SP10 Electric Circuits Revision Support and Guidance Contents: 1) RAG list of SP10 topics. 2) Basic outline of each topic (SP10a-i.) 3) References to other revision resources 4) Practice questions, and critical diagrams.

RAG SP10a-d

RAG SP10 e-i

SP10a Electric Circuits (1) Electric Current Metal atoms have weakly attached outer (or free) electrons which can flow from one atom to the next (current). The channels in metallic structure facilitate this electron flow. Electrons are negatively charged and are ‘pushed’ to the + terminal in a circuit by the voltage (potential difference in charge) - + See: BBC Bitesize – GCSE Physics (Single Science) on current, voltage and resistance (esp. pg2) + lots of short films on current and voltage on You Tube.

SP10b Current and Potential Difference in Series and Parallel Circuits You need to know how current and voltage behave in a series and a parallel circuit See: You Tube: GCSE Physics Revision Current in Parallel Circuits . 2) GCSE Bitesize.

SP10c Current, Charge and Energy Electric charge (Q) measures how many negative electrons flow past a point in a circuit. Charge is measured in coulombs (C). 1C = 1 amp of current flowing for 1 second. Charge = current x time or Q=I.t (C) = (A) x (s) Energy Transferred = charge x potential difference E=Q.V (You must know these equation in triangle form) See: BBC Bitesize – GCSE Physics (Single Science) on current, voltage and resistance (esp. pg1) + lots of short films on current and voltage on You Tube like Doodle Science

Charge (C) = Current (A) x Time (s) or Q = Ixt If you can measure current using an ammeter, and time how long it is flowing, then you can calculate the charge delivered. Example Questions: A current of 2.5 A is switched on for 9 s. How much charge flows ? Current in a lamp is 0.5 A. How long will it take for 10 C of charge to flow through a lamp ? Q Charge I t Current time

Energy (J) = Charge (C) x Potential Difference (V) or E =QxV The potential difference across a lamp is 1.5V. When the circuit is switched on, 600J of energy is transferred in the lamp. How much charge flowed through the lamp ?

Extension Questions (L6): 1: Calculate how much energy is transferred when 8C of charge flows through a potential difference of 3V ? 2: 150J of energy is transferred when 50C of charge flows through a wire. Calculate the potential difference across the wire.

SP10d Resistance Resistance is how much a wire or component slows down the current. It is measured in ohms (Ω) and calculated from the equation : Resistance = Voltage / Current R=V/I (Must know this !) Resistors control the amount of current in electric circuits. See: BBC Bitesize – GCSE Physics (Single Science) on current, voltage and resistance + lots of short films on resistance on You Tube.

Resistance Calculations (R = V/I) Series Circuits 1: What is the total resistance to the current = (Ω) (Ω) 2: What is the current in this circuit ? = A 12V

Resistance Calculations (R = V/I) Parallel Circuits ? Ω A3 (Ω) (Ω) A2 A1 = 1.2 A 6.4V

Resistance Calculations (R = V/I) Parallel Circuits ? Ω A3 1: What is the current at A2 ? = (Ω) (Ω) 2: What is the current at A3 ? = A2 A1 = 1.2 A 3: What is the value of the top resistor ? 6.4V

A 6V drop What size resistor will you put in ? Ammeter reading = 2.4 A. This will blow the LED’s which can take a maximum current = 0.04 A. What size resistor will you put in ? A

SP10e Resistors Fixed resistors, filament lamps, Light Dependent Resistors and thermistors – how do these types of resistor effect current and what are their applications? Make sure you can interpret and explain the graphs for how they effect the current. See: BBC Bitesize – GCSE Physics (Single Science) on resistance + lots of short films on current and voltage on You Tube like Basic Electricity – Resistance and Ohms Law

What is an LDR ? An LDR is a specialised switch ,ie, it can control the current depending on light intensity. The brighter or dimmer the light the lower the resistance and the greater the current.

