Current Electricity

Electricity Electricity is the flow of electrons through a conducting material. Electricity is the flow of electrons through a conducting material. The flow of electricity is called current. The flow of electricity is called current. Current is measured by counting the number of electrons that flow past a point in a given time. Current is measured by counting the number of electrons that flow past a point in a given time.

Current An electric current is moving charges (electrons) An electric current is moving charges (electrons) The brightness of a bulb depends on the current through the bulb (qualitative measure) The brightness of a bulb depends on the current through the bulb (qualitative measure) As the electrons move through the conductor, they collide with the fixed particles of the conductor and the kinetic energy is transferred into heat and light in the resistance As the electrons move through the conductor, they collide with the fixed particles of the conductor and the kinetic energy is transferred into heat and light in the resistance

Current More current means more electrons collide and therefore, more energy is dissipated in the resistance More current means more electrons collide and therefore, more energy is dissipated in the resistance Resistance should be considered an obstacle to flow. Less resistance implies more flow and vice versa Resistance should be considered an obstacle to flow. Less resistance implies more flow and vice versa Current is conserved. What goes in one end must come out the other end Current is conserved. What goes in one end must come out the other end

Electrical Charge and Current Electrical charge is measured in Coulombs (C) Electrical charge is measured in Coulombs (C) 1 coulomb is equal to the amount of charge in 6.25 x electrons 1 coulomb of charge moving past a point in 1 second is equal to 1 Ampere of current 1 Ampere = 1 coulomb/1 second

Calculating Current To calculate current we use the formula; To calculate current we use the formula; I = Q or Q = I x t t I = current and is measured in Amperes (A) Q = charge and is measured in coulombs (C) Q = charge and is measured in coulombs (C) t = time and is measured in seconds (s) t = time and is measured in seconds (s)

Examples If a bulb has 0.25A, how many electrons pass through the bulb in one second? If a bulb has 0.25A, how many electrons pass through the bulb in one second? I = Q/tand Q = Ittherefore Q = (0.25A)(1s) or 0.25C, but 1C = 6.25 x 10^18 So… (0.25A)(6.25 x 10^18) = 1.56 x 10^18 e That is a lot of electrons!

Voltage Voltage (electric potential difference) is measured in Volts (V). Voltage (electric potential difference) is measured in Volts (V). Voltage is the energy per unit charge between two points along a conductor Voltage is the energy per unit charge between two points along a conductor V = E/Q V = E/Q

Five Sources of Electricity Five Sources of Electricity 1. Chemical – chemical sources accumulate charge through chemical processes. 1. Chemical – chemical sources accumulate charge through chemical processes. Ex. batteries Ex. batteries

2. Photoelectric (light) – photoelectric materials emit electrons when they are struck by light. 2. Photoelectric (light) – photoelectric materials emit electrons when they are struck by light. Ex. Solar powered calculators Ex. Solar powered calculators

3. Electromagnetic – a wire moving in a magnetic field generates a current. 3. Electromagnetic – a wire moving in a magnetic field generates a current. Ex. generator Ex. generator

4. Thermoelectric (heat) – the temperature difference between different materials can create a current. 4. Thermoelectric (heat) – the temperature difference between different materials can create a current. Ex. A thermocouple in a furnace Ex. A thermocouple in a furnace

5. Piezoelectric (crystals) – some crystals will create a current when put under mechanical stress (force). 5. Piezoelectric (crystals) – some crystals will create a current when put under mechanical stress (force). Ex crystals found in quartz watches. Ex crystals found in quartz watches.

The Electric Cell The electric cell is a device that has the ability to produce an electric charge for a longer period of time than an electrostatically charged object. The electric cell is a device that has the ability to produce an electric charge for a longer period of time than an electrostatically charged object.

The History 1794 – Luigi Galvani researched the link between muscle contractions and electric “sparks” 1794 – Luigi Galvani researched the link between muscle contractions and electric “sparks”

1800 – Alessandro Volta Discovered that if two different metals are placed in a salt or acid solution a charge was created. The charge produced seemed like it was continuous. This type of cell is called a Volta Cell or Wet Cell

The Wet Cell Two different metals called electrodes are placed in a salt or acid solution called an electrolyte. Two different metals called electrodes are placed in a salt or acid solution called an electrolyte. Able to give off sparks without being recharged Able to give off sparks without being recharged Produces an electric current – a continuous supply or “flow” of electrons. Produces an electric current – a continuous supply or “flow” of electrons.

