rating of a fuse is the maximum current that can pass through it without melting the fuse. A fuse must always have the correct rating. If the rating is too high the fuse will not blow when it should and the appliance could catch fire.

Fuses BY THE END OF THE LESSON: All of you will recall what a fuse is and what it does. Most of you will be able to interpret results from your experiment to describe how fuses work and differ. Some of you will be able to plan to carry out the experiment independently. Fuses

You choose which fuse to use! Teacher notes This multiple-choice activity could be used to assess students’ understanding of the use of fuses. Coloured traffic light voting cards (green=3A, yellow=5A, red=13A) could be used to increase class participation.

Electrical Safety – Circuit Breakers Circuit breakers do the same job as fuses but they are electromagnetic switches which are easy to reset. A house has several circuits and each one is protected by a separate circuit breaker.

Circuit Breakers switch electromagnet Each circuit breaker is an electromagnetic switch which is designed to break the circuit when the current gets too high. switch electromagnet Too much current makes the magnetic field produced by the electromagnet strong enough to open the switch. The circuit breaker is said to ‘trip’ and switches off the current.

What is an RCCD? http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/householdelecrev4.shtml BY THE END OF THE LESSON: All of you will recall what a fuse is and what it does. Most of you will be able to interpret results from your experiment to describe how fuses work and differ. Some of you will be able to plan to carry out the experiment independently.

Door Answer: Back Door

Grabs Grabs Grabs Grabs Answer: Up for grabs

Ci ii Answer: See eye to eye

Electrical Power The power of an appliance tells us how quickly it transforms electrical energy into other types of energy. Power(Watts) = Energy transferred (Joules) Time taken (secs) Or Power (Watts) = Current (amps) x Voltage (volts)

Power formula triangle? A formula triangle helps you to rearrange a formula. The formula triangle for P = IV is shown below. Cover up whatever quantity you are trying to find to leave the calculation required. …which gives the formula… So to find current (I), cover up I…   I = P V x

When doing calculations in exams REMEMBER 3 STEP RULE!!! Step 1: Write down formula (in symbols if you prefer) Step 2: Plug in the numbers Step 3: Find answer and add units EG: Find the power of 2A, 230V heater. 1: P = I x V 2: P = 2 x 230 3: P = 460W

Example Calculate the power of a 12V, 3A light bulb. b) How much energy does this bulb transfer if it is switched on for 1 hour? REMEMBER: 3 STEPS AND WATCH UNITS!!

Worked answers Power = Current x Voltage = 3A x 12V = 36W b) Energy = Power x Time (BUT: 1 hour = 3600 seconds) Energy = 36W x 3600s = 129,600J

Try these!

Calculating Charge and Potential Difference – Higher Tier only.

Calculating Charge. Current is the rate of flow of charge. The faster the charge flows, the higher the current Charge can be calculate using: Charge = Current x time Charge is measured in coulombs (C)

Electrical Energy & Potential difference (voltage). Energy is transferred in a circuit by the moving charges. The potential difference, is the amount of energy each charge carries Potential difference = Energy Charge (Joules) (Volts) (Coulombs)

Worked example – don’t forget 3 steps! Q1) Calculate the charge passing through a motor which has a current of 4A and runs for 15 seconds? Answer: Charge = Current x time Charge = 4A x 15s Charge = 60 C

Worked example contd… Q2) Calculate the energy transfer for a charge of 200C when the potential difference is 12V Answer: Energy = Potential difference x charge Energy = 12v x 200C Energy = 2400J

No Good No Good Answer: Up to no good

BROTHER Answer: Big Brother

Ac caught t Answer: Caught in the act

Quiz Quiz