Electric Current Physics

electric current

the flow of charged particles

• measured by the amount of charges that pass per second

• only flows when there is a potential difference/voltage

What is the equation for current?

I= q/t

• I= current (amps)

• q= charge (coulombs)

• t= time (IN SECONDS)

resistance

• opposition to a flow of current

• unit = ohms (Greek letter)

• variable= R

• due to collisions between electrons and fixed atoms

What are the three parts to an electrical current?

• current, resistance, battery (source of potential difference)

What is the relationship between current, voltage, and resistance?

• higher the resistance —> lower the current (inverse relationship)

• higher the voltage= more potential difference —> faster current (directly proportional)

Ohms law

• V=IR or any variation of this

circuit diagrams

• a way to draw circuits

• short line next to a longer line = battery (voltage)

• resistor = squiggly line

• connect with a wire/ a line

What does an ammeter measure?

• measures amps so CURRENT

• needs to be placed in the circuit/ in series

• if you want to measure the current coming out of a specific resistor then put it in line with that resistor

What does a voltmeter measure?

• volts so the potential difference of a resistor

• has to be placed on either side of a resistor

Electrical power

• work over time = power (units = WATTS)

• P=VI=I²R=V²/R (in ref table)

• the equation you use depends on the the variable you have

What is another way to say power?

• at what rate is electrical energy transformed in the light bulb

• rate of energy

What happens in terms of energy when an electron hits an atom/ the resistance?

• electrical energy is transformed into thermal energy

• resistance = more collisions = more heat generated

Electrical energy

• basically same kind of formula as with power but solved for work and has time (in seconds)

• W=VIT=I²RT= V²T/R

• in ref table

What determines resistance?

• bigger the area —> less resistance (more space for current to travel through)

• longer something is → more resistance (more atoms to bump into)

• material also affects resistance

resistivity

a material’s ability to resist the flow of electrical charge (how much it can resist)

• Variable = slanted p (Greek r)

• Units= Ohms x meters

• will have to look up the resistivity in ref table (like coefficient of friction) depending on the material (also has the units)

• metals have a low resistivity (lets current/ electrons go through them)

resistivity equation

R= slanted p L/A

R= resistance (ohms)

slanted p= resistivity (ohms x meters) (100 cm = 1 m)

L= length (meters)

A= cross sectional area (m²)

• If a problem says 20 Celsius it is a resistivity problem (not actually relevant to solving)

series circuit

one device is connected after another in a simple loop

• current only has one path to take

• voltage is DIFFERENT (add up voltage of each resistor to get total)

• current is the SAME

• the total resistance = just add up the separate resistances

What is the analogy for a series circuit?

• waterfalls that flow from high potential to low potential

• start at different heights and are different heights (have different voltages)

• but the current has to stay the same otherwise the water would run out or overflow (current has to be the same)

How would a break in a series circuit affect the other resistors/ lightbulbs?

• everything would go out

• no elections flowing/ no current

Parallel Circuits

• several paths for the current to take

• resistors are parallel to each other

• the current is DIFFERENT

• voltage is SAME (at the same height = no potential difference in relation to each other)

Parallel circuits waterfall analogy

• the waterfalls are at the same height —> has the same potential difference

• there is a different amount of water flowing in each —> current changes in each resistor

How do you find the total resistance in a parallel circuit?

1/Rt = 1/R1 + 1/R2

• take each resistor and put it under one

• solve BUT you have to FLIP the fraction in order to find Rt NOT 1/Rt

How does a break in a parallel circuit affect the circuit?

• does not affect the other branches

• there are still other paths for the electrons to go

Kirkoff’s Rule

• current going in = current going out

• if 10A is going in and then comes to a junction (multiple paths), the current in has to equal the current going out

• if 10 A is in, 10 A has to go out

Example of Kirkoff’s rule?

• 5A and 7A in (12A) - only 3A out what value has to also be going OUT 9A because 12= 9+3