Example A zinc electrode and a copper electrode are placed in an electrolyte solution of NaCl. This solution contains (Na+) sodium ions and (Cl-) chloride ions. A zinc electrode and a copper electrode are placed in an electrolyte solution of NaCl. This solution contains (Na+) sodium ions and (Cl-) chloride ions.

How this works The copper electrode gives electrons to the positive sodium ions – because it has given away electrons it becomes positively charges. The copper electrode gives electrons to the positive sodium ions – because it has given away electrons it becomes positively charges. The zinc electrode attracts the negatively charged chloride ions and becomes negatively charged. The zinc electrode attracts the negatively charged chloride ions and becomes negatively charged. The conducting loop (usually a metal) provides a pathway for the negative charges (electrons) to flow from the negative Zn electrode towards the positive Cu electrode. The conducting loop (usually a metal) provides a pathway for the negative charges (electrons) to flow from the negative Zn electrode towards the positive Cu electrode.

Example

The Dry Cell Works the same way as the wet cell but the electrolyte solution used is in a paste not a liquid form.

Current Flow in Metal Conductors Electric current is the movement of electrons through a conductor at a certain rate when something is “pushing” them. Electric current is the movement of electrons through a conductor at a certain rate when something is “pushing” them. When an electric cell is attached to a conductor it moves electrons into the conductor When an electric cell is attached to a conductor it moves electrons into the conductor The electric cell pushes electrons from the –ve elecrode (by repulsion) into the conductor which pushes the rest of the electrons towards the +ve electrode (by attraction) The electric cell pushes electrons from the –ve elecrode (by repulsion) into the conductor which pushes the rest of the electrons towards the +ve electrode (by attraction)

Circuits The conducting loop in a cell is often referred to as a circuit. The conducting loop in a cell is often referred to as a circuit. The circuit (conducting loop) can be opened or closed using a switch. The circuit (conducting loop) can be opened or closed using a switch. If the switch is OPEN the conducting loop is “broken” so electrons cannot flow If the switch is OPEN the conducting loop is “broken” so electrons cannot flow If the switch is CLOSED the conducting loop is complete so electrons can flow If the switch is CLOSED the conducting loop is complete so electrons can flow

In an electric circuit, electrons are present in ALL PARTS of the circuit all the time In an electric circuit, electrons are present in ALL PARTS of the circuit all the time In order to move electrons in a wire, we need a potential difference between the ends of the wire, which is created through an accumulation of charge at the ends of the wire In order to move electrons in a wire, we need a potential difference between the ends of the wire, which is created through an accumulation of charge at the ends of the wire As one electron is pushed into one end of the wire, ALL electrons in the wire move simultaneously and one electron moves off the other end As one electron is pushed into one end of the wire, ALL electrons in the wire move simultaneously and one electron moves off the other end

A circuit can be defined as a pathway used to direct an electric current in order to perform a “task”. A circuit can be defined as a pathway used to direct an electric current in order to perform a “task”. Every circuit must have 3 components Every circuit must have 3 components A Source of energy – electric cell or battery A Source of energy – electric cell or battery A Conducting loop – usually a metal wire A Conducting loop – usually a metal wire A Load – something that uses potential energy to do work such as a light bulb, resistor or motor. A Load – something that uses potential energy to do work such as a light bulb, resistor or motor.

Measuring Current and Volts In order to measure current, an ammeter must be placed in series such that the current flows through the meter. In order to measure current, an ammeter must be placed in series such that the current flows through the meter. If placed incorrectly (placed in parallel means it behaves as a low resistance wire – shorts the circuit) If placed incorrectly (placed in parallel means it behaves as a low resistance wire – shorts the circuit) Voltmeters are placed in a circuit in parallel and have a very high resistance Voltmeters are placed in a circuit in parallel and have a very high resistance If placed in series, the large resistance decreases the current (effectively to zero) and the circuit will not work If placed in series, the large resistance decreases the current (effectively to zero) and the circuit will not work

V A

Resistance Resistance is the opposition to the flow of electrons when a current passes through the circuit. Resistance is the opposition to the flow of electrons when a current passes through the circuit. Electron movement is slowed down Electron movement is slowed down Resistance converts electrical energy into other forms of energy such as light or heat Resistance converts electrical energy into other forms of energy such as light or heat Devices called resistors are designed specifically to provide resistance in a circuit. Devices called resistors are designed specifically to provide resistance in a circuit. The unit used to measure resistance is the Ohm (Ω) The unit used to measure resistance is the Ohm (Ω)

