PRE-LECTURE 8: CURRENT, OHM'S LAW AND RESISTANCE

Suppose you place the terminals of a 1.49 V battery across. piece of metal (a wire) and find that 6.3 A of current flows through it. What is the electrical resistance of this metal (in Ohm's)?

Delta-V= I * R Delta-V/ I = R 1.49/ 6.3 = 0.24 ohms

SUMMARY QUESTIONS: What is electric current? how is it defined?

Electric current is defined as the rate of the flow of charge per second. It is measured in coulombs per second (amps).

What is Ohm's law?

Ohm's law says that the larger the voltage across an "ohmic" object (metals), the faster the current will flow through that object, proportionally.

Which of the following statements is consistent with Ohm's law? a. If you reduce the voltage across an object by a factor of two, you will reduce the current flowing through that object by a factor of two. b. If you reduce the voltage across an object by a factor of four, you will reduce the current flowing through that object by a factor of two. c. If you increase the voltage across an object by a factor of two, you will reduce the current flowing through that object by a factor of two. d. If you reduce the voltage across an object by a factor of two, you will reduce the current flowing through that object by a factor of four.

a. If you reduce the voltage across an object by a factor of two, you will reduce the current flowing through that object by a factor of two.

If you double the voltage across a potential difference across a resistor, you will __________ the current (I) flowing through that resistor. a. double b. triple c. quadruple

a. double

Real currents are comprised of ____________ moving through wires. a. electrons b. protons c. none of the above

a. electrons

If you decrease the voltage (V) and decrease the resistance (R) across a wire, the current (I) will ____________. a. increase b. decrease c. stay the same

a. increase

If you increase the voltage (V) across a wire, the current (I) will ____________. a. increase b. decrease c. stay the same

a. increase

If you increase the voltage (V) and decrease the resistance (R) across a wire, the current (I) will _____________. a. increase b. decrease c. stay the same

a. increase

Units of resistance is: a. ohms b. coulombs c. coulombs/second

a. ohms

The difference in potential (deltaVR) across a resistor (wire, light bulb) is directly ______________ to the current (I) running through that resistor. a. proportional b. disproportional c. none of the above

a. proportional

a low current causes the electrons in the wire to move______. a. slow b. fast c. stay the same

a. slow

The more resistive the object is, the ___________ the charge flowing through it. a. slower b. faster c. stays the same

a. slower

Why do the electrons in a current (I) move in a steady direction (bouncing to that direction) when a battery is attached? a. the battery creates an electric field for the electrons to move in (opposite to the electric field) b. the battery creates more force on the electrons c. none of the above

a. the battery creates an electric field for the electrons to move in (opposite to the electric field)

What is an electric current? a. the rate at which charge flows per second. b. the rate at which capacitance flows per second c. the rate at which coulombs flow per second.

a. the rate at which charge flows per second.

electric current in the human body: 1-5 mA (milliamps): a. tingling sensation b. pain c.death

a. tingling sensation

Current (I) is measured in: a. Amps: seconds/coulombs b. Amps: coulombs/second c. Amps; Coulombs/hour

b. Amps: coulombs/second

What happens to the electrons in a current (I) when a battery is attached? a. They remain still b. They move in a direction opposite to the electric field of the battery c. They move in different directions

b. They move in a direction opposite to the electric field of the battery

Wires are made up of atoms that _______ ______ everything in the way which causes them to move down the wire. a. stay still b. bounce off c. none of the above

b. bounce off

If you know the electric potential difference (Voltage) that exists in that object, you will know how much ___________ will flow through that object. a. resistance (R) b. current (I) c. none of the above

b. current (I)

If you decrease the voltage (V) and increase the resistance (R) across a wire, the current (I) through the wire will ____________. a. increase b. decrease c. stay the same

b. decrease

If you increase the voltage (V) and the resistance (R) across a wire, the current (I) will _____________. a. increase b. decrease c. stay the same

b. decrease

If you lower the voltage across a wire, the current (I) will _____________. a. increase b. decrease c. stay the same

b. decrease

delta-V is? a. difference in current b. difference in electric potential c. difference in charge

b. difference in electric potential

Rubber is an example of an: a. excellent conductor b. excellent insulator c. none of the above

b. excellent insulator. Rubber doesn't allow electric charges to flow through easily. Rubber has a resistance of 1 quadrillion microohms.

A high current causes the electrons in the wire to move __________. a. slow b. fast c. stay the same

b. fast

How do we treat electric currents? a. like negative charges, moving to the right b. like positive charges, moving to the right c. like positive charges, moving to the left

b. like positive charges, moving to the right

electric current in the human body: 5-10 mA (milliamps): a. death b. pain c. tingling sensation

b. pain

What can happen to the human body if a current flows through between 10-20 mA? a. you will experience pain throughout the body b. you will experience muscle contractions that will be difficult to let go of the current c. death

b. you will experience muscle contractions that will be difficult to let go of the current

A cubic centimeter of COPPER (excellent conductor) has a resistance of _______ microohms. a. 10 microohms b. 20 microohms c. 1 microohms

c. 1 microohms. Copper allows electric charges to flow through easily.

How much current (I) is flowing in a span of 2 seconds/ 5 minutes/ 1 hour. a. Instantaneous b. Microseconds c. Average

c. Average

Electric current in the human body: more than 1 A (1000+ mA): a. tingling sensation b. pain c. combustion (catch on fire)

c. combustion (catch on fire)

electric current in the human body: 10-20 mA (milliamps): a. tingling sensation b. pain c. muscle contractions

c. muscle contractions

Electric current in the human body: 20-100 mA: a. muscle contractions b. pain c. paralysis, heart stops

c. paralysis, heart stops

Any object that may resist the flow of electric charge is a ____________. a. current b. voltage c. resistor

c. resistor

What happens to the electrons inside of the current (I) when a battery is NOT attached? a. They remain still b. They move in a direction opposite to the electric field of the battery c. They move in different directions

c. they move in different directions

What is electrical resistance?

electrical resistance is the hindering of the flow of current (R). This is measured in ohms (voltage/amps).

When a lot of charge flows quickly, the current is __________.

high

when charge flows slowly, the current is ________.

slow

The larger the resistance (R) in a circuit, the _______ the current tends to move through it.

slower

How can we predict how much current (I) you'll recieve if you contact the terminals of a battery, an electric outlet, or any value of delta-V?

we wouldn't really know unless we knew the difference in electric potential (voltage)(delta-V) through the wire