SPH3U: Electricity Electric Potential Difference

Electric Potential When we turn on a light switch, we can say we are “using electricity,” but what does this mean? What does the light bulb take from the current? To answer these questions, let’s first look at a few analogies.

Electric Potential First, imagine a ball held some distance above the surface of Earth (but still within the earth’s gravitational field). Work must have been done on the ball to lift it this high against the force of gravity. The ball’s gravitational potential energy is increased. If the ball is let go, gravity will make it move back to the Earth, converting gravitational potential energy into kinetic energy as the ball falls.

Electric Potential Now, imagine a small positive charge held at rest a certain distance away from a negatively charged sphere. Work must be done on the small positive charge to overcome the electric force that exists because of the negative sphere. In this case, the small positive charge has an increase in electric potential energy as a result. If let go, the positive charge will move back toward the negative sphere, in much the same way that the ball moved back toward Earth. When it does this it will lose electric potential energy.

Electric Potential In a circuit, when electrons leave the battery (or source) they have a lot of electric potential energy.

Electric Potential As current passes through the circuit, and hits the light bulb, it experiences opposition to the flow, resulting in a loss of electric potential energy. This opposition happens because the electrons collide with the atoms of the light bulb as they move through the light bulb.

Electric Potential As current passes through the circuit, and hits the light bulb, it experiences opposition to the flow, resulting in a loss of electric potential energy. This opposition happens because the electrons collide with the atoms of the light bulb as they move through the light bulb.

Electric Potential Since the electric current loses energy it also loses electric potential, resulting in an electric potential difference (V) between two points, A and B. This can be represented as where W is the amount of work that must be done to move a small positive charge, Q, from point A to point B.

Electric Potential in a Circuit

Electric Potential Difference The Electric Potential Difference (V) is defined as: the change in electric potential when a charge is moved between two points in an electric field. The SI unit for electric potential difference is the volt (V). 1 Volt is the electric potential difference between two points if it takes 1 J of work per coulomb to move a positive charge from one point to the other. 1 V = 1 J/C

Electric Potential Difference Because of the units in which it is measured, electric potential difference is often referred to as “voltage.”

Electric Potential Difference A 12-V car battery is a battery that does 12 J of work on each coulomb of charge that flows through it. Electric potential difference between two points in a circuit is measured with a device called a voltmeter. To measure electric potential difference, a voltmeter is connected across the source of electricity and the bulb in the circuit. This type of connection is called a parallel connection.

Connecting a Voltmeter

Electric Potential The electrical energy lost or work done by a charge, Q, going through a potential difference, V, can be written ΔE = QV Since it is often easier to measure the current and the time during which it lasts, we can use the equation Q = IΔt and, substituting in the first equation, we get an expression for the electrical energy lost by a current, I, through a potential difference, V, for a time interval, Δt. ΔE = VIΔt

Example Problem 1 A 12-V car battery supplies 1.0 × 10 3 C of charge to the starting motor. How much energy is used to start the car?

Example Problem 1 A 12-V car battery supplies 1.0 × 103 C of charge to the starting motor. How much energy is used to start the car?

Example Problem 2 If a current of 10.0 A takes 300 s to boil a kettle of water requiring 3.6 × 10 5 J of energy, what is the potential difference (voltage) across the kettle?

Example Problem 2 If a current of 10.0 A takes 300 s to boil a kettle of water requiring 3.6 × 10 5 J of energy, what is the potential difference (voltage) across the kettle?

Electricity and Safety Electricity is dangerous because of its effect on the life support organs of the human body. The primary factors determining the effect of electricity on the body are the amount and path of the current passing through the body.

Electricity and Safety Currents of less than 0.02 amp may produce sensations ranging from tingling to sharp pain (like the current generated by our little hand- held generator). A more serious effect occurs if the current causes muscles to contract. A person touching a live wire with their outstretched hand may literally not be able to let go of the wire due to the current's effect on the muscles. Currents from 0.03 to 0.07 amp will begin to impair the ability of the person to breathe.

Electricity and Safety The most dangerous range of currents is from 0.1 to about 0.2 amp. Currents in this range can cause death by initiating fibrillation (uncontrollable twitching) of the heart, which stops the regular flow of blood to the rest of the body. Currents much larger than 0.1 amp do not result in fibrillation - instead they stop the heart completely. If the duration of the current is short, the heart will usually start to beat by itself after the current is removed.

Electricity and Safety Because of the equation V = IR, we know that Voltage and Resistance determine the current. Therefore, high voltages and low resistance will produce a high current, which is dangerous.

Electricity and Safety As examples, an AA battery provides voltage of 1.5 volts, a car battery 12 volts and an electrical outlet 110 volts. The resistance (which is measured in ohms) of the human body can range from one hundred to one million ohms. Wet skin has a much lower resistance to current than dry skin.

Electricity and Safety This is why electrical appliances warn against use while in the shower or bath; although the voltage of the appliance may not be sufficient to send large currents through a dry body, the same voltage may result in a very dangerous current in a wet body. If the resistance of a body is as low as 100 ohms, a voltage as small as 20 volts can lead to fatal currents.

Homework Read Section 11.3 pages Add to your notes from your readings. Answer: Page 513 # 1-5