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REVISION NOTES name cell battery lamp switch resistor variable resistor PRACTICAL ELECTRICITY symbol NATIONAL 4/5 notes provides electrical energy to the charges flowing around a circuit two or more cells connected in series converts electrical energy into light (+ heat) energy. It is an OUTPUT device breaks a circuit to turn it on or off reduces the flow of charge and so the current same as a resistor, but resistance can be changed voltmeter measures the voltage (potential difference) across part of a circuit ammeter measures the current in a circuit diode converts electrical energy to light energy. An OUTPUT device. Uses very low currents converts electrical energy to kinetic energy. An OUTPUT device converts electrical energy to sound energy. An INPUT device converts light energy to electrical energy. An INPUT device contains a thin wire which melts if the current gets too high. A safety device. a ‘valve’ which only allows charge to flow one way capacitor used to store electric charge thermistor a type of resistor. As the temperature goes up, its resistance goes down LDR a type of resistor. As the light level goes up, the resistance goes down relay a type of switch. Low voltage side powers electromagnet. This closes switch on high voltage side transistor (NPN) an electronic switch. Switches “on” at about 0.7 V MOSFET an electronic switch. Switches on at about 2.0 V LED motor loudspeaker photovoltaic (solar) cell fuse REVISION NOTES PRACTICAL ELECTRICITY NATIONAL 4/5 Series circuits (N4/5) A series circuit has only one ‘loop’. There is only one path for the current. In a series circuit, the current is the same at all points. In a series circuit, the voltages (potential differences) across the components in the circuit add up to the supply voltage. We can write this as: Vs = V1 + V2 Example Current is the same at all points – so current at Y = 0. 5 A. Vs = V1 + V2 so 6 V = 4.5 V + V2 so V2 = 1.5 V Parallel circuits (N5) A parallel circuit has more than one ‘loop’. The current can takes more than one path. In a parallel circuit, the currents in each branch add up to the current from the supply. We can write this as: Is = I1 + I2 The voltage across each branch is the same, and is equal to the supply voltage. Example Is = I1 + I2 so Is = 2.0 A + 3.0 A so Is = 5.0 A Voltage across each branch is the same, so Vs = 12.0 V. Using voltmeters and ammeters (N4/5) Ammeters are connected in series to measure the current in a circuit. Voltmeters are connected across a component to measure the potential difference (voltage) across that component. Resistors in series (N4/5) To find the resistance of several resistors in series, simply add them up. RTotal = R1 + R2 + R3 Resistors in parallel (N5) This is a little more complicated. 1 1 1 1 = + + +⋯ 𝑅𝑇 𝑅1 𝑅2 𝑅3 Example A 10 resistor and a 15 resistor are connected in parallel. 1 𝑅𝑇 So = 1 𝑅1 + 1 1 𝑅2 𝑅𝑇 = 1 10 + 1 1 15 𝑅𝑇 = 3 30 + 2 1 30 𝑅𝑇 = 5 𝑅𝑇 30 1 = 30 5 RT = 6 Voltage, current and resistance (N4/5) These three quantities are linked by Ohm’s Law: V=IxR Example A 20 resistor is connected to a 3 V battery. Calculate the current in the resistor. R = 20 V=3V I=? V=IxR 3 = I x 20 I = 3/20 = 0.15 A Special resistors (N4/5) The resistance of resistors is usually fixed. However, resistance can sometimes change depending on the conditions. For a light-dependant resistor (LDR): as the Light goes Up the Resistance goes Down (LURD) For a thermistor: as the Temperature goes Up the Resistance goes Down (TURD) The LED (light-emitting diode) (N5) Like diodes, LEDs only allow current to pass in one direction. They must be fitted the right way round in a circuit – ‘pointing’ towards the negative. LEDs use very little power and so are ideal for different types of lighting. LEDs can only handle small currents, so must be fitted with a resistor in series to reduce the current. Here is an LED, rated at 10 mA and 2 V, connected to a 6 V supply. 2 V across the LED means 4 V across the resistor (VS = V1 + V2) 10 mA (= 0.01 A) in LED means 0.01 A in the resistor (series circuit) So using Ohm’s Law for the resistor: V=IxR 4 = 0.01 x R so R = 400 Voltage dividers (N5) 12 V Two resistors in series will share the supply voltage. The resistor with the largest resistance will take the larger share. If R1 = R2 = 50 , the voltage across each resistor will be 6 V. If R1 = 20 and R2 = 40 , then R1 ‘gets’ 4 V and R2 ‘gets’ 8 V. (R2 ‘gets’ twice as much voltage because it’s twice the size of R1). If R1 = 5 k and R2 = 1 k, then R1 ‘gets’ 10 V and R2 ‘gets’ 2 V. (R1 ‘gets’ five times as much voltage because it’s five times the size of R1). When it’s difficult to spot the relationship, you can use: 𝑉2 = ( 𝑅2 ) 𝑥 𝑉𝑠 𝑅1 + 𝑅2 to find V2 and then 𝑉𝑠 = 𝑉1 + 𝑉2 to find V1. Transistors (N5) Transistors are electronic switches. They are switched ON and OFF by altering the voltage across them. There are two main types: NPN switches ON at 0.7 V MOSFET switches on at 2.0 V MOSFETs can handle larger currents than NPNs. Control circuits (N5) We can combine special resistors, voltage dividers and transistors to make control circuits. These circuits will respond to changes in light or temperature and switch on an output device like an LED or a motor. This circuit switches on the LED if the temperature gets too high. As the temperature rises, the resistance of the thermistor falls. (TURD) So the voltage across the thermistor falls. This means the voltage across the variable resistor goes up (as the two resistors share the supply voltage). When the voltage across the variable resistor rises above 0.7 V, the NPN transistor switches ON and the LED comes on. By adjusting the variable resistor, we can adjust the temperature at which the LED comes on. By switching the position of the two resistors, we can make a circuit that turns on the LED if the temperature gets too low. Control circuits, capacitors and timing (N5) Capacitors take time to charge. As a capacitor charges the voltage across it increases. So in this example, as time passes, the capacitor gradually charges. The voltage across it gradually rises. When the voltage reaches 0.7 V, the NPN transistor switches on and the LED lights. We have built a time delay into the circuit.