Ohm’s Law

Charge Pump Positive terminal of the battery pumps up the charges and gives them a lot of potential energy. This energy is given up to the load as the charges pass through the load. Charge returns to the negative terminal of the battery with much less potential energy

voltage V E per unit charge volt (v) v = 1J/1C Quantity Symbol Definition Unit Fact voltage V E per unit charge volt (v) v = 1J/1C

voltage V E per unit charge volt (v) v = 1J/1C current I Quantity Symbol Definition Unit Fact voltage V E per unit charge volt (v) v = 1J/1C current I Rate of flow of charge amperes (amps) 1 amp = 1C/s

voltage V E per unit charge volt (v) v = 1J/1C current I Quantity Symbol Definition Unit Fact voltage V E per unit charge volt (v) v = 1J/1C current I Rate of flow of charge amp (a) 1 a = 1C/s resistance R Opposition to current Ohms (Ω)

Electric Current vs. Water Flow

Factors affecting Resistance in wires thickness: The thicker the wire the lower the resistance. length: The shorter the wire the lower the resistance. type of metal: Gold, silver, and copper have the lowest resistance temperature: The lower the temperature, the lower the resistance.

Physics Challenge When do light bulbs blow? When they have been burning a while or when they are first turned on? First turned on Why? Temperature of the filament is cold, so resistance is low. This means that the current is high. Weak filaments will melt and break under this surge of current

Length of Wire demo Work like dimmers in dining rooms Variable speed blenders and fans

Voltage, Resistance, and Current Vary voltage Relationship? I α V In a circuit, current is directly proportional to voltage.

Voltage, Resistance, and Current Change resistance Relationship? I α 1/R Current is inversely proportional to resistance.

Using the Multimeter Measuring Current: Current flows through the circuit so connect it inside the circuit. (Series) Use the 10A setting Make sure the leads are plugged into the 10A and COM ports

Using the Multimeter Measuring Voltage: Voltage is potential difference between two points so measure around the device. (Parallel) Use the 20 V setting Make sure the leads are in the COM and V ports.

Using the Multimeter Measuring Resistance: Resistance is the opposition to current flow through a device. (Parallel) Use the 200 Ω setting Make sure the leads are in the COM and Ω ports.

Ohm’s Law V = IR R = V/I 15:05

Problem Set 1 1. What is the resistance of an electric frying pan that draws 12 amps of current when connected to a 110 v circuit? 2. How much current is drawn by a 25 Ω lamp when a voltage of 12 v is applied? 3. What is the voltage of a battery if it produces a current of 0.75 amps in a 12 Ω resistor?

Effects of Current on the Body 0.001 a (1 ma) – barely felt 0.005 a (5 ma) – painful 0.010 a (10 ma) – muscles contract 0.015 a (15 ma) – loss of muscle control 0.100a (100 ma) – can be fatal if current goes through heart

Safety Notes Electricians work with one hand in their pocket to avoid current passing through the heart. Make first touch with back of your hands so muscles contract away from wire. 100 Ω – 500,000 Ω

Video Problem Set 2 For each problem, calculate the current running through the body and describe its effect: 1. Your hands are wet (R= 2400Ω) and you touch the terminals of a 12v battery. 2. Your R = 7000Ω and you insert a paper clip into a 110v electrical socket. 3. A 9.0v portable CD player falls into a hot tub where your R = 100Ω?

Video Problem Set 3 1. Why isn’t the bird electrocuted? 2. If you fell from a tree onto the same wire, would you be electrocuted? 3. Should you touch the bird to rescue him?