Transcript Slide 1
cell ☺ ☺ ☺ energy ☺ electron ☺ ☺ ☺ ☺ lamp ☺ ☺ ☺ Coulomb of charge (electrons) Think of it as a “bag of electrons” (containing 6000000000000000000 electrons!) ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺☺ ☺ ☺ ☺ ☺ ☺☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ ☺ Current ☺ ☺ ☺ ☺ ☺ ☺ The rate of flow of electric charge (number of Coulombs flowing past a point in the circuit every second). I = ΔQ/Δt ☺ ☺ I’m counting how many coulombs of electrons go past me every second ☺ A ☺ ☺ 1 Amp = 1 coulomb per second In a series circuit Current is the same at any point in the circuit 2.5 A 2.5 A 2.5 A 2.5 A In a parallel circuit The current splits (total current stays the same) 2.5 A 2.5 A 1.25 A 1.25 A Voltage(emf) ☺ ☺ ☺ V ☺ ☺ ☺ I’m checking the difference in energy (per coulomb) between the 2 red arrows ☺ ☺ ☺ ☺ ☺ 1 Volt = 1 Joule per coulomb Voltage (p.d.) ☺ ☺ I’m checking the difference in energy (per coulomb) before and after the lamp ☺ ☺ ☺ ☺ V ☺ ☺ ☺ ☺ ☺ 1 Volt = 1 Joule per coulomb p.d. and e.m.f Electric potential difference between two points is the work done per unit charge to move a small positive charge between two points. Electromotive force is the total energy difference per unit charge around the circuit (it is the potential difference when no current flows in a circuit). In a series circuit The sum of the p.d.s across the lamps equals the emf across the cells 9V 3V 3V 3V In a parallel circuit In a simple parallel circuit, p.d. across each lamp equals the e.m.f. across the cells 5V 5V 5V Resistance Measures how difficult it is for current to flow. Measured in Ohms (Ω) V Resistance = voltage/current A R = V/I Ohm’s Law • V = IR V I X R Resistance • R is proportional to the length of wire – WHY? RαL • R is inversely proportional to the cross sectional area of wire – WHY? R α 1/A • R depends on the type of material – WHY? Resistivity R = ρL A where R = resistance in Ohms L = Length of conductor in metres A = cross sectional area of conductor in m2 ρ = resistivity of the material in Ohms.meters Example The resistivity of copper is 1.7 x 10-8 Ωm. What is the resistance of a piece of copper wire 1 m in length with a diameter of 0.1mm? radius = 0.05mm = 5 x 10-5m cross sectional area = πr2 = 3.14x(5 x 10-5)2 = 7 x 10-9 m2 R = ρL/A = (1.7 x 10-8 x 1)/ 7 x 10-9 = 2.42 Ω Resistance of a lamp Vary the voltage and current using a variable resistor (rheostat). Plot a graph of resistance against current V Resistance = voltage/current A R = V/I Resistance of a lamp • As the current in a lamp increases, its resistance increases. Why? Ohmic behaviour • p.d. is proportional to the current Metal wires at constant temperature Non-Ohmic behaviour • p.d. is not proportional to the current Power The amount of energy used by a device per second, measured in Watts (Joules per second) A V Power = voltage x current P = VI Power dissipated in a resistor/lamp • P = VI • From Ohm’s law, V = IR • So P = VI = I2R • From Ohm’s law also, I = V/R • So P = VI = V2/R Total energy So the total energy transformed by a lamp is the power (J/s) times the time the lamp is on for in seconds, E = VIt E = energy transformed (J) V = Voltage (also called p.d.) I = current (A) t = time (s) Electronvolt • Electronvolt – the energy gained by an electron when it moves through a potential difference of one volt. Internal resistance • Connecting a voltmeter (VERY high resistance) across the terminals of a cell measures the EMF of the cell (no current flowing) V Internal resistance • We have assumed so far that the power source has no resistance…….not a good assumption! Internal resistance • In actuality the p.d. across a cell is less than the EMF due to energy lost in the INTERNAL RESISTANCE Internal resistance • To help us visualize this, a cell is represented as a “perfect” cell attached in series to the internal resistance, given the symbol r. Internal resistance • The p.d. across a cell (V) is then equal to the EMF (ε)minus the voltage lost across the internal resistance (=Ir) V = ε - Ir Example • A cell of emf 12V and internal resistance 1.5 Ω produces a current of 3A. What is the p.d. across the cell terminals? • V = ε - Ir • V = 12 – 3x1.5 • V = 7.5 V Adding resistances In series and parallel Ideal meters • Voltmeters – infinite resistance! • Ammeters – Zero resistance! The potential divider • The potential divider is at its simplest two series resistors connected to a battery and the voltage is measured between the negative terminal and a point between the two resistors. • However if we use battery the principle will be the same but formula will be different Light depended resistor (LDR) • A Light Dependent Resistor is a device which has a resistance which varies according to the amount of light falling on its surface. • High resistance in the dark but a low resistance in light Light depended resistor (LDR) Suppose the LDR has a resistance of 500 , 0.5 , in bright light, and 200 in the shade When the LDR is in the light, Vout will be: In the shade, Vout will be: