Electric Current
Circuits ILD Electrical Power Batteries in Parallel
In a comon dc circuit, electrons move at speeds of:
a) centimeters per second.
b) the speed of a sound wave.
c) thousands of kilometers per second.
d) the speed of light.
e) none of these.
When like charges undergo a net motion in some specific direction-- e.g., electrons travel along a wire-- a current is said to be flowing. The path that charges follow in being pushed by voltage is called a circuit.
In the absence of very large voltages (potential differences), current needs a "charge highway" (e.g., wire) to travel along. A break in the charge highway, that charges can't be pushed across, results in a broken circuit.
All electrical appliances-- including light bulbs, stereos and televisions, electric heaters and golf carts-- require current flow in order to function. Electric current is measured in units of amperes, defined as:
Electrons in motion comprise the electric currents we use every day. In most metals the nuclei of atoms (protons & neutrons) are bonded together and some of their electrons are relatively free to move around. This is why metals are used to manufacture wire. Placing a voltage across both ends of a wire causes the electrons to accelerate along the wire. They can be pushed through the filament of a lamp and cause something useful to happen-- light!
An idealized voltage source (battery or power plant) is assumed to be a very large resevoir of charge (electrons). The voltage source does work on the electrons to move them, increasing the electron's kinetic energy. When the electrons travel they can do work on other things, giving up their energy gained from the voltage source.
If you could somehow see microscopically inside the filament of a light bulb carrying electric current, you would see electrons accelerating in the direction of current flow. They would occasionally run into the atoms making up the filament, and would be scattered like one pool ball hitting another.
This scattering of electrons reduces their energy and transfers it to the atoms of the filament. The atoms of the filament can be thought of as vibrating from the collision. Some of this energy of vibration would go towards making light. Most would be lost as heat transfer to the surroundings.
Electrons colliding with atoms amounts to a resistance to their passage through a material.
As mentioned before, most metals are good conductors of electricity (electrons), so they resist the flow of electrons less than other materials and are used to manufacture pathways for electron flows-- wires. Other materials, e.g. most plastics, ceramics and rubber products are highly resistant to electrons passing through them. These materials are used to clad wires, preventing the electrons from leaking out of the wire or jumping to another wire.
Some poor resistors (good conductors):
Some good resistors (poor conductors):
One analogue for voltage, current flow and resistance is show below:
The green part of this analogous model shows a way to visualize what happens as electrons pass through conducting materials. Marbles on the top have more gravitational potential energy than do those at the bottom. Hence, they roll down the slanted surface. This is analgous with the case of electrons in a wire connected to a battery, which gain electrical potential energy in the battery and "roll" through the wire.
As the marbles roll down, they collide with some of the posts, and lose energy to the posts. The posts vibrate from the collisions. This is analgous to electrons colliding with atoms in a material, losing energy to the atoms, and heating the atoms as a result. The filament of an incandescent heats up following this analogy, and the result is light emission as the atoms cease to vibrate (before the next electron-atom collision).
A material that has more resistance to the flow of electrons can be thought of as having more posts per area of material. Wires, which have low resistance to electricity (and are called good conductors) have fewer "posts."
The above analogy falls short in that you might imagine that the marbles would travel faster in the feeder trays (wires) than in the resistive material (light bulb element).
In the case of electricity, the current is the same everywhere in the wire, which means that the same number of electrons are crossing per second at each location along the path. In real circuits, the resistive circuit elements cause traffic jams which propogate backwards (and forwards) along the current path.
Also, don't think that the electrons coming out of one end of a wire are the same ones going in the other end, as is true for our "marble-electrons."
Circuits ILDLet's explore current "flow" through some real battery+bulb circuits.
When speaking of macroscopic objects in motion, we defined Power as work done per unit time (units of energy in J (joules) per time (seconds). A joule per second is a watt (W).
Recall that voltage is energy/charge (units joules/coulomb) and that current is charge moving over time (units are coulombs/second). Thus voltage times current, V x I, is energy per time, or power:
Some of the power supplied to any circuit is lost as "heat," the increase in average kinetic energy of atoms and molecules of metals, air, water, pins hit by marbles, etc. In electrical circuits this process is known as "Joule heating," named after James Prescott Joule. Using Ohm's law to substitute for voltage, one can see that power is given by:
Fuses exploit this principle. If a "short" occurs in a circuit, a path for electricity to flow that has too little resistance, then current will increase through the entire circuit. As long as the fuse has the least tolerance for heat, the increased current will heat the element of the fuse until, eventually, it will melt, breaking the circuit.
Batteries in ParallelConsider an analogy to a battery... a mass on a table.
This particular table can also lift the mass from its base to its top.... giving the mass gravitational potential energy (the potential for gravity to do work on the mass if it falls). So this special table is analogous to a battery, and the mass is like a charge that is pushed through a connected circuit by the "charge push" of the battery.
What's involved in hooking up two batteries in parallel?
Using our table analogy, we simply slide in another table, with its own mass, next to the first. The second table can do work on its own mass, but it can't do work on the mass on the first table. The second table's mass, when dropped, will have no more energy than the mass dropped from the first table. The "charge push" (voltage) will not change when the second "battery" is wired in parallel to the first.
How do we rig up our tables to represent two batteries in series. We stack the second table on the first. The first takes its mass and lifts it to the top. The second special table lifts the mass up even higher. Now, when the mass is dropped, more work can be done on it (as evinced by the higher speed it attains in falling twice the original distance). So the "charge push" (voltage) is doubled!
Previous Lecture 
Next Lecture 
Back to Course Page. 
