Chapter 20 - Electric Circuits Physics

Electric Current • Electric current is simply the flow of electric charges • Charge flows when there is an electric potential difference, or difference in potential (voltage), between the ends of a conductor. (Think of water pipes. Water can only flow if there a difference in gravitational potential energy between two points) • The flow of charge will continue until both ends reach a common electric potential. When there is no potential difference, there is no longer a flow of charge through the conductor • The unit of electric current is called the ampere (A) – named after André-Marie Ampère (1775–1836), French mathematician and physicist

flow of charge - When the rate of flow of charge past any cross section of wire

cross section is 1coulomb per second, the current is 1 ampere

( ) => Electric current(I) = rate of charge flow = ( ) amount of charge q I = [A]change in time t 풒

휟풕 Q1) A wire carries a current of 2.0A. How much charge passes by a point in 55 seconds? a) 110C b) 100C c) 90C d) 80C

Q2) 175C of charge pass a point in a wire in 35s. What is the current in the wire? a) 1A b) 3A c) 5A d) 8A

Q3) How much charge, in coulombs, passes through a cross section of a particular wire every second if the current in the wire is 3A? How many electrons would this charge be equivalent to? (q= ne, 1e = 1.6×10-19C) a) 5C, 3.2×1019e-/s b) 3C, 1.9×1019e-/s c) 5C, 1.9×1019e-/s d) 3C, 3.2×1019e-/s

1 Voltage Sources and Resistance i) Voltage Sources • Charges do not flow unless there is a potential difference(=Voltage) • A battery is one example of a voltage source. • A battery has two terminals. One terminal is marked ‘+’ and the other is marked ‘-’. • If you connect a wire between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (and wear out the battery very quickly -- this also tends to be dangerous, especially with large batteries, so it is not something you want to be doing).

ii) Resistance

In physics, resistance is defined as the opposition to the flow of charges through a conductor. When charges flow, it has kinetic energy. By placing a resistor, like a light bulb, the resistor converts some of the kinetic energy of the charges in motion into heat and light.

When rubbing your hands together, the friction converts some of the kinetic energy of motion into heat energy. The filament of the bulb does the same thing, converting the electrical energy into heat and light.

The electrical resistance(R) of a circuit component or device is defined as the ratio of the voltage(V) applied to the electric current(I) which flows through it:

R = [Ω] 푉 The unit for resistance is called ohm [Ω] (after Georg Ohm, German, 1789 – 1854) 퐼

Ohm’s Law *~ important! If the resistance is constant over a considerable range of voltage, then the current in a circuit is directly proportional to the voltage across the circuit, and is inversely proportional to the resistance of the circuit

( ) current(I) = ; Ohm’s law ( ) 퐯퐨퐥퐭퐚퐠퐞 퐕 퐫퐞퐬퐢퐬퐭퐚퐧퐜퐞 퐑 I = 푽

푹 *~Ohm’s law is not universal. Materials that obey Ohm’s law are mostly conductors. Semiconductors do not obey Ohm’s law~*

2 I = 푽

푹 Q4) Find the current through a bulb that has a resistance of 3Ω and a potential difference of 9V across it. a) 12A b) 9A c) 6A d) 3A

Q5) A component has a potential difference of 3V across it and a current of 2A through it. Calculate its resistance. a) 1.5Ω b) 3.0Ω c) 4.5Ω d) 6.0Ω

Q6) Find the current through a 3Ω resistor with a voltage of 1.5V a) 0.5A b) 2.5A c) 5.5A d) 8.5A

- The main cause of electric shock in the human body is current, not the voltage. - The resistance of your body depends on its condition and ranges from 100Ω if you are soaked with salt water to about 500000Ω (500 kΩ) if your skin is very dry. - If you touched the two electrodes of a battery with dry fingers, the resistance your body would normally offer to the flow of charge would about 100000Ω. - You usually would not feel 12V, and 24V would just barely tingle. If your skin were moist, 24V could be quite uncomfortable.

Effect of Various Electric Currents on the Body Current in Amperes Effect 0.001 Can be felt 0.005 Painful 0.010 Involuntary muscle contractions 0.015 Loss of muscle control

0.070 If through heart =

3 I = 푽

푹 Q7) If the resistance of your body were 100000Ω, what would be the current in your body when you touched the terminals of a 12V battery?

Q8) If your skin were very moist so that your resistance was only 1000Ω, and you touched the terminals of a 24V battery, how much current would you draw?

Working with Circuit Diagrams

+ Battery Resistor

_ Switch

Current Flow in a Circuit • It is customary to describe current as flowing from a higher potential to a lower potential. • This is opposite the directions of electron flow (electrons are attracted to the higher potential and move from the point of lower potential).

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