Circuit Analysis: Videos:

- https://youtu.be/KsqmxApA9-s (recap) - https://youtu.be/-w-VTw0tQlE (crash course on circuits) - https://youtu.be/JbJ6aGYa63k (Explanation of Kirchhoff laws) - https://youtu.be/Z2QDXjG2ynU (example of Kirchhoff’s laws) - https://www.youtube.com/watch?v=vf8HVTVvsdw (Potential dividers) - https://youtu.be/k077CwKGiwk (emf) - https://youtu.be/R1QN_0fcZ_8 (emf and battery internal resistance)

Notes:

Series circuit: has only one path for electrons to flow, which makes the current the same all throughout.

Parallel circuit: has multiple paths for the current to flow, hence, the current is different at different points of the circuit. But the voltage across each component remains constant (work is independent of path and voltage is W/q).

Complex circuit: a combination of series and parallel.

Common mistake: students tend to forget that after the addition of terms is done, they need to take the reciprocal of the sum to get the Req.

Ammeter: an electrical device that measures current - Has to be connected in series, because it has very low internal resistance. - Ideal ammeters have zero resistance

Voltmeter: measure potential difference across a device

- Has to be connected in parallel because it needs to see the difference of voltage going in and voltage going out

- An ideal voltmeter should have an infinite internal resistance because it shouldn’t take up current. We do that because we want ideal theoretical readings.

Kirchhoff’s laws: Junctions rule: the total current entering a junction will be equal to the current leaving, due to conservation of charges

I1+I2+I3=I4+I5 Iin=Iout

Loop rule: the summation of all potential difference (voltage) in a closed circuit is zero, because work done in a closed cycle is zero. (V=W/displacement), the sum of the loads is equal to the sum of the sources

Vin=Vout • Source: device delivering energy into a system (usually a cell), • Load: device extracting energy from a system (usually a resistor)

Rules to follow in loop law:

1) For resistors: If the loop is with the direction of current à Load If the loop is against the direction of the current à Source

2) Across cell: if the loop is from inside the cell (- to +) à Source If the loop is inside the cell (+ to -) à Load

• When using loop law in a circuit that is parallel, or parallel at some point, don’t get

Req, leave the branches • Don’t replace a loop equation within another loop equation, loop equation + junction

equation.

Potential dividers: potential divider is a simple circuit that uses resisters (thermistors / LDR's) to supply a variable potential difference, so resistance can be different from section to section in the wire:

Vout= ( "#$%&'("*+,+&'-.*)Vin

Conductor acting as a potential divider in a circuit:

• You can use he formula of the resistance of a wire: R= ρL/A • Which will result in giving us a ratio of length multiplied by Vin:

Vout= 012013

4,-

• Vin is usually the emf.

Cells and Batteries: Difference between primary and secondary cells: primary cells are the ones that cannot be charged but secondary cells are the ones that are rechargeable. Connecting cells:

• Connecting cells in series: increases input voltage but also increases resistance • Connecting cells in parallel: decreases internal resistance, but the input voltage does

not increase. (it reaches a maximum point) Emf: (electromotive force) it is the electric potential across the cell when no current is passing. Emf = energy/ unit charge supplied by a source A battery or a cell usually has internal résistance due to the motion of the electrolyte chemical.

• When solving, consider the internal resistance of a cell/ battery as a separate resistor. • The power dissipated is at the maximum value when the internal resistance is equal to

the external.

To find the emf and internal resistance of a cell, there is an experiment to be done. A simple circuit with a variable resistor and internal resistance is constructed. At varying points of resistance, the voltage and current are recorded to create a graph.

find: emf and internal resistance

• Emf= Ir + V {the emf is equal to the voltage drop created by the internal resistance plus the voltage across the variable resistor} (from loop law)

• V= -Ir + emf • V= -rI +emf

This looks like the formula: y = mx +b which means that: the internal resistance magnitude is equal to the slope of the graph the emf is the point at the y- axis at which the voltage starts.

Reminders for solving:

• Brightness is proportional to voltage • Remember that an electron volt is a measure of energy (Joules) • Use Kirchhoff’s laws when you are presented with a circuit including more than one

cell • the potential divider method is easiest when you have one cell.

Problems:

- http://www.saburchill.com/physics/questions/quest021.html (doesn’t contain solution)

- https://studylib.net/doc/8934111/ib-physics-hl-multiple-choice-review-electric-circuits

- https://peda.net/jao/lyseo/isac/sciences/physics/course-2017-2018/5eam/revision-questions/topic-5-revision:file/download/87ef30b75689747bbe905a862941b56302d30ab7/Topic_5_Electricity_31.1.2018.pdf (includes questions ahead of this topic)

- https://gradegorilla.com/microIB/5circuits/M_cells.php