ELECTRICAL Currents & Energy

Remember that an electrical charge is when there are more or less electrons than protons in an atom. The SI unit of charge is the coulomb “C”. One C is equal to 6.24 billion billion(6.24 x 1018) electrons. 6.24 billion billion electrons might seem like a great number of electrons, but it represents only the amount of charge that passes through a common 100-W light bulb in about one second. where Q1 represents the quantity of charge on object 1 (in Coulombs), Q2 represents the quantity of charge on object 2 (in Coulombs), and d represents the distance of separation between the two objects (in meters). The symbol k is a proportionality constant known as the Coulomb's law constant. The value of this constant is dependent upon the medium that the charged objects are immersed in. In the case of air, the value is approximately 9.0 x 109 N • m2 / C2. If the charged objects are present in water, the value of k can be reduced by as much as a factor of 80. It is worthwhile to point out that the units on k are such that when substituted into the equation the units on charge (Coulombs) and the units on distance (meters) will be canceled, leaving a Newton as the unit of force.

A continuous flow of charge is called an electric current A continuous flow of charge is called an electric current. Electric current is measured in amperes, “A”. An ampere is the flow of 1 coulomb of charge per second. A current carrying wire doesn’t have a net electric charge. The same # of electrons enter as the same # electrons exit. Net Charge = 0

Electric currents can be produced in many ways Electric currents can be produced in many ways. One way is through chemical reactions in a battery. A battery is a container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power. A cell is a device that produces an electric current.

Every cell contains a mixture of chemicals that conducts a current. electrolyte- the mixture of chemicals in a cell that conducts a current. Chemical reactions in here convert chemical energy into electrical energy. Every cell also contains a pair of electrodes made from 2 different conducting materials that are in contact with the electrolyte. Electrode- part of a cell through which charges enter or exit.

Cell are divided into 2 groups: Wet Cells and Dry Cells 1. Wet Cells contain liquid electrolytes.

2. Dry Cells contain electrolytes that are solid or pastelike.

Why does an electric current exist between the two electrodes Why does an electric current exist between the two electrodes? The electric current exists because a chemical reaction causes a difference in charge between the two electrodes. The difference in charge means that an electric current – a flow of electric charges – can be produced by the cell to provide energy. Electric potential = electrical potential energy/charge . Since potential energy is measured in joules and charge is measured in coulombs 1 volt = 1 joule/coulomb Volts are commonly called voltage The energy per unit charge is called the potential difference and is expressed in volts (V).

Current Revisited: Current is the continuous flow of charge, but it is more precisely defined as the rate at which charge passes a given point. There are two different types of current. Direct Current (DC) are where the charges always flow in the same direction. The electric current produced by batteries and cells is DC.

provided to households changes directions 120 times each second. In Alternating current (AC) the charges continually switch from flowing in one direction to flowing in the reverse direction. Alternating current is used in homes today because it is more practical for transferring electrical energy. In the U.S., the AC provided to households changes directions 120 times each second. https://www.youtube.com/watch?v=xyQfrzBfnDU

In addition to voltage, resistance also determines the current in a wire. Resistance- the opposition to the flow of electric charge; expressed in Ohms (Ω); represented by the letter R You can think of resistance a “electrical friction.” The higher the resistance of a material, the lower the current is in it. Therefore, as resistance increases, current decreases if the voltage is kept the same. An object’s resistance varies depending on the object’s material, thickness, length, and temperature.

How you should be thinking about electric circuits: Voltage: a force that pushes the current through the circuit (in this picture it would be equivalent to gravity)

How you should be thinking about electric circuits: Current: the actual “substance” that is flowing through the wires of the circuit (electrons!)

How you should be thinking about electric circuits: Resistance: friction that impedes flow of current through the circuit (rocks in the river)

The relationship between voltage difference, current, and resistance in a circuit is known as Ohm’s Law I = V / R I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms) Georg Simon Ohm (1787-1854)

Other Ways to produce electricity: A Photocell is the part of a solar panel that converts light into electrical energy. Thermal energy can be converted to electrical by a Thermocouple. The temperature difference within the loop causes charges to flow.

Would This Work?

Would This Work?

Would This Work?

The Central Concept: Closed Circuit

circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires

Simple Circuits Series circuit Parallel circuit All in a row 1 path for electricity 1 light goes out and the circuit is broken Parallel circuit Many paths for electricity 1 light goes out and the others stay on

1 2 3 The current decreases because the resistance increases. Ohm’s Law says that I=V/R. The voltage in the system is constant, resistance increases.

PARALLEL CIRCUIT Place two bulbs in parallel. What do you notice about the brightness of the bulbs? Add a third light bulb in the circuit. What do you notice about the brightness of the bulbs? Remove the middle bulb from the circuit. What happened?

measuring current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A

measuring current A A This is how we draw an ammeter in a circuit. PARALLEL CIRCUIT SERIES CIRCUIT

measuring voltage The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V

measuring voltage V V This is how we draw a voltmeter in a circuit. SERIES CIRCUIT PARALLEL CIRCUIT

OHM’s LAW Voltage Current Resistance Parallel Circuit Voltage Current Measure the current and voltage across each circuit. Use Ohm’s Law to compute resistance Series Circuit Voltage Current Resistance Parallel Circuit Voltage Current Resistance

measuring current SERIES CIRCUIT current is the same at all points in the circuit. 2A 2A PARALLEL CIRCUIT 2A 2A current is shared between the components 1A 1A

fill in the missing ammeter readings. ? 3A 3A 4A ? 1A ? ? 4A 4A 1A ? 1A

The voltage decreases because the current is decreased The circuit is no longer complete, therefore current can not flow The voltage decreases because the current is decreased and the resistance increases.

The current remains the same The current remains the same. The total resistance drops in a parallel circuit as more bulbs are added The current increases.

Series and Parallel Circuits Series Circuits only one end of each component is connected e.g. Old Christmas tree lights Parallel Circuits both ends of a component are connected e.g. household lighting

copy the following circuits and fill in the missing ammeter readings. ? 3A 3A 4A ? 1A ? ? 4A 4A 1A ? 1A

measuring voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter, a voltmeter is connected across the components Scientist usually use the term Potential Difference (pd) when they talk about voltage.

measuring voltage V V V V

series circuit voltage is shared between the components 3V 1.5V 1.5V

parallel circuit voltage is the same in all parts of the circuit. 3V

measuring current & voltage copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits.

measuring current & voltage

measuring current & voltage b) 6V 4A A V A V A

answers a) b) 6V 6V 4A 4A 6V 4A 4A 3V 3V 2A 4A 6V 2A

Voltage, Current, and Power One Volt is a Joule per Coulomb (J/C) One Amp of current is one Coulomb per second (6.24 x10^18 electrons/second). If I have one volt (J/C) and one amp (C/s), then multiplying gives Joules per second (J/s) this is power: J/s = Watts So the formula for electrical power is just: More work is done per unit time the higher the voltage and/or the higher the current P = VI: power = voltage  current