Potential Energy Physics 12

Potential Energy In grade 11 physics we looked at Gravitational Potential Energy In grade 11 physics we looked at Gravitational Potential Energy Formula: Eg = mgh Formula: Eg = mgh W = F g d W = F g d Work and Energy are measured in Joules Work and Energy are measured in Joules When an object falls, its Eg is converted from gravitational potential energy to kinetic energy or work. When an object falls, its Eg is converted from gravitational potential energy to kinetic energy or work.

Electric Potential Energy Electrical Potential Energy (EQ) is stored by doing work against electrostatic force. This can be done by separating attractive/unlike charges (as in a battery) or forcing together like charges (as in a capacitor). Electrical Potential Energy (EQ) is stored by doing work against electrostatic force. This can be done by separating attractive/unlike charges (as in a battery) or forcing together like charges (as in a capacitor). If you move a charge in an electric field, you must apply a force. Applying a force causing displacement means that work is done. If you do work, then the object gains energy. Moving a charge is an example of electric potential energy. If you move a charge in an electric field, you must apply a force. Applying a force causing displacement means that work is done. If you do work, then the object gains energy. Moving a charge is an example of electric potential energy.

Electric Potential Energy Note: Eq can be + or – (depends on the charge) Note: Eq can be + or – (depends on the charge) ** Write on your formula sheet! ** Write on your formula sheet!

Voltage (V) The work done per unit charge in moving a charge between two points in an electric field The work done per unit charge in moving a charge between two points in an electric field Voltage is a measure of how electrical potential energy is delivered Voltage is a measure of how electrical potential energy is delivered Voltage is also called electrical potential difference Voltage is also called electrical potential difference

Voltage V = W/q or V = Ee/q V = W/q or V = Ee/q Units of V = volts Units of V = volts 1 Volt = 1 Joule/Coulomb 1 Volt = 1 Joule/Coulomb

Example: Batteries Batteries achieve separation of charge in an anode and cathode separated over a distance. Batteries achieve separation of charge in an anode and cathode separated over a distance. Batteries are not rated by how much potential energy they contain but by how much energy they can deliver as current flows (voltage) Batteries are not rated by how much potential energy they contain but by how much energy they can deliver as current flows (voltage)

Example 1 What is the electric potential energy stored between charges of +8.0 microColoumbs and +5.0 microColoumbs that are separated by 20.0cm? What is the electric potential energy stored between charges of +8.0 microColoumbs and +5.0 microColoumbs that are separated by 20.0cm? +1.8 J +1.8 J

Example 1 A battery has a potential difference of 18.0 V. How much work is done when a 64.0 C charge moves from the anode to the cathode? A battery has a potential difference of 18.0 V. How much work is done when a 64.0 C charge moves from the anode to the cathode? 1150J 1150J

Example 2 A potential difference of 10V exists between 2 points, A and B, within an electric field. What is the magnitude of charge that requires 2.0 x J of work to move it from A to B? A potential difference of 10V exists between 2 points, A and B, within an electric field. What is the magnitude of charge that requires 2.0 x J of work to move it from A to B? C C

Electron volts One electron-volt (eV) is the energy needed to move one elementary charge through a potential difference of 1V. One electron-volt (eV) is the energy needed to move one elementary charge through a potential difference of 1V. 1 eV = 1.6 x J 1 eV = 1.6 x J

Example 3 A charge of 2.0 x C is moved through a potential difference of 10 volts in an electric field. How much work, in eV, was required to move this charge? A charge of 2.0 x C is moved through a potential difference of 10 volts in an electric field. How much work, in eV, was required to move this charge? W = 1.25 x eV W = 1.25 x eV

Try these 1) How much electrical energy is required to move a 4.0 microCoulomb charge through a potential difference of 36V? 1) How much electrical energy is required to move a 4.0 microCoulomb charge through a potential difference of 36V? 2) In an electric field, 0.90 J of work is required to bring 0.45 Coulombs of charge from point A to point B. What is the electric potential difference (voltage) between points A and B? 2) In an electric field, 0.90 J of work is required to bring 0.45 Coulombs of charge from point A to point B. What is the electric potential difference (voltage) between points A and B? 3) A single proton is moved through a potential difference of 10 volts in an electric field. How much work, in electronvolts, was required to move this charge? 3) A single proton is moved through a potential difference of 10 volts in an electric field. How much work, in electronvolts, was required to move this charge?

HINTS Remember that work is equal to change in energy (look on your formula sheet for formulas for kinetic energy, gravitational, etc) Remember that work is equal to change in energy (look on your formula sheet for formulas for kinetic energy, gravitational, etc)

Voltaic Cell One of the silver, zinc and salt water soaked discs became known as a voltaic cell One of the silver, zinc and salt water soaked discs became known as a voltaic cell A voltaic cell requires: A voltaic cell requires: Cathode (positive end)Cathode (positive end) Anode (negative end)Anode (negative end) Electrolyte (to allow the movement of charge)Electrolyte (to allow the movement of charge)