Electricity and Circuits 28/07/2019 Electricity and Circuits 2016 EdExcel GCSE Physics Topic 10 - + W Richards The Weald School

The structure of the atom recap 28/07/2019 28/07/2019 ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”)

Circuit Symbols Variable resistor + - + - Diode Switch Bulb A V 28/07/2019 28/07/2019 Variable resistor + - + - Diode Switch Bulb A V Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

Circuit building practice 28/07/2019 1. 2. 3. 4. A 5. V

Energy and charge 28/07/2019 28/07/2019 The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around: Energy transferred = charge x voltage (in J) (in C) (in V) E Q V By definition, 1V = 1 Joule per coulomb

Example questions 28/07/2019 28/07/2019 In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred? In another circuit a voltage of 10V is applied and 200C of charge flows around the circuit. How much energy is transferred? 48J of electrical work is done by a 6V battery. How much charge flowed around the circuit? Another battery transfers 20C of charge and does 100J of electrical work. What was the battery’s voltage? 600J 2000J 8C 5V

Electric Current 28/07/2019 28/07/2019 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” and it has the same value at any point in this circuit e-

Charge (Q) 28/07/2019 28/07/2019 As we said, electricity is when electrons move around a circuit and carry energy with them. Each electron has a negative CHARGE. Charge is measured in Coulombs (C). We can work out how much charge flows in a circuit using the equation: Q T I Charge = current x time (in C) (in A) (in s)

Example questions Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50 28/07/2019 28/07/2019 Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50 250 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

Example questions Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 28/07/2019 28/07/2019 Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5 40 50 0.2 250 180 3 60 A circuit is switched on for 30s with a current of 3A. How much charge flowed? During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed? 90C 200s 300C

Example questions on two equations 28/07/2019 28/07/2019 In a radio circuit a voltage of 12V is applied and a charge of 100C flows. How much energy has been transferred? In the above circuit the radio drew a current of 0.5A. How long was it on for? A motor operates at 6V and draws a current of 3A. The motor is used for 5 minutes. Calculate: a) the charge flowing through it, b) the energy supplied to it A lamp is attached to a 12V circuit and a charge of 1200C flows through it. If the lamp is on for 10 minutes calculate a) the current, b) the energy supplied to the bulb. 1200J 200s 900C, 5400J 2A, 14,400J

What happens to Current in a series circuit? 28/07/2019 28/07/2019 What happens to Current in a series circuit? The current here will be… If the current here is 2 amps… ?A ?A And the current here will be… ?A In other words, the current in a series circuit is THE SAME at any point

What happens to Current in a parallel circuit? 28/07/2019 28/07/2019 What happens to Current in a parallel circuit? A PARALLEL circuit is one where the current has a “choice of routes” ?A 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… In other words, the current in a parallel circuit SPLITS UP down each “strand”

Current in a parallel circuit 28/07/2019 28/07/2019 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… 28/07/2019 28/07/2019 3A ?A 6A 4A 2A 1A each

Voltage in a series circuit 28/07/2019 28/07/2019 If the voltage across the battery is 4.5V… V …and these bulbs are all identical… V V …what will the voltage across each bulb be? 1.5V In other words, voltage in a series circuit “SPLITS UP” over each component

Voltage in a series circuit 28/07/2019 28/07/2019 If the voltage across the battery is 4.5V… V V …what will the voltage across two bulbs be? 1.5V

Voltage in a parallel circuit 28/07/2019 28/07/2019 If the voltage across the batteries is 4V… What is the voltage here? 4V V And here? 4V V In other words, the voltage in a parallel circuit STAYS THE SAME across each “strand”

Summary Current is THE SAME at any point 28/07/2019 28/07/2019 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 28/07/2019 28/07/2019 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V

An example question: 6A 9V A1 6A 9V A2 4A V1 2A A3 V2 V3 4.5V 4.5V 28/07/2019 28/07/2019 6A 9V A1 6A 9V A2 4A V1 2A A3 V2 V3 4.5V 4.5V

Another example question: 28/07/2019 28/07/2019 3A 10V A1 3A 1.2A A2 V1 6.7V A3 1.8A V2 V3 5V 5V

Resistance V R I Resistance = Voltage (in V) (in ) Current (in A) Georg Simon Ohm 1789-1854 28/07/2019 28/07/2019 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)

Resistance Resistance is anything that opposes an electric current. 28/07/2019 28/07/2019 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

An example question: Ammeter reads 2A A 28/07/2019 28/07/2019 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? Example Questions 28/07/2019 28/07/2019 3A 6V 12V 4V 2A 1A 2V 3A What is the resistance of these bulbs?

Varying Resistance 28/07/2019 28/07/2019 What happens if you increase the amount of resistance in a circuit? The same effect is seen when using a variable resistor: Increase the resistance:

Using Variable Resistors in circuits 28/07/2019 V A What will happen to the reading on the ammeter if the resistance of the variable resistor is increased? How will this affect the brightness of the bulb? What will happen to the reading on the voltmeter?

Resistors in Series and Parallel 28/07/2019 3A 6V 12V 3A In SERIES, the total resistance is given by the sum of the individual resistances: RTotal = R1 + R2 What is the total resistance in this circuit? In PARALLEL the total resistance is less than the resistance of the individual resistors as the current has more “routes” to flow through. What is the highest value the resistance of this circuit could be?

