11-Electric Circuits

Current (I) is the rate at which charge passes through a cross section of wire with the equation I = Δq/Δt

Current is measured in Amperes (A) or amps with an ammeter where 1 Amp = 1 C/s

Current is generated by a potential difference (ΔV) across a conducting path. In circuits with batteries, like we often use, the ideal potential difference of the battery when no current is running through the circuit is also called electromotive force or EMF (ε)

How current flows: 

• The direction of conventional current (what we are talking about when we say "current") is in the direction positive charges would drift in a circuit, which would be from high potential to low (or out the positive end of a battery). 

• Conventional current is opposite the actually movement of electrons, which are the charged particles that actually move in a conductor (since the protons in metal atoms are locked in place in the nucleus). 

• Charges do not move in a straight, orderly line through a circuit despite the way current is often animated. The charges are in constant random motion traveling through conducting atoms that are in constant random motion, but they drift with an average speed in the direction described above. 

A circuit is made of electrical components like those shown in the diagram that are connected with wires or a conducting path. 

• A single circuit may contain several electrical loops that through which current may flow. 

• closed circuits have no breaks in the path (for example, switches are closed) and therefore allow charges to flow

• open circuits have a disrupted pather where charges cannot flow. This can be due to an open switch or a component that is damaged or removed without reconnecting the wires.

• Short circuit is a loop where charges can flow without changing potential - typically this is an alternate path to a loop  that was designed to have current, but, due to the short circuit, the components get no ΔV and therefore no current flows through it. 
  Practice tracking potential through a circuit using level 1 and 2 of the Know Your Potential simulation at PhysicsClassroom.com.

When drawing a circuit, use the symbols shown in the diagram and represent the wires connecting the components as straight wires with 90 degree angles, when possible. 

• The location of the components in physical space is much less important to the diagram than the connections (where junctions - splits and joining - or wires are and the order of components along the loop path.) 

Resistance R measures how much an object opposes electric current and is measured in ohms Ω.

Resistivity ρ indicates how strongly a material resists charge flow based on molecular structure and is measured in Ωm.

Resistivity is a fundamental property of the material (silver has a low resistivity as compared to tungsten, but a 1Ω resistor can be made of either one) where resistance is a property of a circuit component (2 resistors of the same material can have separate resistances, 50 Ω and 100 Ω).