INTRODUCTION TO CIRCUITS NOTES
WHAT IS A CIRCUIT? For electricity to flow from a battery to light up a light bulb, there must be a complete path from the positive terminal on top of the battery to the negative terminal on the bottom of the battery. The word circuit comes from the same root as the word circle you can think of it as a circular loop. A complete path is called a closed circuit. Electrons are flowing. If there is a gap or hole or opening in the path, it is called an open circuit and the light bulb will not light up. Electrons are stationary.
BASIC ELECTRIC CIRCUIT Circuit when there is a continuous and conducting path between the positive and negative terminals of a battery; through which charge can move or flow. Electrons are pushed away from the negative terminal of the battery and pulled towards the positive terminal. The battery provides the force (we call it Voltage) which moves the charges (we call this Current).
ELECTRIC CURRENT Current describes the amount of charge (number of electrons) flowing in a circuit each second. Symbol: I Unit: Coulombs per second or Amperes or Amps 1 C/s=1 A I = Q t Current = Charge time
ACTUAL CURRENT VS. CONVENTIONAL CURRENT Actual current is electrons (-) flowing from the to the + of a battery. Conventional current (the one we will use) describes the movement of + charges from the + to the of a battery.
VOLTAGE Without a voltage source, current will not flow in a circuit. Another name for voltage is potential difference it is when two sides of a circuit have different amounts of potential (like + and -). Voltage the change in electric potential energy per Coulomb of charge. Symbol: V Unit: Joules per Coulomb or Volts 1 Volt = 1 Joule/Coulomb
VOLTAGE CONTINUED Increasing the voltage in a circuit causes the current to increase. A battery doesn t provide charge, it provides energy to the charges already there. A typical battery (AA, AAA, D) provides 1.5 V In the U.S., household voltages are usually between 110 V and 120 V
ELECTRICAL RESISTANCE In almost all circuits, the electrons flow with some opposition or resistance. Resistance- opposition to flow of charge. You can think of electrical resistance as similar to friction. Symbol: R Unit: Ohms (Ω the Greek letter omega) Conductors such as metals have very low resistance in general.
RESISTANCE MATHEMATICALLY DEFINED Mathematically, resistance in a circuit is defined as the amount of voltage needed to produce one amp of current (volts per amp) Resistance = Voltage Current R = V I Increasing resistance causes current to decrease Resistance is determined by wire length, thickness, and material Current in a circuit is determined by voltage and resistance: I = V R
EXAMPLE 1: A SMALL HEATER HAS A RESISTANCE OF 200 Ω. WHEN THIS HEATER IS PLUGGED INTO A 120 V OUTLET, HOW MUCH CURRENT FLOWS THROUGH THE HEATER?
EXAMPLE 1 SOLUTION R=200 Ohms V=120 V I=? I = V R = 120V 200Ω = 0.6 A
EXAMPLE 2: LIGHT BULB B HAS 4 TIMES THE RESISTANCE OF LIGHT BULB A, HOW WILL THE CURRENTS THROUGH THE TWO BULBS COMPARE IF THEY ARE CONNECTED TO THE SAME BATTERY?
EXAMPLE 2 SOLUTION V B =V A (same battery) R B =4R A I A = V A R A I B = V A = I A 4R A 4
EXAMPLE 3: A FLASHLIGHT USING 2 1.5 V BATTERIES HAS 0.1 A OF CURRENT WHEN IT IS TURNED ON. HOW MUCH CURRENT WOULD FLOW IF YOU USED 3 BATTERIES INSTEAD OF 2?
EXAMPLE 3 SOLUTION I = V R Using 3 batteries instead of 2 would increase voltage by a factor of 1.5. I = 1. 5V R = 1. 5 0. 1A = 0. 15 A
ENERGY IN A CIRCUIT A battery gives energy to electrons, as they move through a device, they give their energy up to another form (light, heat, etc.)
ELECTRIC POWER Rate of energy transfer in a circuit Symbol: P Unit: Watts (W) = Joules/second Equation: Power = Current Voltage P = IV
EXAMPLE 4: WHAT IS THE CURRENT IN A 60 WATT LIGHT BULB WHEN IT IS PLUGGED IN TO A 120 V OUTLET?
EXAMPLE 4 SOLUTION P=60 W V=120 V I=? I = P V P = IV = 60 W 120 V = 0.5 A
EXAMPLE 6: THE RESISTANCES OF SEVERAL DEVICES ARE GIVEN BELOW. WHICH WILL USE THE MOST POWER? HAIR DRYER - 12Ω LED 1440Ω
EXAMPLE 6 SOLUTION Assume each device is plugged into the same outlet (voltage doesn t change). Increasing resistance decreases current I = V R P=IV, so if voltage stays the same, but current decreases power also decreases. The hair dryer uses the most power since it has the smallest resistance.
CIRCUITS ANALOGY #1 ROCKS ROLLING DOWN A HILL Imagine a rock laying on level ground. It will not move anywhere (no current) since there is no difference in elevation (voltage). However, an object at the top of a hill could roll down (current flowing) because of the elevation difference (voltage). If there were bushes (resistance) on the hill, fewer rocks would make it to the bottom each second (current)
CIRCUITS ANALOGY #2 WATER FLOWING THROUGH A PIPE OR HOSE The pump provides a pressure difference (voltage) and causes water to flow (current) through the pipes. If there are a lot of kinks in the hose (resistance), not as much water gets through (current).