Current-Voltage graphs 28/07/2019 28/07/2019 Voltage on powerpack/V Current/A Voltage/V 12 10 8 6 4 2 -2 -4 -6 -8 -10 -12

Current-Voltage graphs 28/07/2019 V A Method: Connect up the circuit as shown with a powerpack voltage of 2V Measure the voltage across the bulb and the current through the bulb Increase the voltage to 4V and repeat the measurements Repeat up to 12V Repeat with “negative” voltages from the powerpack

Current-voltage graphs 28/07/2019 28/07/2019 I V I V I V 3. Diode 1. Resistor 2. Bulb A diode only lets current go in one _______ – it has very _____ resistance in the other direction Current increases in proportion to _______, provided the temperature doesn’t change As voltage increases the bulb gets ______ and _______ increases Words – resistance, high, voltage, hotter, direction

LDRs and Thermistors 28/07/2019 28/07/2019

Two simple components: 28/07/2019 28/07/2019 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES 2) Thermistor – resistance DECREASES when temperature INCREASES Resistance Amount of light Resistance Temperature

Using LDRs in circuits A 28/07/2019 V A Here’s a circuit that has been left in a room. Will the lamp come on when the room is light or dark? Explain your answer.

Understanding Resistance 28/07/2019 What happens inside a conducting material? The following model of a metal wire could help: Electrons Ions At normal temperatures, with no current flowing, electrons hurtle around continuously. What happens if you connect a battery?

Using a Battery Apply a voltage: Electrons Ions Negative Positive 28/07/2019 Apply a voltage: Electrons Ions Negative Positive This time we can see that the electrons are accelerated from negative to positive. This is an electric current and happens because the voltage “pushes” the electrons. The collisions between the electrons and the ions cause the wire to heat up as the current is doing work on the wire and this heat energy is dissipated to the surroundings.

Using this heating effect 28/07/2019 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

Energy and Power 28/07/2019 The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second Energy transferred (J) = power (W) x time (s) OR Energy (J) = current (A) x voltage (V) x time (s) E = I x V x t

Some example questions 28/07/2019 A battery gives out a current of 0.2A and has a voltage of 1.5V. If it is used for 30 seconds how much energy has it transferred? An electric fire runs at a voltage of 230V and a current of 8A. If it is left on for 2 hours how much electrical energy has it transferred? A toaster transfers 20,000J of electrical energy. If it runs at a voltage of 230V and a current of 2A how long was it on for? A light bulb is left on overnight for 8 hours. If it transfers 1,000,000J of energy and runs on a voltage of 230V what current did it draw? 9J 13.2MJ 43.5s 0.15A

Electrical Power revision 28/07/2019 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? 2300W 2A 0.26A

Another Equation for Power 28/07/2019 Here’s another way of calculating power – use this one if you’re given a value for resistance: Power = I2R A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor. A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor. A 2KW heater has a resistance of 20Ω. Calculate the current through it. 40W 220MW 10A

Recap: The Conservation of Energy 28/07/2019 There are many laws of physics, but one of the most important ones is: Energy cannot be created or destroyed, it can only be converted from one form to another What are the energy changes for: An electric light bulb? An electric motor? A battery?

DC and AC 28/07/2019 28/07/2019 V DC stands for “Direct Current” – the current only flows in one direction. Batteries supply DC. Time 1/50th s AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz). Household electricity is AC 230V T V

Wiring a plug 1. Earth wire 4. Live wire 5. Fuse 2. Neutral wire 6. 28/07/2019 28/07/2019 1. 2. 3. 4. 5. 6. Earth wire Live wire Fuse Neutral wire Cable grip Insulation The live wire of a plug alternates between positive and negative potential relative to the Earth The neutral wire of a plug stays at a potential close to zero and is used to complete the circuit

What differences can you spot and can you explain them? Two different plugs 28/07/2019 From a 100W bench lamp From a 2000W kettle What differences can you spot and can you explain them?

Words – large, live, melt, live, circuit, fuse, current Fuses 28/07/2019 28/07/2019 Fuses are connected to the _____ wire as this wire carries the ______. If there is a fault in an appliance which causes the ____ and neutral (or earth) wire to cross then a ______ current will flow through the _____ and cause it to _____. This will break the _______. This is a 3A fuse – what would be different if it was a 13A fuse? Words – large, live, melt, live, circuit, fuse, current

Words – fuse, fault, metal, surges, touch Earth wires 28/07/2019 28/07/2019 Earth wires are always used if an appliance has a _____ case. If there is a _____ in the appliance, causing the live wire to ______ the case, the current “_______” down the earth wire and the ______ melts. Some appliances do not need earth wires as they are “double insulated” (i.e. have a plastic case). Words – fuse, fault, metal, surges, touch

Words – circuit, safer, reset, repairs, quickly Circuit breakers 28/07/2019 Residual Current Circuit Breakers (RCCBs) are often used with fuses and are also connected to the live wire. They have some advantages over fuses: They are _____ – they don’t get hot They react more _______ They can be switched off for _____ They are easy to _____ Each RCCB is attached to a certain circuit, so if one switches off you can see which ______ has a fault Words – circuit, safer, reset, repairs, quickly

Potential Differences between wires 28/07/2019 Here's a plug again. What would a voltmeter read if you connected it across these two wires? The live and neutral wire The live and earth wire The neutral and earth wire 230V 230V 0V Any connection between the live wire and the earth wire will encourage a large current to flow, which could cause electric shock or a large heating effect.

Which devices transfer the most power and why? Recap of Power 28/07/2019 28/07/2019 Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster 230 4 Fire 8 Hoover 460 2 Hairdryer 1150 Computer 100 Stereo 80 Which devices transfer the most power and why?

Example Questions Complete the following table: 28/07/2019 28/07/2019 Complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Toaster 920 230 4 Fire 1840 8 Hoover 460 2 Hairdryer 1150 5 Computer 100 0.43 Stereo 80 0.35 Devices that use heating are the most powerful. What is the change in stores of energy for these devices?