Measuring current

• Electric current: rate of flow of electrons past a specific point in a circuit

• Measured with: Ammeter• Symbol: I• Unit: ampere (A)• 1 A = 6.2 x 1018 electrons/s = 1 C/s C= coulomb

Ammeter

• Must be connected IN SERIES with a load to measure the current through that load

Potential Difference

• The difference in electric potential energy per unit charge measured at two different points

• the force that moves electrons• the amount of energy given to electrons to

move them• also called Voltage• Measured with: Voltmeter• Symbol: V Unit: volt (V)

• There is potential difference between the two terminals of an electric cell

• Electrons leave the negative terminal with electric potential energy to operate the load

• They return to the positive terminal with no potential energy because it was used to energize the load

• The electrons are “re-energized” in the electric cell

Electrons leaving electric cell have a lot of potential energy

Electrons returning to the positive terminal have lost their potential energy/ given it to the lamp/load

Voltmeter

• The device used to measure potential difference

• Must be connected in PARALLEL with a load or energy source

•The negative side of the voltmeter is connected to the negative side of the battery

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Electrical Resistance

• The ability of a material to oppose the flow of electric current; measured in ohms ()

• All materials have some internal resistance• Greater resistance lower current warmer

material• Electrons flowing through the material bump

into atoms that make up the material and some of the electrical energy they carry is converted into thermal energy

Factors that Affect Resistance

1) Type of material• Conductors such as copper and silver allow electrons to

pass through easily and thus has low electrical resistance2) Cross-sectional area• Wire’s with a larger diameter have lower resistance than narrower wires• Electrons have more room to move freelyd R

3) Length• Internal resistance increases as the wire length

increases because electrons have to travel through more material

• l R

Ex. Extension cords are made using large-diameter wire to reduce the resistance as result of their length. If not, they would pose a fire threat due to overheating

4) Temperature• As electrons move through the wire they

bump into atoms and release thermal energy• The wire’s atoms gain energy, vibrate faster

and more collisions result• More collisions increased resistance• T R

Ohmmeter

• Device used to measure resistance across a load

• Must be connected in PARALLEL with a load• The ohmmeter provides an electric current

therefore the circuit should not be powered to measure resistance

How Stuff Works - Toaster• When a toaster is plugged in

current flows through a copper wire with little resistance

• The heating element is made up of an alloy with much greater resistance

• The electrons slow down, bump into each other a lot and release thermal and light energy – the glowing element

• Same is true for elements on the stove

Resistors in Circuits

• An electrical device that reduces current in a circuit

• Used to adjust brightness of lamps, to protect devices from current overload

Fill in this chart in your notesElectrical Quantity

Electrical quantity symbol

Unit of Measurement

Unit of Measurement Symbol

Device used to measure quantity (if applicable)

Electrical Energy

Joules J

Electric current I Ampere A Ammeter

Potential Difference

V volt V Voltmeter

Resistance R Ohms Ohmmeter

• Potential difference/voltage (V): The difference in electric potential energy per unit charge measured at two different points

• Electrical Resistance (R): the ability of a material to oppose the flow of electric current; measured in ohms ()

• Electric current: rate of flow of electrons past a specific point in a circuit

Ohm’s Law

• Georg Ohm discovered a mathematical relationship between potential difference and current in the 1800s

• R = V / I I = V / R V = I x R• As the potential difference across a load

increases, so does the current

Sample Problem – calculate the resistance across a load

V

An electic cell with a potential difference of 6.0V is connected in a circuit with a lamp. A current of 2.0 amperes flows. All the wires are resistance-free. What is the resistance of the lamp?

V = I R R=V/I R = 6.0 V / 2.0 AR = 3.0 Therefore the resistance of the lamp is 3.0 Ohms

Sample Problem 2 – calculating the voltage across a load

• A toaster oven has a 24. 0 resistor that has 5. 0 A of current going through it when the toaster is on. Calculate the potential difference across the resistor

• R = 24. 0 • I = 5. 0 A• V = IR• V= (5. 0 A)(24. 0 )• V= 120 V• The potential difference across the resistor is 120V

Sample Problem 3 – calculating the current across a load

• A laptop computer adapter has a voltage of 19V. It has a resistance of 4.0Ω. The adapter gets warm when operating. Determine the current through the adapter.

• R=4.0Ω• V=19V• I=V/R• =12V/4.0Ω• I=3.0A• Therefore the current through the adapter is 3.0A