Electric Current and Circuits

Electric Current and Circuits

Potential Energy • Movement of a positive test charge within an electric field is accompanied • by changes in Potential Energy: • Moving the charge AGAINST the direction of an electric field is like moving • a mass upward within Earth’s gravitational field. (requires work by an outside force) • Would increase the PE of the object • Moving the charge in the SAME direction of an electric field is like a mass falling • downward within Earth’s gravitational field. (occurs without an outside force) • Would decrease the PE of the object

Electric Potential • Electric Potential = the amount of Potential energy per charge Electric Potential = Potential Energy charge

e e e e e e e • Electric current is the continuous flow of electrical charge • Current flows when there is potential difference (voltage) between two points • To create this difference, work must be done on the charge by a charge pump e e e e e e e e e e e e electron e

e e e e e e e Charge Pumps (sources of pumping electrons): • Photocell - converts light energy into electricity Examples: solar calculator, solar cell • Battery - converts chemical energy into electrical energy • Generator – converts mechanical energy into electrical energy Examples: steam turbine, nuclear electric plant

e e e e e e e Batteries • The first battery was created by physicist Alessandro Volto by stacking alternating layers of zinc, brine-soaked cloth, and silver. • It was the first device to have a lasting current (flow of charge)

Anatomy of a battery • Each battery has 2 terminals – positive and negative • A cathode connects to the positive terminal • An anode connects to the negative terminal Collectively, these are called electrodes and are where chemical reactions take place

Electric Circuit: • A complete conducting path through which electrons can flow • For a current to flow it must include a charge pump and be closed This is an open circuit. No current would flow while open

A load is a device using electrical energy. In a load, work is done by the electrons, and the PE of the electrons decreases. Since it opposes the flow of current, a load is a type of resistor Basic Circuit Open Switch Conductor Closed Switch + Battery Load - A light is an example of a load Fuse Resistor

Electric energy (from charge pumps) can be converted into: • Heat • Light • Sound

Activity 1 • Draw a simple circuit diagram with a battery (don’t forget the electrode signs), a load, and a closed switch. Load Battery Closed Switch

Fact or fiction? • When you turn on a light switch, electrons move rapidly from the socket to the lamp to the bulb. Fiction

What electrons really do • Electrons move slowly bouncing around in many directions as they move in one direction overall. Overall Direction Electron Drift

Heat is always a side effect of electric current (because of electron drift). Overall Direction Electron Drift

A potential difference (V) between two points creates an electric field • An electric field moves through a circuit at the speed of light, setting electrons in motion The electrical field caused by the voltage makes all electrons start to move simultaneously

Voltage (V) – energy per unit charge • Unit: Volt (V) • 1V = 1J/C Current(I) – rate of flow of charge • Unit: Amp (A) • 1A = 1C/s Resistance(R) – opposition to current • Unit: Ohm (Ω)

Factors affecting resistance of a wire • 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 most bulbs blow and why? • When first turned on (cooler). Less resistance = more current. • A large surge of current can cause a weak filament to break.

In a circuit, current is directly proportional to voltage (I α V) and inversely proportional to resistance (I α 1/R) . Ohm’s Law: V = I R

Problem Set 1 • What is the resistance of an electric frying pan that draws 12 amps of current when connected to a 110v circuit? • How much current is drawn by a 23Ω lamp when a voltage of 12v is applied? • What is the voltage of a battery if it produces a current of 0.75 amps in a 12Ω resistor?

Problem Set 1 • What is the resistance of an electric frying pan that draws 12 amps of current when connected to a 110v circuit?

Problem Set 1 2. How much current is drawn by a 23Ω lamp when a voltage of 12v is applied?

Problem Set 1 3. What is the voltage of a battery if it produces a current of 0.75 amps in a 12Ω resistor?

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

Safety Notes • Electricians often put their hand in their pocket to avoid current going through heart (just in case) • Electricians touch a wire they think is not live with the back of their hand so they are not stuck clinching the wire if they were mistakenly wrong

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

Problem Set 2 • Your hands are wet (R= 2400Ω) and you touch the terminals of a 12v battery.

Problem Set 2 2. R = 7000Ω and a paper clip is inserted into an 110v electrical socket.

Problem Set 2 3. A 9.0v portable CD player falls into a hot tub where your R = 100Ω

Problem Set 3 If your pet Peety the parakeet perches on an electric wire. • 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 reach up and touch the bird to rescue him?

Problem Set 3 If your pet Peety the parakeet perches on an electric wire. 1. Why isn’t the bird electrocuted? No potential difference between feet = no current 2. If you fell from a tree onto the same wire, would you be electrocuted? No potential difference between hands = no current 3. Should you reach up and touch the bird to rescue him? No, you would be the path from the bird to the ground and both of you would be toast. There would be potential difference in this case.

Electric Power- • Rate of converting electric energy into other energy forms • P = W power = work t time • The unit for power is the watt 1w = 1J/s • Another power equation P=IV

Problem Set 4 • An appliance draws 13 A when connected to a 110 v circuit. a) What is the power of the appliance b) What is its resistance 2. What is the power of a light bulb that has a resistance of 190 ohms in a 120 v circuit?

Problem Set 4 • An appliance draws 13a when connected to a 110 v circuit. a) what is the power of the appliance b) What is its resistance

Problem Set 4 2. What is the power of a light bulb that has a resistance of 190 ohms in a 120 v circuit?

“Power” companies sell us energy. The unit they use for energy is the kilowatt hour. Physics challenge: What is the smallest denomination coin you could use to pay for the energy used by a 60w bulb burning for 8 hours? (at a cost 16.0412 cents per kilowatt hour)

The multimeter • Used to measure current, voltage, resistance, etc. • The black lead is plugged into the common port • The red lead is switched depending on what you are trying to record

Measuring Voltage • Set up the multimeter to detect voltage. • The black cord should be connected in the middle slot of the multimeter (this will stay here for all readings) • The red cord will be placed in the slot to the right of the black that has V or Voltage as one of its symbols • Turn the knob to read Voltage for a direct current. Look for V and the symbol for direct current (The symbol for direct current is like an equal sign with the bottom line dashed) _______ _ _ _ _ • The voltmeter should be placed outside the circuit

Measuring Voltage • You are measuring the potential difference from one side of the resistor to the other • The volt meter should not be part of the circuit • Touch each probe to opposite sides of the resistor

Set up the multimeter to detect current • Set up the multimeter to detect current. • The black cord should be connected in the middle slot of the multimeter (this will stay here for all readings) • The red cord will be placed in the slot to the left of the black that has A for a reading of Amperes. • Turn the knob to read Amperes for a direct current. Look for A and the symbol for direct current.

To read current • The current must flow through the multimeter and it should be part of the circuit • Make it part of the circuit so that electrons must flow through it. • Open the circuit and attach one probe to each open end • This will once again make it a closed circuit with the multimeter acting as an ammeter in series.

Direct Current (dc) • Electrons move in one direction • Electrons follow a gradient to where there are less electrons (but we don’t draw it this way)

Alternating Current (ac) • Electrons change direction • In USA, direction changes 60 times per second

Conventional way to draw current • Show current moving from the negative to positive Load Battery Closed Switch

Series circuits • Series circuit: • Resistors are connected in a single path • Current must go through every resistor in order Circuit diagram:

Electric Current and Circuits