Electricity and Circuits

Electricity and Circuits Chapter 20-23

Electrostatics • Study of non-moving electric charges • Positive charges – protons • Negative charges – electrons • Like charges repel • Opposite charges attract • Much stronger than the force of gravity, but can cancel out (gravity can only attract and therefore cannot be canceled) • Depends inversely on the square of the distance and directly on the amount of charge involved

Creating Charged Particles • Most objects are electrically neutral (their positive and negative charges cancel out) • But, charges can be transferred between objects, resulting in an object becoming charged • Usually done by electrons moving between materials • Things with loosely held electrons (like metals) carry charge well, we call these conductors

Separation of Charges • Instead of actually gaining or losing charges, an object can rearrange its charges so all the negatives are on one side and all the positives are on the other • When two oppositely charged objects are near each other, they can create a path for electrical discharge to occur between them (like lightning connect the ground to the clouds)

Electric Fields • Electric force is a field force (contact isn’t necessary for it to be exerted) • Measured as how a positive charge will be treated in the test field • Can interact with the fields of other electric charges to create complex patterns • By forcing a charged particle into a like charged field, you create electric potential energy (or by pulling a charged particle away from an opposite charged field)

Electric Equilibrium • Like all objects, charges want to reach the lowest possible energy state (equilibrium) • This causes charges to move to create a net electrical force of zero • You are safe inside a metal car that has been struck by lightning because the charge spreads equally around the exterior to give a charge of 0 inside

Electric Current • Caused a difference in electric potential energy (voltage) • Current flows from high voltage to low voltage until equal, then flow stops • If you continue to pump charges into one object, you can maintain the difference and therefore maintain the current flow • That’s what batteries and electric outlets do

Current Flow • Unit is an ampere • Too much current can burn through a circuit • Directly proportional to voltage difference and inversely proportional to resistance • Resistance of objects can change due to temperature (hot has less resistance) or becoming wet (damp has less resistance) or the thickness (thick wires have less resistance)

Circuits • In order for the current to flow, it needs to have a closed loop to move through • As the charges are moved, their energy can be converted into other forms by the devices that are plugged into it • Within the circuit, charge and energy are always conserved

Series Circuits • Only one path for electrical flow exists • All current must pass through the same points • The voltage drop across each device depends on its resistance since the current must be the same at all points • Resistance for the circuit is the sum of the individual resistors • If one resistor is broken, or the flow is interrupted at any point in the circuit, the entire circuit ceases to work (like Christmas lights)

Parallel Circuits • Multiple current paths are available • The current will be split between the paths • The voltage drop each device receives will be identical since each has its own connection to the power source • Multiple resistors connected in parallel decreases the overall resistance of the circuit (and increases the current) • One device will not knock out the whole circuit, but overheating is more likely with more resistors connected

Circuits in Your Home • Usually a combination of both types • Short circuit occurs when a low resistance path gets formed (energy will choose the path of least resistance) • Safety devices include: • Fuse – melts if too much current flows • Circuit breaker – metal bends up to create hole if too much current flows • Grounding line – creates a low resistance path to the ground to keep a person from being shocked

Electricity and Circuits