Battery A combination of two or more dry cells that act as an energy source for a circuit A combination of two or more dry cells that act as an energy source for a circuit 2 types of batteries may be used in a circuit 2 types of batteries may be used in a circuit 1. Cells in series 2. Cells in parallel

Cells in Series Cells in Series Two or more dry cells connected end to end Two or more dry cells connected end to end Connecting dry cells in series INCREASES the voltage but DOES NOT CHANGE the current produced by the battery Connecting dry cells in series INCREASES the voltage but DOES NOT CHANGE the current produced by the battery Ex. If two 1.5V cells are connected in series we would have a 3.0V battery Ex. If two 1.5V cells are connected in series we would have a 3.0V battery

Cells in Parallel (last longer) Cells in Parallel (last longer) 2 or more dry cells connected side by side so that their + electrodes are joined and their – electrodes are joined 2 or more dry cells connected side by side so that their + electrodes are joined and their – electrodes are joined Connecting dry cells in parallel DOES NOT CHANGE the voltage produced by the battery but it INCREASES the current produced by the battery Connecting dry cells in parallel DOES NOT CHANGE the voltage produced by the battery but it INCREASES the current produced by the battery Each cell only has to contribute a portion of the current available to the circuit. This “shared” load allows the current to be delivered for a longer period of time. Each cell only has to contribute a portion of the current available to the circuit. This “shared” load allows the current to be delivered for a longer period of time.

Series Circuits Circuits constructed in a way that all of the current must pass through each of the loads in the circuit Circuits constructed in a way that all of the current must pass through each of the loads in the circuit

Resistors in Series Two resistors placed in series have a greater resistance than a single resistor Two resistors placed in series have a greater resistance than a single resistor The total resistance of resistors in series is the sum of the resistors: The total resistance of resistors in series is the sum of the resistors: R total = R 1 + R 2 + R 3 … R total = R 1 + R 2 + R 3 …

Parallel Circuits Circuits constructed in a way in which the loads are parallel to each other creating BRANCHES in the circuit Circuits constructed in a way in which the loads are parallel to each other creating BRANCHES in the circuit Current has a choice of branches to travel through. Current has a choice of branches to travel through. The branch with the lower resistance will have the higher current passing through it The branch with the lower resistance will have the higher current passing through it

The current before the junction and after the junction have the same value The current before the junction and after the junction have the same value The sum of the current running through each branch is equal to the total current running through the entire circuit The sum of the current running through each branch is equal to the total current running through the entire circuit

Resistors in Parallel When resistors are connected in parallel the total resistance is less than that of a single resistor. When resistors are connected in parallel the total resistance is less than that of a single resistor. The electrons have an extra path to follow (like opening another door) The electrons have an extra path to follow (like opening another door) The equivalent resistance of resistors in parallel is given by the formula: The equivalent resistance of resistors in parallel is given by the formula: 1/R eq = 1/R 1 + 1/R 2 + 1/R 3 …

Power, Energy and Time In physics the word Power is defined as the energy per unit time In physics the word Power is defined as the energy per unit time Electrical power describes the amount of electrical energy that is converted into light, heat, sound or motion every second. Electrical power describes the amount of electrical energy that is converted into light, heat, sound or motion every second. The symbol for power is P. The equation that defines power in mathematical form is The symbol for power is P. The equation that defines power in mathematical form is P =E/t P =E/t

Units of Power Since the unit for energy is joule ( J ) and the unit for time is the second ( s ), power can be expressed in joules/second Since the unit for energy is joule ( J ) and the unit for time is the second ( s ), power can be expressed in joules/second Joules/second has been named Watt ( W ) in honour of James Watt. Joules/second has been named Watt ( W ) in honour of James Watt. When one joule of electrical energy is converted into heat and light by a light bulb every second, the power of the bulb is said to be one watt. When one joule of electrical energy is converted into heat and light by a light bulb every second, the power of the bulb is said to be one watt. Note that the energy delivered to a circuit must be used in the circuit, or the wires will burn Note that the energy delivered to a circuit must be used in the circuit, or the wires will burn

Nothing is perfect If an electrical device were perfect, all the electrical energy that it uses would be converted into the desired form of energy. If an electrical device were perfect, all the electrical energy that it uses would be converted into the desired form of energy. However some energy is always converted into heat. However some energy is always converted into heat. You can determine the efficiency of an electrical device by using the following relationship: You can determine the efficiency of an electrical device by using the following relationship: Percent efficiency = Useful energy output X 100% Total energy input Total energy input