Transcript unit3-1
Notes p.1 Electricity Revision Current = charge transferred per second I = Q/t Potential Difference = difference in energy per coulomb of charge between 2 points in a circuit. Symbols Quantities – Symbols – Units – Unit Symbols Circuit Rules Series Circuits Parallel Circuits Current & Voltage Current and Voltage Current and Voltage Current and Voltage + - How many paths from – to + ? Only 1 … so what type of circuit is it? A series circuit! Voltage in Series Vs + - V1 V2 •Voltage is a measure of the energy carried by the charges in a circuit. •Voltage is measured in Volts (V) •We use a voltmeter to measure voltage. V Voltage in Series •1 lead to COM, 1 lead to V W. Dial to 20 V. •DO NOT break the circuit. •Connect voltmeter “on top of” components. Vs Eg) V1 V2 Current in Series A + A A A •Current is a measure of how quickly the charges move around the circuit. •It is measured in Amperes (A)…”amps” •We use an ammeter to measure current. A More Revision of Circuits (not in notes) 12 V + - P (0J) X (0J) 1. 200 W A2 100 W Q (8J) 100 W A1 R (12J) Y (12J) Supply Voltage (12V) is a measure of how much “electrical potential” energy each coulomb of charge gains as it passes through the supply. In the circuit above each Coulomb gains 12 J of energy. i.e. 12 V = 12 Joules per Coulomb (J/C) 1. Supply Voltage (12V) is a measure of how much “electrical potential” energy each coulomb of charge gains as it passes through the supply. In the circuit above each Coulomb gains 12 J of energy. i.e. 12 V = 12 Joules per Coulomb (J/C) 2. Voltage (or potential difference) across a component: This tells us how much energy each coulomb “loses” in the circuit’s components. (This energy is not really “lost”, it is transformed e.g. to heat in the resistor! e.g. to light and heat in a bulb! In the circuit • At R – each coulomb has 12 J or energy. • At Y – each coulomb has 12 J or energy. • At Q – each coulomb has 8 J or energy (as 4 J were used in the 100 W resistor, 8 J used in the 200 W resistor). • At P – each coulomb has 0 J or energy. • At X – each coulomb has 0 J or energy. Then • voltage across QR = p.d. across QR = 4 V (or 4 J/C) • voltage across QP = p.d. across QP = 8 V (or 8 J/C) • voltage across XY = p.d. across XY= 12 V (or 12 J/C) Then •voltage across QR = p.d. across QR = 4 V (or 4 J/C) •voltage across QP = p.d. across QP = 8 V (or 8 J/C) •voltage across XY = p.d. across XY= 12 V (or 12 J/C) 3. Current is a measure of the amount of charge, Q, measured in Coulombs, which passes a point per second. Reminders I = Q t I = V R NB ALWAYS CONSIDER WHICH RESISTORS AND VOLTAGES (p.d.’s) AFFECT THE CURRENT AT ANY POINT FOR CALCULATIONS. e.g. At A1 – supply voltage + total resistance of the circuit. 1/RT = 1/R1 + 1/R2 =1 + 1 100 300 RT = 75 W RT = 75 W VT = 12 V I =? I=V R = 12 75 = 0.16 A e.g. at A2 – Voltage across PR + resistance at PR need to be used. I=V R RT = 300 W = 12 VT = 12 V 300 I =? = 0.04 A Problems on CIRCUITS REVISION A - D. Revision of Circuits Use the Standard Grade Physics books to remind yourself of how to find a) resistance in parallel. 1/RT = 1/R1 + 1/R2 b) voltage across a voltage divider circuit. V1 = R1 x VS (R1 + R2) V1 = V2 c) the value of the resistor in series with an LED. VR = VS – VLED THEN……… R = VR I Problems 5 – 20on page 55 of the Problem booklet. R1 R2