Physics 121 Electricity Magnetism Lecture 7 Current Resistance

Physics 121: Electricity & Magnetism – Lecture 7 Current & Resistance Dale E. Gary Wenda Cao NJIT Physics Department

Definition of Current is the flow of electrical charge, i. e. amount of charge per second moving through a wire, i = dq/dt. q It is a scalar, not a vector, but it has a direction—positive in the direction of flow of positive charge carriers. q Any way that you can get charges to move will create a current, but a typical way is to attach a battery to a wire loop. q Charges will flow from the + terminal to the – terminal (again, it is really electrons that flow in the opposite direction, but current is defined as the direction of positive charge carriers). q Units: ampere 1 A = 1 C/s 11 A - 3 A = 8 A Total current out in 8 A October 17, 2007

Current in a Circuit q What is the current in the wire marked i in the figure below? 11 A - 3 A = 8 A Total current out in 8 A October 17, 2007

Current At Junctions q What is the current in all of the wire sections that are not marked? 5 A 3 A 2 A 6 A 8 A October 17, 2007

Try One Yourself 1. What is the current in the wire section marked i? 1 A A. B. C. D. E. 1 A. 2 A 2 A. 5 A 7 A. Cannot determine from information given. 3 A 2 A i 6 A October 17, 2007

Current Density When we care only about the total current i in a conductor, we do not have to worry about its shape. q However, sometimes we want to look in more detail at the current flow inside the conductor. Similar to what we did with Gauss’ Law (electric flux through a surface), we can consider the flow of charge through a surface. To do this, we consider (charge per unit time) per unit area, i. e. current per unit area, or current density. The units are amps/square meter (A/m 2). q Current density is a vector (since it has a flow magnitude and direction). We use the symbol. The relationship between current and current density is q High current density in this region Small current density in this region October 17, 2007

Drift Speed q Let’s look in detail at one happens when we connect a battery to a wire to start current flowing. Current Thermal motions of electrons—no net drift Electrons in direction opposite to i Signal t ravels t hrough the wire at speed o the f light _ 1. 5 V battery + October 17, 2007

Drift Speed The drift speed is tiny compared with thermal motions. q Thermal motions (random motions) have speed q Drift speed in copper is 10 -4 m/s. q Let’s relate drift speed to current density. q A vd L Total charge q in volume V n density of charge carriers time to drift a distance L +e means J and vd in same direction -e means J and vd in opposite directions ne is carrier charge density r October 17, 2007

Increasing the Current 2. When you increase the current in a wire, what happens? A. The number of charge carriers stays the same, and the drift speed increases. The drift speed stays the same, and the number of charge carriers increases. The charge carried by each charge carrier increases. The current density decreases. B. C. D. October 17, 2007

Resistance is defined to be. That is, we apply a voltage V, and ask how much current i results. This is called Ohm’s Law. q If we apply the voltage to a conducting wire, the current will be very large so R is small. q If we apply the voltage to a less conducting material, such as glass, the current will be tiny so R is very large. q q The unit of resistance is the ohm, W. (Greek letter omega. ) 1 ohm = 1 W = 1 volt per ampere = 1 V/A Resistor V Circuit Diagram R + i i small, large, so large R small 1. 5 V battery _ glass wire filament October 17, 2007

Current Through a Resistor 3. What is the current through the resistor in the following circuit, if V = 20 V and R = 100 W? A. 20 m. A. 5 m. A. 0. 2 A. 200 A. 5 A. B. C. D. E. V Circuit Diagram R October 17, 2007

Current Through a Resistor 4. If the current is doubled, what changes? A. V The voltage across the resistor doubles. Circuit Diagram The resistance of the resistor doubles. The voltage in the wire between the battery and the resistor doubles. The voltage across the resistor drops by a factor of 2. The resistance of the resistor drops by a factor of 2. B. C. D. E. October 17, 2007 R

Resistivity and Conductivity Rather than consider the overall resistance of an object, we can discuss the property of a material to resist the flow of electric current. q This is called the resistivity. The text uses (re-uses) the symbol r for resistivity. Note that this IS NOT related to the charge density, which we discussed earlier. q The resistivity is related not to potential difference V and current i, but to electric field E and current density J. High resistance Definition of resistivity q Units V/m over A/m 2 = Vm/A = ohm-meter =W m q Note that the ability for current to flow in a material depends not only on the material, but on the electrical connection to it. Note use (re-use) of s for conductivity. NOT surface charge density. Low resistance Definition of conductivity October 17, 2007

