Potential EnergyPotential Energy

Physics 12Physics 12

Potential EnergyPotential Energy In grade 11 physics we looked at In grade 11 physics we looked at

Gravitational Potential EnergyGravitational Potential Energy Formula: Eg = mghFormula: Eg = mgh W = FW = Fggdd

Work and Energy are measured in JoulesWork and Energy are measured in Joules

When an object falls, its Eg is converted When an object falls, its Eg is converted from gravitational potential energy to from gravitational potential energy to kinetic energy or work.kinetic energy or work.

Electric Potential EnergyElectric Potential Energy Electrical Potential Energy (EQ) is stored Electrical Potential Energy (EQ) is stored

by doing work against electrostatic force. by doing work against electrostatic force. This can be done by separating This can be done by separating attractive/unlike charges (as in a battery) attractive/unlike charges (as in a battery) or forcing together like charges (as in a or forcing together like charges (as in a capacitor).capacitor).

If you move a charge in an electric field, If you move a charge in an electric field, you must apply a force. Applying a force you must apply a force. Applying a force causing displacement means that work is causing displacement means that work is done. If you do work, then the object done. If you do work, then the object gains energy. Moving a charge is an gains energy. Moving a charge is an example of electric potential energy.example of electric potential energy.

Electric Potential EnergyElectric Potential Energy Note: Eq can be + Note: Eq can be +

or – (depends on or – (depends on the charge)the charge)

** Write on your ** Write on your formula sheet!formula sheet!

rqkqE

dFEW

Q

eQ

21

Voltage (V)Voltage (V) The work done per unit charge in The work done per unit charge in

moving a charge between two points moving a charge between two points in an electric fieldin an electric field

Voltage is a measure of how Voltage is a measure of how electrical potential energy is electrical potential energy is delivereddelivered

Voltage is also called electrical Voltage is also called electrical potential differencepotential difference

Voltage Voltage V = W/q or V = Ee/qV = W/q or V = Ee/q Units of V = voltsUnits of V = volts 1 Volt = 1 Joule/Coulomb1 Volt = 1 Joule/Coulomb

Example: BatteriesExample: Batteries Batteries achieve separation of Batteries achieve separation of

charge in an anode and cathode charge in an anode and cathode separated over a distance.separated over a distance.

Batteries are not rated by how much Batteries are not rated by how much potential energy they contain but by potential energy they contain but by how much energy they can deliver as how much energy they can deliver as current flows (voltage)current flows (voltage)

Example 1Example 1 A battery has a potential difference A battery has a potential difference

of 18.0 V. How much work is done of 18.0 V. How much work is done when a 64.0 C charge moves from when a 64.0 C charge moves from the anode to the cathode?the anode to the cathode?

1150J1150J

Example 2Example 2 A potential difference of 10V exists A potential difference of 10V exists

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

0.0020 C0.0020 C

Electron voltsElectron volts One electron-volt (eV) is the energy One electron-volt (eV) is the energy

needed to move one elementary needed to move one elementary charge through a potential difference charge through a potential difference of 1V.of 1V.

1 eV = 1.6 x 101 eV = 1.6 x 10-19-19 J J

Example 3Example 3 A charge of 2.0 x 10A charge of 2.0 x 10-3-3C is moved C is moved

through a potential difference of 10 through a potential difference of 10 volts in an electric field. How much volts in an electric field. How much work, in eV, was required to move work, in eV, was required to move this charge?this charge?

W = 1.25 x 10 W = 1.25 x 10 1717 eV eV

Try theseTry these 1) How much electrical energy is required to 1) How much electrical energy is required to

move a 4.0 microCoulomb charge through a move a 4.0 microCoulomb charge through a potential difference of 36V? potential difference of 36V?

2) In an electric field, 0.90 J of work is required to 2) In an electric field, 0.90 J of work is required to bring 0.45 Coulombs of charge from point A to bring 0.45 Coulombs of charge from point A to point B. What is the electric potential difference point B. What is the electric potential difference (voltage) between points A and B?(voltage) between points A and B?

3) A single proton is moved through a potential 3) A single proton is moved through a potential difference of 10 volts in an electric field. How difference of 10 volts in an electric field. How much work, in electronvolts, was required to much work, in electronvolts, was required to move this charge?move this charge?

HINTSHINTS Remember that work is equal to Remember that work is equal to

change in energy (look on your change in energy (look on your formula sheet for formulas for kinetic formula sheet for formulas for kinetic energy, gravitational, etc)energy, gravitational, etc)

Voltaic CellVoltaic Cell One of the silver, zinc and salt water One of the silver, zinc and salt water

soaked discs became known as a soaked discs became known as a voltaic cellvoltaic 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 Electrolyte (to allow the movement of

charge)charge)