Unit 9 Review Electricity Physics
Electrical Force
A force that one charge exerts on another. When the charges are the same sign, they repel; when the charges are opposite, they attract. -act between 2 or more "charges" -repel and attract -large compared to gravitational forces Fe=Kg1g2/d^2 F+ F- BIG
Both Electrical Force and Gravitational Force
-act the same everywhere in the observance universe -act in fields surrounding the mass or charge -act over a distance (contact isn't required)
Current
-electron flow through a wire in series, the current is the same everywhere (always equal)
Series Circuit
-single loop for current to flow
Voltage drop
-the amount the voltage changes when it passes through a resistor in a series circuit, the voltage drops add together to equal total voltage
Equivalent resistance
-total resistance in a circuit, Rt, in a series circuit, the total resistance is equal to the sum of all the resistors
If eight electrical devices are connected in a series circuit, then the number of current paths is
1 The number of current paths through a SERIES circuit will ALWAYS be ONE!
Three resistors with values of 8 Ω , 2 Ω , a n d 16 Ω respectively are connected in parallel. What is their equivalent resistance?
1.45 Ω Yes. Using the equation 1/Rt= 1/R1 + 1/R2 + 1/R3 you should get the correct answer.
If a hair dryer expends 30 J of energy in 20 seconds, how much is the power rating of the hair dryer?
1.5 W You needed to use the equation E= P x t in order to get this problem correct.
What is the power rating of a toaster with a current of 8 A and 16 V?
128 W You needed to use the equation P= I x V in order to get this problem correct.
Four resistors with values of 5 Ω , 8 Ω , 10 Ω , a n d 3 Ω respectively are connected in series. What is their equivalent resistance?
26 Ω Yes. The correct equation is Rt= R1 + R2 + R3 + R4
If four electrical devices are connected in a parallel circuit, then the number of current paths is
4 In a PARALLEL circuit, the number of current paths is equal to the number of resistors you have.
A computer dissipates 1,200 watts of power for 45 seconds. The amount of electrical energy used by the computer is approximately
5.4 x 10^4 J Yes! You needed to use the equation E=P x t in order to get the correct answer.
An electric microwave is operated by applying a potential difference of 30 V across a nichrome wire with resistance of 5.0 \Omega Ω . What current does it draw?
6 A
What is the current through an electric skillet using 200 W plugged into a 30 V outlet?
6.67 A You need to use the equation P=I x V in order to get the correct answer.
Types of Current: AC
-alternating current -current switches direction
Types of Current: DC
-direct current -flows in only one direction -usually a battery source
A device that measures current in a circuit is called a(n) __________ .
Ammeter
What causes electrical force?
Charge is the property of an object that causes electrical force
If you decrease the length of a wire, what effect will it have on resistance?
IT WILL DECREASE THE RESISTANCE The longer the wire, the higher the resistance will be. If you decrease the length, you are also decreasing the resistance.
Current Flow: what direction does conventional current flow?
It flows from the + electrode to the - electrode
Open circuit
NO FLOW WHATSOEVER
You are given 3 light bulbs arranged in a parallel circuit. A rabbit comes along and gently removes one of the light bulbs from the circuit. What happens to the other two light bulbs?
NOTHING HAPPENS TO THE OTHER TWO BULBS Removing one branch has no effect on the voltage to the other two branches, so the other two branches continue to burn with the same intensity. Total current in the circuit will decrease, but that does not change the brightness of the bulbs.
A voltmeter is always connected in
Parallel (Outside the circuit)
An ammeter is always connected in
Series
Coulomb's Law
The electric force between two charged objects is directly proportional to the charge on each body and inversely proportional to the square of the distant between them.
How many types of charges exist? What are they?
There are two types of charges protons are positively charged and electrons are negatively charged. Charge is measure in Coulombs (C)
A voltmeter measures
Voltage
Electrical System Components
Voltage source: battery, power supply or generator Conductors: wire or connectors Load: resistor, appliances, etc Control element: switch, volume control, etc
Wall Plugs
alternate 60 times each second or frequency = 60 Hz
If you break the circuit at R1
the current still has a path to follow
If you are given a 10 V battery, wire, and five identical light bulbs, in which configuration would you put them in in order to result in the brightest burning bulbs?
ARRANGE THE LIGHT BULBS IN A PARALLEL CIRCUIT Arranging resistors (light bulbs) in parallel will result in a greater current going through each of the bulbs which will cause them to burn brighter!
If you add more resistors in a parallel circuit, what will happen?
INCREASE THE CURRENT DRAWN If you add resistors in a parallel circuit, then you are decreasing the total resistance therefore increasing the current drawn. Remember: If you add more resistors in PARALLEL then the overall resistance will DECREASE! Also, current & resistance are INVERSELY related!
What does it mean to conserve charge?
No electrons are created or destroyed they just transfer from one object to another or the net electric charge in an isolated system never charges. (there are not any 1/2 charges)
Compare Series vs Parallel
Series Circuit -single path -voltage drops add to equal voltage -current constant throughout circuit -total resistance=sum of resistors Parallel Circuit -multiple paths -voltage is the same in each branch -currents add -total resistance is smaller than the smallest resistor
Moving charges-outline the process that creates current
Step 1-charges are surrounded by electric fields Step 2-fields exert forces on other charges Step 3-force cause charges to move Potential difference:provides the energy to move a charge Step 4- moving charges creates a current
Resistance
Symbol: R Unit: Ohms Cause: It is caused by collisions between electrons and atoms in a material Resistance depends on material, length, and cross-sectional area When the wire length increases resistance increases When the cross-sectional area of the wire increases the resistance decreases Voltmeters measures voltage and ammeters measure current Ammeters are connected in the main loop of the circuit (SERIES) -Ammeters have low resistance and measure in amps or mA Voltmeters are connected in its own loop in a circuit (PARALLEL) -Voltmeters have high resistance and measure potential difference or voltage across circuit elements
You are given 5 light bulbs arranged in a series circuit and observe the brightness of the light bulbs. You then take all the circuit pieces apart and rebuild a series circuit with only 4 of the original 5 light bulbs (mostly because you dropped the 5th light bulb and it broke). What do you notice about the circuit with only 4 light bulbs vs the original 5 light bulb circuit?
THE BULBS IN THE 4 LIGHT BULB CIRCUIT ARE BRIGHTER The more resistors you have in series, the more the overall resistance will be. If you take away one of the resistors, you are decreasing the overall resistance which increases the current through it.
If the battery voltage of a series circuit does not change, what will happen if you add more resistors?
THE CURRENT WILL DECREASE If you add more resistors in a SERIES circuit, then the overall resistance will increase. Resistance and current are inversely related.
Coulomb's Law: Breakdown of equation
The Fe in Coulombs Law stands for electrical force the unit is N The ke stands for constant The value of kc with units: Nm^2/C^2 Each q stands for charges the unit is c The d stands for distance the unit is m
Gravitational Force
an attractive force that acts between any two objects -act between 2 or more "masses" -only attracts -small compared to electrical forces F=Gm1m2/d^2 small
Current and Voltage
charges move due to forces and voltage difference
Electric Potential
measured in volts often called voltage or electric potential difference, created by an electric field, electric potential is the the potential energy in each charge
Parallel Circuit
multiple loops for current to flow through
Electrical force can _________ or __________
repel or attract
