Danger High Voltage! Your friend starts to climb on this... You shout Get away! That s High Voltage!!! After you save his life, your friend asks: What is Voltage anyway?
Voltage... Is the energy (U, in Joules) that a unit of (+) charge (1 Coulomb) would have at a given location. The formal term for it is: The Electric Potential (...or sometimes just the Potential ) Voltage is measured in Volts (V) = Joules/Coulomb (J/C) ΔU = q ΔV
Voltage Near a Charge V = k q/r P If more than one charge, then Voltage at point P is V(P) = kq1/r1 + kq2/r2 Where: r1 = dist. from q1 to P r 2 = dist. from q 2 to P (Note: q can be + or - ; r is always +) Figure 20-4
Consider 2 positive charges on the x-axis: What is the Voltage at the origin, where x = 0?
Consider 2 positive charges on the x-axis: What is the Voltage at other points along the x-axis, V(x)?
Positive everywhere. (and infinite at x = +1 & -1!)
Electric potential of two point charges of opposite sign.
Electric Field due to a point charge. 2-D Suppose the Voltage at Point X is: V = 2 Volts. X Where else would the voltage be the same? draw
Lines that connect points of equal Voltage (V) are called: Equipotential lines They are like contours of elevation on a topographic map.
Equipotential Surfaces & Electric Field On a map, contours mark constant elevation; a ball will roll downhill: perpendicular to the curves. The closer together the curves, the steeper the slope.
Heat maps are another form of contour maps
Figure 20-7 Equipotential lines on a capacitor. Where are they the highest?
Figure 20-5A Surface Plot
Figure 20-5B
Capacitors and Capacitance A capacitor is two conducting plates separated by a small distance. Use a battery to make a voltage difference between the plates (eg. ΔV=12 V) This causes charge to build up on each of the plates. But How much charge builds up? A A
Capacitance Well, the higher the Voltage, V, the more charge (Q) we can expect. Q ~ V The amount of charge that builds up on a capacitor, for a given voltage is defined as Capacitance =Capacitance (Note: Capacitance is always positive)
Analogy: Holding Water The word Capacitance is related to capacity. The capacity to hold charge is similar to the capacity of a container to hold water.
Capacitance & Units The units of Capacitance (C) are Farads (F) Most real-life capacitors have a Capacitance much less than 1F: (eg. mf, µf, nf, pf) Warning! The symbol for the quantity Capacitance (C) is the same as for Coulombs (C), which is the unit of charge. Capacitance = Charge per Voltage, the units are: 1 Farad = 1 Coulomb/Volt (F = C/V)
In many electronic devices, the capacitor is the largest electrical component. Some Capacitors
Chapter 21 Electric Current & DC Circuits Optional: Ammeters and Voltmeters (21-8) Optional: RC Circuits (21-9) Water
U
Reservoirs store potential energy in water at high elevation. This energy comes from the Sun.
Figure 21-3
Electric Current Electric current is the flow of electric charge from one place to another. If a charge ΔQ flows past a point in time Δt, then there is a current: In an electric circuit, charge always returns to its starting point (eg., a battery).
Current is defined as the amount of positive charge that flows in a chosen direction. (regardless of the type of particle that s actually moving) Examples: -In electronic devices, electrons (-) flow -In human nerve cells, ions (eg. Ca+, Na+, K+) flow If the chosen direction is up, then: -Positive charge flowing up or -Negative charge flowing down have the same effect.
Figure 21-2
Parts of a Circuit: 1. Battery: Provides charges with potential energy 2. Wire: made of a conductor 3. Load: component that does something, eg a light bulb 4. Switch: creates an open circuit (off) battery has + and - poles or terminals A circuit with no load is called a short circuit
Short Circuit! In a short circuit, a high current flows from one terminal of the battery to another....so high that the wire can heat up and start a fire. To prevent this, a fuse can be placed in the circuit. The fuse will blow if the current exceeds a certain level (eg 12 Amps)
A blown fuse results in an open circuit: no current flows. It must be replaced. A circuit breaker is a switch that prevents short circuits.