Energy and the Electric Potential - Lesson III.2.4

Key Terms:

electric potential difference |chassis ground |

Recall the change in potential energy of a ball when you lift it from point A to B. Gravitational force is acting on the ball. If you lift the ball against the force of gravity from A to B work is done, and thus it's potential energy is increased. Work done by the hand to lift the ball from A to B equals the difference between the electric potential energy at B and at A: The diagram below will help to clarify the analogy between electric potenital energy and gravitational potential energy. In this diagram a positive test charge +q is situated between two oppositely charged plates. Because of the charge on the plates, the test charge will undergo an electric force from the + plate (directed to the - plate). The hand in the drawing is providing the external force required to move the test charge towards the + plate (neglect gravitational potential energy) because of the electric force (like charges repel). At locations A and B, the test charge q has electric potential energies. The electric potential difference (V) is the work done per unit charge as a charge is moved between two points in an electric field. The SI unit of electric potential: joule/coulomb = volt (V). Note: Electric potential difference uses the variable V as well as the unit volt. Do not refer to electric potential as Volts (even though the units are volts). When you use V in this situation it stands for electric potential. Example 1: The work done to move the test charge (q = +3.0 x 10-5 C) at a constant speed from A to B is + 4.5 x 10-6J. Find the difference in the electric potential energies of the chagre between A and B. Determine the potential difference between A and B. Solution: The difference in potential energy between points A and B is equivalent to the work done in moving the charge from A to B. Therefore, + 4.5 x 10-6J is the difference in potential energy. The potential difference between A and B is equal to the difference in potential energy divided by the chage: Example 2: A positively charged object with a charge of 4.0 x 10-7 C is moved from it's original position A 90 cm closer to a positively charged plate. The electric force of repulsion is 2.5 x 10-2 N. Calculate the work done to move the charge from A to B. Calculate the work done per unit charge in moving from A to B. Solution: a. b. 3. A 0.30 gram sphere with a positive charge of 3.5 x 10-6C accelerates from point A towards point B undergoing an electric force of 5.9 x 10-4N. The distance between A and B is 8.0 cm. Caclulate: Note: When entering answers use two decimal places (with answers that have a decimal, round the 2nd digit). Enter in the answer written in scientici notation for example: 2 x 10-4 Joules should be entered in 2*10^-4 J. The work done by the force on the sphere as it moves from A to B.    The potential difference between A and B.     The velocity of the sphere when it reaches B.     The Potential of the Earth The earth has an endless supply of electrons. Electrons can be added or taken away without its potential being changed. The potential of the earth is arbitrarily said to be zero. An object connected directly to the ground ican be described as being earthed. Discuss with the class items that are grounded and why. In electrical circuits the ground means zero voltage when compared with the rest of the circuit. This is sometimes called a chassis ground. Ex: The computer is plugged into the wall. All electrical components inside of the computer are connected to the box or chassis which in turn is connected to the ground wire in the electrical computer. The electrical chord that connects the computer tower to the wall has three wires in it. One of the wires is a ground wire which is connected to the third prong on the electrical cord (middle round prong). This prong plugs into the wall which connects to the electrical receptacle. The receptacle ground is connected to a copper wire which leads through the wall back to the breaker box (usually in the basement). This copper wire is connected to another very thick copper wire which is connected to a copper rod buried into the ground. This electrical ground helps protect the very sensitive electronic components inside the computer as well as human life when people come in contact with an electrified object. Providing a path for electrons to flow if a "short" occurs inside the computer or any other device is a safety measure. Discuss with the class why some people remove the third prong from the plug-in and why this may pose a safety concern. Voltmeter The meter shown here is a digital multimeter or DMM. This meter can measure current AC or DC, voltage AC or DC, resistance, and continuity. This DMM in the voltmeter setting can be used to measure the electric potential difference between two points in an electric circuit. The voltmeter is connected in parallel (across) within a circuit to measure voltage. The black test lead is the negative or 0 potential or ground lead, and the red test lead is the positive potential lead. When using a DMM it is important to start with the highest setting and then move the dial to lower setters to measure the voltage. Having a lower setting on the DMM and trying to measure a larger voltage can damage the multimeter. The internal resistance of the Voltmeter is very high so a negligible amount of current will flow through the meter which will not disturb the circuit.