Electricity & Circuits

I. Electricity Electricity is electrons moving through a conductor.

How is that different from static electricity? In static electricity, electrons don’t move Remember conductors allow the flow of electricity or heat and insulators prohibit the flow of electricity or heat

II. Potential Difference (V) (A.k.a.- Voltage) The push that causes electrons to move across a circuit. Voltage is the difference of electric potential between 2 points. Electrons move from high potential to low potential. Electrons ONLY move when there is a potential difference and a closed circuit Unit = Volts (V)

Voltage Source A battery or electrical outlet is a source of electric potential - not charge The net charge on a current-carrying conductor is zero.

III. Current (I) The flow of electric charges (energy) through conductor. Unit = Ampere or Amp. (A) Electrons moves from negative to positive Conventional current is the flow of energy from positive to negative

Types of Current Direct Current (DC) charge will flow in one direction - from high to low potential. Ex: Battery Alternating Current (AC) current will flow “back and forth” in the circuit. Ex: Generator, Electrical Outlet

IV. Electrical Resistance (R) A material’s resistance to the flow of electrons. Resistors change electrical energy into thermal energy and light. I.e. – Light bulbs All materials have some resistance. Things that influence resistance: Making wires thinner, longer, or hotter increases resistance Unit = Ohms (Ω)

V=IR V. Ohms’ Law Voltage equals Current times Resistance: Current and Resistance are inversley related

VI. Electrical Power The rate at which electrical power is converted to another form of energy Electrical power is expressed in watts (W) 1000 W = 1kW Power = current X voltage difference P = IV Electrical Energy = Power X Time (E=Pt) Units = kWh

Test Problem An immersion heater has a resistance of 4.0 ohms while drawing a current of 2.0 amperes. How much electrical energy is delivered to the heater during 100. seconds of operation?

VII. Electrical Circuits Require a complete path from positive to negative for current to flow A circuit requires a power source, resistor, and a conductor. An open circuit does not allow a current to flow

Series Circuits The current has only one loop to flow through The loss of one component results in an open circuit Voltage depends on resistance VT = V1 + V2 + V3 … Current is constant RT = R1 + R2 + R3 ...

Series Schematic 7Ω 9Ω 5Ω 7Ω + 9Ω + 5Ω = 21Ω What is the total resistance of the circuit? 7Ω 9Ω 5Ω 7Ω + 9Ω + 5Ω = 21Ω

Example: When the switch is on, what is the total resistance of the circuit if R1=2Ω and R2=4. What is the total current of the circuit?

Test Problem What must be inserted between points A and B to establish a steady electric current in the incomplete circuit represented in the diagram A. switch B. voltmeter C. resistor D. Source of potential difference

Test Problem A s B Which series circuit diagram above would the light bulbs be the dimmest? Why?

Parallel Circuits Multiple branches for current to flow through Loss of a component does no affect the rest of the circuit Current depends on resistance I = I1 + I2 + I3… Voltage constant (1/RT) = (1/R1) + (1/R2) + (1/R3) R-1T = R-11 + R-12 + R-13 …

Parallel Schematic R = 9/3 = 3 Ω 9Ω V = 18 V What is the total resistance of the circuit? 9Ω V = 18 V 1/R = 1/9Ω + 1/9Ω + 1/9Ω = 3/9 Ω R = 9/3 = 3 Ω

Example: When the switch is on, find the following: If R1=2 and R2=4, calculate the total current. Find the current at R1 and R2.

Which set of resistors has the smallest equivalent resistance?

Connecting an Ammeter and Voltmeter An ammeter is connected in series. A voltmeter is connected in parallel.