Electric Circuits
The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.
What is an electric current? An electric current is a flow of microscopic particles called electrons flowing through wires and components. - + In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell.
simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. wires cell lamp switch To make the circuit, these components are connected together with metal connecting wires.
Electrical Circuit Symbols Electrical circuits often contain one or more resistors grouped together and attached to an energy source, such as a battery. The following symbols are often used: + - + - - + - + - Ground Battery - + Resistor
circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires
circuit diagrams In circuit diagrams components are represented by the following symbols; switch Cell Battery Bulbs ammeter voltmeter motor resistors
Laboratory Circuit Symbols Rheostat Emf - + V Source of EMF Voltmeter Ammeter Rheostat
Electromotive Force A source of electromotive force (emf) is a device that uses chemical, mechanical or other energy to provide the potential difference necessary for electric current. Power lines Battery Wind generator
Water Analogy to EMF Low pressure Pump Water High pressure Valve Water Flow Constriction Source of EMF Resistor High potential Low potential Switch R I + - The source of emf (pump) provides the voltage (pressure) to force electrons (water) through electric resistance (narrow constriction).
Electric Current Electric current [I] is the rate of the flow of charge Q through a cross-section A in a unit of time t. A + - Wire +Q t One ampere A is charge flowing at the rate of one coulomb per second.
types of circuit There are two types of electrical circuits; SERIES CIRCUITS PARALLEL CIRCUITS
PARALLEL CIRCUITS The components are connected side by side. The current has a choice of routes. If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.
Electric Resistance Suppose we apply a constant potential difference of 4 V to the ends of geometrically similar rods of, say: steel, copper, and glass. 4 V Steel Copper Glass Is Ic Ig The current in glass is much less than for steel or iron, suggesting a property of materials called electrical resistance R.
Factors Affecting Resistance 1. The length L of the material. Longer materials have greater resistance. 1 W L 2 W 2L 2. The cross-sectional area A of the material. Larger areas offer LESS resistance. 1 W 2A 2 W A
Factors Affecting R (Cont.) 3. The temperature T of the material. The higher temperatures usually result in higher resistances. R > Ro Ro 4. The kind of material. Iron has more electrical resistance than a geometrically similar copper conductor. Ri > Rc Copper Iron
I = V / R Ohm’s Law Georg Simon Ohm (1787-1854) I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms) Georg Simon Ohm (1787-1854)
Ohm’s Law Ohm’s law states that the current I through a given conductor is directly proportional to the potential difference V between its end points. Ohm’s law allows us to define resistance R and to write the following forms of the law:
Ohm’s Law Review R=V or V = IR or I = V I R R= resistance (ohms ) ( Ω ) V = Voltage (volts ) ( V ) I = Current (amps) ( A ) R=V or V = IR or I = V I R
Example 1 The lights of a lantern are powered by a 24 V battery. What is the resistance if they draw 0.80 A of current when turned on? 24 V = .80A R= V = I 30 Ω
Example 2 When a 3-V battery is connected to a light, a current of Example 2 When a 3-V battery is connected to a light, a current of .006 A is observed. What is the resistance of the light filament? R = 500 W Source of EMF R=? I= + - V = 3 V .006 A The SI unit for electrical resistance is the ohm, W: