ELECTRIC CURRENT (I)
• The movement of charge.
• SI unit is the Ampere (A, Amps)
• Two types: direct and alternating
• Direct – batteries
• Alternating – wall outlets, light fixtures
•We use “conventional current”
• The direction of movement of a proton
• EVEN THOUGH we know protons don’t move!!!!
𝐼 =𝑞
𝑡
EXAMPLE: A STEADY CURRENT OF 2.5A EXISTS IN A WIRE FOR 4.0 MIN. A. HOW MUCH CHARGE PASSES A POINT IN THE CIRCUIT?B. HOW MANY ELECTRONS IS THIS?
TASK: USE THE WIRES, LIGHT, BATTERY AND SWITCH. ATTACH ALL OF THESE PARTS TO MAKE THE BULB LIGHT.
• 1. Draw a picture of your setup
• 2. Open the switch. What happens?
• 3. Switch the battery (turn it around). What happens?
• 4. Can you tell which way current is flowing?
CIRCUIT – A CONTINUOUS CONDUCTING PATH CONNECTED BETWEEN THE TERMINALS OF A BATTERY
• Circuit – comes from the word “circle”
• Why does this make sense?
• Closed circuit – continuous conducting path , current flows
• Open circuit - a break in the path; no current flows
• Instead of a picture – we draw circuit
diagrams with symbols
• Cause THIS AINT ART!!!!
• Always in a block or triangle
FLOW OF CURRENT IN A CIRCUIT:
• For conventional current:
• Negative terminal has 0 voltage
• As charge flows to + terminal, voltage is gained
• Wires are considered perfect conductors (THEY AREN’T
THOUGH!!!) so they don’t take away voltage.
• Resistors and bulbs take voltage
• When the charge reaches the negative terminal again,
there is NO voltage
OHM’S LAW – IN METAL CONDUCTORS RESISTANCE (R) IS A CONSTANT INDEPENDENT OF POTENTIAL DIFFERENCE (V)
• Resistance (R) – friction of a circuit
• Slows down current
• Measured in Ohms (Ω)
• Decreases current
• Not all resistors are “Ohmic”
• We engineer resistors to behave differently at different voltages (Semiconductors)
• “Ohmic” resistors have a linear relationship!!
𝑉 = 𝐼𝑅
EXAMPLE: A small flashlight bulb draws 300 mA from its 1.5 V battery.
• A. What is the resistance of the bulb?
• B. If the battery becomes weak and the voltage drops to 1.2 V, what would
happen to the current?
ELECTRIC POWER (P) -
• Measured in Watts (W)
• How else could we write the equation, using Ohm’s Law?
• Example: Calculate the resistance of a 40.0 W headlight designed for 12 V.
𝑃 = 𝐼𝑉
HOW DOES THE ELECTRIC COMPANY CHARGE?
• An electric heater draws a steady 15.0 A on a 120 V line. How much power
does it require?
• How much does it cost per month if it operates for 3.0 hours per day and the
electric company charges 9.2 cents per kWh?
RESISTORS IN PARALLEL AND SERIES
• Parallel
• Current splits at junctions
• Same voltage across each branch
• Series
• Connected end to end
• Same Current throughout
1
𝑅𝑒𝑞=
1
𝑅1+
1
𝑅2+⋯
𝐼𝑇 = 𝐼1 + 𝐼2 +⋯
𝑅𝑒𝑞 = 𝑅1 + 𝑅2+…
𝑉𝑇 = 𝑉1 + 𝑉2 +⋯
EXAMPLE 1:
1. Series or parallel?
2. Calculate equivalent
resistance.
3. Calculate potential
difference and
current across each
resistor.
EXAMPLE 2
1. Series or parallel?
2. Calculate equivalent
resistance.
3. Calculate potential
difference and
current across each
resistor.
EXAMPLE 3:
1. Series or parallel?
2. Calculate equivalent
resistance.
3. Calculate potential
difference and
current across each
resistor.
EXAMPLE 4:
1. Series or parallel?
2. Calculate equivalent
resistance.
3. Calculate potential
difference and
current across each
resistor.
COMPLEX CIRCUITS – CONTAIN SERIES AND PARALLEL ELEMENTS• Combine parallel elements first – replace with equivalent resistor
• Redraw your diagram each time you combine resistors.
• Work backward to find I and V for each element.