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Transcript

Current, Voltage and resistance Objective: to recap GCSE circuits To know the circuit laws for current, voltage and resistance. Starter: table to fill out Current The net flow of charged particles In a metal this is due to electrons, when the circuit is not complete then the electrons just drift around and there is no net flow. Current The net flow of charged particles When a potential difference is applied the electrons start to drift in the positive to neagtive direction. This causes a net movement not a rapid flow. Annoyingly conventional current is positive to negative. Representing current direction Current Another way of thinking of current is the rate of flow of charge. Or π πΌ= π‘ πβππππ πΆπ’πππππ‘ = π‘πππ Units of current = Amps (A) Units of charge = coulombs (C) Unit of time = seconds (s) How does current behave in series and parallel circuits? Set up some circuits and fill out the sheet (15mins) Current is the same everywhere in a series circuit Current splits up at each junction/branch in a parallel circuit Voltage Voltage, also known as potential difference, is a measure of the energy provided to the charge carriers. It can be defined as the amount of work done per unit charge. V (V) = W (J) Q (C) Voltage is measured as a difference in potential between two points. Thus a voltmeter must be connected in parallel and used to measure the difference in potential across a device. If a cell supplies 23 coulombs of charge with 776 J of energy, what is its voltage? V= 776 23 = 33.7 V How does voltage(P.D) behave in series and parallel circuits? Set up the circuits and record your answers on the sheet Voltage (P.D) is divided amongst the components in a series circuit Voltage (P.D) is the same across each branch in a parallel circuit Resistance Resistance is a measure of the opposition a material exerts against the flow of electrons. The resistance of a material can be calculated from the current and voltage passing through it. R (Ξ©) = V (V) I (A) Calculate the resistance in this simple circuit. 9.7 V 3.2 A R= 9.7 3.2 = 3.0 Ξ© How does resistance behave in series and parallel circuits? R2 R1 Equivalent resistance R3 R4 R5 R6 RT R7 When designing or analysing circuits, complex combinations of resistors are common. To perform calculations, for example to find a suitable fuse to protect the circuit, it is easier to use a value for the total resistance of the circuit, RT. RT can be called the equivalent resistance because it is the single resistor that is equivalent to the complex combination. Resistors in series To work out the equivalent resistance of resistors in series, the resistor values can just be added together: 10 Ξ© 20 Ξ© equivalent resistance = 10 + 20 + 15 = 45 Ξ© In general for a number of resistors, n: RT = R1 + R2 + β¦ + Rn 15 Ξ© Resistors in parallel To work out the equivalent resistance for resistors in parallel, a more complex equation must be applied: 1 RT = 1 R1 + 1 R2 + β¦ + 1 Rn For example: 1 RT 10 Ξ© 1 RT 20 Ξ© 15 Ξ© 1 = = 10 + 1 20 + 6+3+4 60 = RT RT 1 13 60 = 60 13 Ξ© = 4.62 Ξ© 1 15 Resistor combinations