Electric Current and Circuits Ch 18 Electric Current

Electric Current and Circuits Ch. 18

Electric Current • • • A net flow of charge Variable = I Unit = Ampere (A) I = Δq/Δt Conventional current is the direction a positive charge would flow

Potential Difference • Just like a ball will not fall if there is not a difference in gravitational potential, an electron would not move (ie no current generated) if there is not a difference in electric potential • To have a current, you need a potential difference.

EMF • Potential difference maintained by an ideal battery • EMF is measured in volts (V) • Measure of the work done by the battery per unit of charge • W = Ԑq

Current, Water, and Batteries • Water runs down an incline passing through a current is at the water wheel. When the water resistor current back up to bottom, a battery person carries the water the top.

Batteries and Voltage • A 9 V battery keeps a positive terminal that is 9 V higher in potential difference than the negative terminal. • The battery does 9 J of work for every C it pumps through. The battery does work by converting stored chemical energy into electric energy.

More about Batteries • Batteries come in different EMFs (voltages) (1. 5 V, 6 V, 9 V, etc) and different sizes (AAA, C, D…) • The common batteries all have 1. 5 V. This means a larger batter can last longer or supply charge faster than a smaller one.

Types of Currents • Direct Current – The current in any branch always moves in the same direction • Alternating Current – The currents periodically reverse directions.

Electrons and Current • Since current was defined (by Albert Einstein) to be the direction a positive charge would flow… • Electrons move in the direction opposite the current.

Resistance • The current (I) that flows through a conductor is proportional to the potential difference (ΔV) that supplies it. (Ohm’s Law) • Some materials allow current to flow more freely than others. A measure of how well the current flows is called resistance. • R = ΔV/I • Or more commonly… V = IR • Resistance is measured in ohms (Ω)

Resistance of Materials • R = ρL/A • Long wires provide more resistance than short wires • Skinny wires provide more resistance than fat wires • When in doubt, think of a water hose.

Superconductors • Materials with a resistivity approaching zero when cooled to a very low temperature (close to absolute zero) • Resistance also increases when the temperature increases.

Resistors • In a circuit, resistors are materials that cause a drop in voltage • Typically the resistance is known

Kirchhoff’s Rules • At a junction, the current entering the junction is equal to the current leaving a junction. • The net voltage drop around a circuit is zero. All the potential created by the battery must be used up by the resistors.

Series Circuits • The same current flows through each resistor

Series Circuits • The total resistance in a series circuit is a sum of all the individual resistors connected in series • RT = R 1 + R 2 + R 3 + … • The total resistance is larger than any of the individual resistances

Series Circuits • Things that are connected in series have the same current, but different voltages (unless they have the same resistance)

Series Circuits • For a Resistor V = IR • For a capacitor V = Q/C • For multiple capacitors in series the total capacitance is • 1/C = 1/C 1 + 1/C 2 + 1/C 3 + …

Parallel Circuits • Resistors are wired so that the potential difference across them is the same.

Parallel Circuits • Things that are connected in parallel have the same voltages, but different currents (unless they have the same resistance). • Benefits to parallel circuits… – When one light bulb goes out, the current still has a path to travel through so the other light bulbs stay lit.

Parallel Circuits • 1/RT = 1/R 1 + 1/R 2 + 1/R 3 + … • The total resistance for a parallel circuit is smaller than any of the individual resistors. • Capacitors in a parallel circuit: C = C 1 + C 2 + C 3 + …

Drawing Circuits • Things you must have… – Battery – long side is the positive terminal and short side is negative terminal. The current leaves the positive end. – Wire – Resistor – Drawn as zig zag lines, not light bulbs. Each resistor must be labeled. – Switch – to open or close the circuit (not always necessary)

Solving Circuit Problems • Simplify the resistors • Assign variables to the current in each branch (I 1, I 2, I 3…) and choose a direction for each. Draw the circuit with the current flow indicated by arrows. • Apply the Junction Rule • Apply the loop rule – If your loop goes against the current in a resistor, V is +. If your loop goes with the current, V is – – If your loop goes from – to + terminal in a battery, the voltage is +. From + to – is a negative voltage.

Electric Power • P = IV • P = I 2 R • P = V 2/R