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Learning Objectives

Learning Objectives. Define Voltage and average electric current. Use the expression of electric power to determine its value. Solve questions related to Current and Power. Explain what is an electric resistor and factors affecting on it. Learning Outcomes.

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Learning Objectives

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  1. Learning Objectives • Define Voltage and average electric current. • Use the expression of electric power to determine its value. • Solve questions related to Current and Power. • Explain what is an electric resistor and factors affecting on it.

  2. Learning Outcomes • Students can understand about the Voltage and current and can solve problems related to Power , Current and Voltage. • Students can learn about Resistor.

  3. Voltage • Voltage is a representation of the electric potential energy per unit charge. • Voltage is a scalar quantity. The SI unit of voltage is the volt, such that 1 volt = 1 joule/coulomb.

  4. ELECTRIC CURRENT • The electric current is the rate at which charge flows through cross sectional area.

  5. Average current • ΔQ is the amount of charge that flows through an area A in a time interval Δt and direction of flow is perpendicular to the Area. Iav=ΔQ/Δt • S.I unit of electric current is coulomb/ second ΔQ-------- charges flowing Δt --------- time during which charge was flowing

  6. Charge carriers • Current is the result of flow of both positive and negative charges, these charges are known as “Charge Carriers”. • Example: In a metal charge carriers are electrons. • total energy for charges = ΔU= q Δv • If charge is negative ΔU= -q Δv • The energy carried by an electric current is related to the voltage, E =qV.

  7. Section Current and Circuits 22.1 Electric Power Power is equal to current times the potential difference. • Units of Power is watts, (W, measures the rate at which energy is transferred) • If a generator transfers 1 J of kinetic energy to electric energy each second, it is transferring energy at the rate of (p =E/t units J/s), or 1 W. PowerP= IV

  8. Example 1 • The amount of charge that passes through the filament of a certain light bulb in 2.0 s is 1.67C. • A. Find the average current in the light bulb ? • B. the number of electrons that passes through the filament in 5.0s?

  9. Q 17.1 • A 9.0 v battery delivers a current of 1.34A to the light bulb filament of packet flash light. • A. how much charge passes through the filament in 2.0min? • B. how many electron passes through filament?

  10. Resistor & Resistance • A resistor is a device designed to have a specific resistance. • Resistance • The property determining how much current will flow is called resistance.

  11. Resistance • The resistance, R, is defined as the ratio of electric potential difference, V, to the current, I. • Resistance = R= V/I • Resistance is equal to voltage divided by current. • Units of Resistance is Ohms(W) • One ohm (1 Ω ) is the resistance permitting an electric charge of 1 A to flow when a potential difference of 1 V is applied across the resistance.

  12. Example

  13. Factors depend on Resistance • The resistance of any material with a uniform cross-sectional area is determined by the following factors: • Material • Length • Cross-sectional Area • Temperature

  14. Section Current and Circuits 22.1 • The table below lists some of the factors that impact resistance.

  15. Learning Objectives • Define resistivity and its relation( between resistance, resistivity and material dimensions area and length). • Recognize the internal resistance of sources of electricity (emf). • State Ohm’s law • Use Ohm’s law to solve related problems

  16. Resistivity • Resistivity is the resistance per unit volume. • Resistivity is a measure of how strongly a material opposes the flow of electric current. • r = R A/L • r = resistivity (W*m)

  17. Example

  18. Internal Resistance of source(emf) • Emf refers to voltage generated by a battery. • The internal Resistance of the battery is very small r is known as emf electromotive force. • total resistance of the circuit is • Rtotal= r + R

  19. Emf • Electromotive "force" is not a force (measured in newtons) but a potential, or energy per unit of charge, measured in volts. Or • how much work would be done by moving an electric charge. • The electromotive force (EMF) is the maximum potential difference between two electrodes of a galvanic or voltaic cell.

  20. Ohm’s Law • The unit for resistance is named for German scientist Georg Simon Ohm, who found that the ratio of potential difference to current is constant for a given conductor. V=IR

  21. Ohmic devices: Obys Ohm’s Law (Most metallic conductors obey Ohm’s la) • Many important devices, such as transistors and diodes in radios and pocket calculators, and lightbulbs do not obey Ohm’s law. • Example: A 1-m length of a typical wire used in physics labs has a resistance of about 0.03 Ω.

  22. Example • When V is 6 V and R is 30 Ω, the current is 0.2 A. • According to Ohm’s law, • the greater --------voltage (placed across a resistor) the larger the current passing through it. • If the current through a resistor is cut in half, the potential difference also is cut in half.

  23. Resistance Effect • Greater resistance……… • to reduce the current replace the 30-Ω resistor with a 60-Ω

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