More on Resistivity q Since resistivity has units of ohm-meter, you might think that you can just divide by the length of a material to find its resistance in ohms. since resistivity is Resistance from resistivity q Dependence on temperature: you can imagine that a higher temperature of a material causes greater thermal agitation, and impedes the orderly flow of electricity. We consider a temperature coefficient a: October 17, 2007

Resistivity of a Resistor 5. A. B. C. D. E. Three resistors are made of the same material, with sizes in mm shown below. Rank them in order of total resistance, 4 greatest first. I. I, III. I, II. II, I, III, I. 4 II. 5 Each has square cross-section 2 6 III. 3 October 17, 2007

Ohm’s Law Ohm’s law is an assertion that the current through a device is always directly proportional to the potential difference applied to the device. q A conducting device obeys Ohm’s law when the resistance of the device is independent of the magnitude and polarity of the applied potential difference. q A conducting material obeys Ohm’s law when the resistivity of the material is independent of the magnitude and direction of the applied electric field. 4 Does not obey Ohm’s Law Slope = R R=1000 Slope = 1/RW Potential difference (V) q 2 0 -2 -4 -2 0 2 Current (m. A) October 17, 2007

Electric Power Recall that power is energy per unit time, (watts). Recall also that for an arrangement of charge, dq, there is an associated potential energy d. U = dq. V. q Thus, Rate of electrical energy transfer q Units: 1 VA = (1 J/C)(1 C/s) = 1 J/s = 1 W q In a resistor that obeys Ohm’s Law, we can use the relation between R and i, or R and V, to obtain two equivalent expressions: Resistive dissipation q In this case, the power is dissipated as heat in the resistor. October 17, 2007

Superconductivity In normal materials, there is always some resistance, even if low, to current flow. This seems to make sense—start current flowing in a loop (using a battery, say), and if you remove the battery the current will eventually slow and stop. q Remarkably, at very low temperatures (~4 K) some conductors lose all resistance. Such materials are said to be superconductors. In such a material, once you start current flowing, it will continue to flow “forever, ” like some sort of perpetual motion machine. q Nowadays, “high-temperature” superconductors have been discovered that work at up to 150 K, which is high enough to be interesting for technological applications such as giant magnets that take no power, perhaps for levitating trains and so on. q October 17, 2007

Ohm’s Law 6. The three plots show voltage vs. current (so the slope is R) for three kinds of device. What are the devices? A. Resistor, superconductor, diode Diode, superconductor, resistor Resistor, diode, superconductor Diode, resistor, superconductor Superconductor, resistor, diode C. D. E. II. Potential difference (V) B. III. Current (m. A) October 17, 2007

How Do Batteries Work? A battery is a source of charge, but also a source of voltage (potential difference). q We earlier saw that there is a relationship between energy, charge, and voltage. q Thus, a battery is a source of energy. We describe a battery’s ability to create a charge flow (a current) as an electromotive force, or emf. q We need a symbol for emf, and we will use an E, but it needs to be distinguishable from electric field, so we will use a script E. q The unit of emf is just the volt (V). q Other sources of emf are, for example, an electric generator, solar cells, fuel cells, etc. Here is a case where two emf Ea i sources are connected in opposing R i directions. The direction of i Eb indicates that Ea > Eb. In fact, emf i a charges emf b. q October 17, 2007

Summary q q q q q Current, i, is flow of charge (charge per unit time), units, amperes (A). Net current into or out of a junction is zero. Current density, J, (current per unit area) is a vector. J is proportional to the density of charge carriers, ne, and the drift speed of the carriers through the material. Resistance, R, (units, ohms, W) is the proportionality between voltage V applied, and current, i. Ohm’s Law states that R is a constant. It is not always a constant, but if not, the device does not obey Ohm’s Law. Resistivity (r) and conductivity (s) are properties of materials. Resistivity units, ohm-meter. Resistance is related to resistivity by Electric power P (units watts, W) is. For resistors: October 17, 2007