Lesson 4

 Charge does not flow on its own. An electric charge has a certain amount of electrical potential energy because of the electric field set up by the power supply.  Work is done by the power supply to increase the electric potential energy of each coulomb of charge from a low to a high value.  As the charge flows through the load, its energy decreases.

 A load converts electrical energy into another form of energy. You can compare this to the water flowing past a water wheel. The wheel converts some of the energy of the water into motion. The water has more energy before the wheel than after the wheel.

 The electrical potential energy for each coulomb of charge in a circuit is called the electric potential difference (V)  Voltage

 The formula can be rearranged  Energy  Charge

 Where E is the energy required to increase the electric potential of a charge, Q. Potential difference is often called voltage.

 One volt (V) is the electric potential difference between two points if one joule of work (J) is required to move one coulomb (C) of charge between the points.

 What is the potential difference across an air conditioner if 72 C of charge transfers 8.5 x 10 3 J of energy to the fan and compressor?  Q = 72 C  E = 8.5 x 10 3 J  V = ?

 V = 1.2 x 10 2 V  Therefore, the potential difference or voltage in the air conditioner is 1.2 x 10 2 V

 A static electric shock delivered to a student from a “friend” transfers 1.5 x 10 1 J of electric energy through a potential difference of 500 V. What is the quantity charge transferred in the spark?  E = 1.5 x 10 1 J  V = 500 V  Q = ?

 Q = 0.03 C  Therefore, the charge transfer between friends is 0.03 C.

 Recall that and  So E = VQ and Q = It  Therefore, E = VIt

 One 1.5 V (AA) battery runs a portable MP3 Player that draws 5.7 x A of current for about 6 hours before it runs out. How much energy does the battery transfer?  V = 1.5 V  I = 5.7 x A  E = ?

 E = 185 J  Therefore, the battery transfers 185 J of energy

 A coffee maker draws about 5.0 A of current for 270 s using 1.6 x 10 5 J of energy. What is the potential difference across the coffee maker?  I = 5.0 A  t = 270 s  E = 1.6 x 10 5 J  V =?

 V = 119 V  Therefore, the potential difference across the coffee maker is 119 V

 Potential difference between any two points can be measured using a voltmeter. A voltmeter must be connected in parallel with a load in the circuit in order to compare the potential before and after the load.

 Electrical energy always originates from some other form of energy.  Some common sources include:  Voltaic cells – Chemical potential energy released during a reaction as electrons are driven between two different metals

 Piezo-electricity – Crystals that produce a small electric potential when a mechanical force is placed on them.

 Thermoelectricity – Two different types of metal joined together and subjected to temperature differentials.

 Photo electricity – Light energy absorbed by electrons of certain metals causes charge flow.

 Electromagnetic induction in generators – Kinetic energy forces conductors to rotate in a magnetic field.

1. Explain the difference between current and voltage. 2. You go to a store and buy a 12 V car battery. All the batteries are 12 V, but they differ in cranking amps. What should you look for in a winter battery? 3. A 12 V car battery delivers 1. 3 x 10 4 J of energy to the starter A. How much charge does it deliver? B. How many electrons does the battery transfer C. Given that you keep the key turned for 2.5 s (time to turn the car over in order to start the car), how many amps are delivered to the starter motor? 4. Lightning transfers charge between a charged cloud and the ground. If the voltage difference between two is 1.3 x 10 8 V and 3.2 x 10 9 J of energy is transferred, find A. The total charge moved between the two potential energy surfaces. B. The number of electrons making up the charge C. The current delivered if the lightning takes 25 microseconds to hit the ground.