1. Resistance

2. RESISTANCE: http://www.youtube.com/watch?v=5Qh-4Wm51pw (1.09 min)
(circuit; current; potential difference)

3. RESISTANCE:
“ Electrical Resistance is the ability of an object to resist the flow of electrons (ie. To limit the current).”
Electrons moving through the circuit collide with atoms.
These collisions can make it harder for the electrons to flow.
The more difficult it is for the electrons to flow  the greater the resistance.

4. RESISTANCE:
Copper has a low electrical resistance, making it good to use for electric wiring.
good conductor low resistance
Plastic has a very high resistance.
Insulator high resistance
Resistance allows electrical energy to be converted into other forms of energy such as light (light bulb) or heat (element of electric stove).

5. The resistance of an object depends on 4 factors:
Type of materials
(the more freely e- move  less resistance)
Cross-sectional area
(larger X section lower resistance, (stove cord vs kettle cord).
Length • (shorter lengthlower, resistance)
Temperature
(generally: hotter higher resistance)
e- are deflected by atoms that are moving more rapidly.

6. Circuit Water Analogy

7. Ohm’s Law This is called Ohm’s Law. Resistance is measured in ohms, Ω.
The current flowing through a resistor at a constant temperature is directly
proportional to the
voltage across the
resistor.
So, if you double the
voltage, the current
also doubles.
This is called Ohm’s Law.

8. Ohm’s Law
The resistance of an object/load is defined as the ratio of the voltage across the load to the current through the load.
R= R- Resistance (Ohm, Ω)
V- Voltage (Volt, V)
V=IR I- Current (Amp, A)
Example: What is the resistance of the heating coil of an electric heater, if a current of 12.5 A runs through it when it is connected to a wall outlet? (wall outlet 120V) R=9.60Ohms
Bicycles with battery operated lights often have different size bulbs for the front and rear lights. The filament in the front lamp has a resistance of 3 ohms. It takes a current of 0.6A. What voltage does it work at? 1.8V V I

9. RESISTORS:
Resistors are electrical components that are placed in circuits to control the flow of electrons around a circuit.
An ohmic resistor has a constant resistance regardless of potential difference.
Resistance can be determined by a graph of voltage vs current, slope will be the resistance
(similar to the density graphs we did in chemistry).
Do the following:
Worksheet on Resistance Problems
Page 570 # 3 Graph
Read page #2, 3, 5-7 p 566