Electricity!Potential Difference, Current,

Resistance, Ohm’s Law, Circuits

Circuits, and symbols to learn

Wire

Resistor Light Bulb

Battery

Moving Charges

• What makes charge move?– Electric Potential Difference!

Electric Potential Difference

• The difference in Potential between two points. Positive charge moves from high potential to low potential.

– Unit: Volts (V)

Electric Potential Difference

• What is the potential difference between the two ends of a 1.5V battery?

Electric Potential Difference

• Why aren’t the birds getting shocked?

Electric Current

Electric Current

Electric Current

Electric Resistance

• Some objects are better conductors than others. In these better conductors the charge has less resistance in moving through the material.

• Electric Resistance: the measure of how hard it is for charge to move.– Unit: Ohms (Ω)

Electric Resistance

increasing the resistance - decreases the currentdecreasing the resistance - increases the current

•What affects Electrical Resistance?

Electric Resistance

• What affects Electrical Resistance?

• Resistance is lower when you increase the surface area.– Why?

Electric Resistance

• Resistance is lower for a shorter wire– Why?

Electric Resistance

• Resistance is lower for a shorter wire– Why?

• For most conductors – Resistance is higher when at higher temperatures– Why?

Electric Resistance

• Resistance is lower for a shorter wire– Why?

• For most conductors – Resistance is higher when at higher temperatures– Why?

• Exception: Carbon…much lower resistance at higher temperatures

Electric Resistance

• Superconductors have ZERO resistance at very low temperatures

– Applications for a superconductor

Ohm’s Law

• How do we relate Electric Potential Difference, Current, and Resistance?

• Ohm discovered that the current in a circuit is directly proportional to the voltage across the circuit and inversely proportional to the resistance of the circuit.

Ohm’s Law

or

units:

Ohm’s Law and Electric Shock

Ohm’s Law and Electric Shock

• Current (the flow of electrons) is what hurts us when we get shocked.

Ohm’s Law and Electric Shock

• Current (the flow of electrons) is what hurts us when we get shocked.

• How much current is enough to give us pain?0.005 A

Ohm’s Law and Electric Shock

• Current (the flow of electrons) is what hurts us when we get shocked.

• How much current is enough to give us pain?0.005 A

• To kill us?

Ohm’s Law and Electric Shock

• Current (the flow of electrons) is what hurts us when we get shocked.

• How much current is enough to give us pain?0.005 A

• To kill us?

Lightning Strike

• Typical voltage for a lightning strike:~ 3 x 108 V

Lightning Strike

• Typical voltage for a lightning strike:~ 3 x 108 V

• Typical human resistance? Lets find out!

Lightning Strike

• Typical voltage for a lightning strike:~ 3 x 108 V

• Typical human resistance? Lets find out!

• Current?

Direct Current or DC

• Direct Current: flow of charge that always flows in one direction.

– Example: a battery. Electrons always moves from one end of the battery to the other through the circuit.

Alternating Current or AC

• Alternating Current: electrons in the circuit first move in one direction then in the opposite, alternating back and forth about relatively fixed positions.

– Example: The wall socket. The voltage and current of nearly all of these AC circuits alternates back and forth at a frequency of 60-hertz (Hz). The voltage of most of these circuits is 120 V.

– Why?

Source of Electrons in a Circuit

Source of Electrons in a Circuit

• Electrons already exist in the conductor as a part of it. The voltage just provides the energy for the motion of the electrons!

Power

• What do we remember about power?

Power

• What do we remember about power?

• Power = Energy/time (Watts = Joules/second)– The work done over time, or the transfer of

energy over time.

Electric Power

• electric power = current x voltage 1 W = (1 A) x (1 V)

P = V I

Electric Power

• electric power = current x voltage 1 W = (1 A) x (1 V)

P = V I =

• And… V= IR

Electric Power

• electric power = current x voltage 1 W = (1 A) x (1 V)

P = V I = I 2R

• And… V= I R

Electric Companies and Power

• Remember: Power = Energy/time – Electric companies bill you for the amount of

energy you use…

Electric Companies and Power

• Remember: Power = Energy/time – Electric companies bill you for the amount of

energy you use…

• Energy = Power x Time

Electric Companies and Power

• Remember: Power = Energy/time – Electric companies bill you for the amount of

energy you use…

• Energy = Power x Time

• Kilowatt-hour