Thermistor A thermistor is a resistor sensitive to temperature. Its resistance decreases as the temperature increases. Uses, fan in computer.

Resistance in a filament lamp -  + + -

SP10f Transferring Energy When current encounters resistance, heat is produced OR energy is transferred/transformed (electrical energy  heat energy). You have calculated the energy transferred in the water heaters and toasters you have made. Must know equation: Energy transferred (J) = Current (A) x pot. diff. (V) x time (s)

Blast that water: 100 ml water – how hot can it go Blast that water: 100 ml water – how hot can it go ? = how much energy can be transferred ? Variables to get the water hot: Calculating energy transferred: Voltage used = …6.7…. V Current used = …3.8….. A Heating time = …390…. s

Modelling Power Lines Investigation Lamp Brightness ( /10) Voltage Readings (V) Current Readings (A) Resistance Values (Ω) a = 8 V1 = 9 A1 = 0.3 Close to Power Pack = 30 b = 5 V2 = 6.7 A2 = 0. 14 Far from Power Pack =47.85 1 2 A A V V 6-7V a b 1 2

Energy Calculations: E = V x I x t 1) Calculate energy transferred when a TV using 230 V and a 0.9 A current is switched on for 1 minute. 2)The current in a lamp is 0.4 A when connected to a 230 V supply. How long does it take to transfer 2000 J of energy. Energy Voltage Current Time

Energy Calculations: E = V x I x t 3) 900 J of energy is transferred when there is a 0.5 A current in a circuit for 20 minutes. Calculate the potential difference or voltage across the circuit. Energy Voltage Current Time

SP10g Power Power is the amount of electrical energy transferred per second (Watts , W). A washing machine working hard to spin dry heavy wet washing uses (transfers) more electrical energy per second than an electric tooth brush. Electrical power (W) = Current (A) x Pot. Diff. (V) (Must know !)

Which Electrical Appliance is the most powerful ? Power Drill Milk Whisk P = VI V = 3V I = 0.5 A P = …………. P = ……W P V I Electrical Power = Voltage x Current P=VI P= 230V x 2A …. P=………W

Calculating Electrical Power P=IV and P=I2R Example Question: An electric cable has a resistance of 900Ω and a current of 3A. Calculate the power transferred in kW.

Exam Style Question (L6+) A 46W electric blanket has a resistance of 1150Ω. Calculate the current used in A. (tip: use the√ function)

SP10h Transferring Energy by Electricity DC (Direct Current) is supplied by cells and batteries. It is a constant, 1 directional flow of electrons. AC (Alternating Current) is produced by AC generators in power stations. The current ‘pulses’ back and forth at a frequency of 50 times per second (Hz) and has a potential difference of 230 V (mains voltage). See: GCSE Bitesize-Generating Electricity + animations of AC generators on You Tube.

AC and DC on Oscilloscope (Alternating Current and Direct Current) Current flowing - to + DC flowing in one direction AC trace on the oscilloscope: Voltage constantly changing Current changes direction Mains supply at frequency of 50Hz. Y-axis can show current or voltage +2 +1 0 A 1 Period -1 -2 Current flowing + to - 0.25 0.5 0.75 1.0 1.25 1.5 1.75 2.0 Time (s)

SP10i Electrical Safety What do the live, neutral, and earth wires do and what voltages do they carry ? The fuse is a safety component but how does it work and how do you select the correct size (3A, 5A, 13A)

Fuse Prevents You Turning into Toast Fuse Prevents You Turning into Toast ! How can the giant mouse make the metal outer case live and how would the fuse protect you ? 3 Pin Plug E N L Fuse

Calculating Power Ratings and Fuses for Domestic Appliances Fuse Size Appliance Voltage (V) Current (A) Power (W) Washing Machine 230 2980 Dishwasher 8 Hairdrier 0.8 Kettle 1840 Light Bulb 0.26 Microwave 3.69 13A To convert W to kW, divide by 1000 or move . 3 places to left