Transcript Electric Current and Circuits
Electric Current and Circuits
Can you get a light bulb to light?
• Using the equipment given to you, try to find as many ways as possible to get the light bulb to light.
• Diagram four ways in which you were able to get the light bulb to light. Label the battery, the wire, and the bulb.
• Diagram at least three ways in which you were not able to get the bulb to light.
Question: 1. From your observations, what conditions seem to be necessary in order for the bulb to light?
Successful Attempts
Potential Energy review 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
Potential Energy review 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 amount of charge A large amount of potential energy at a place where large amounts of charge are present results in a LARGE electric potential A small amount of energy for a given amount of charge results in a SMALL electric potential
Electric Potential The difference between two such points (one high potential and the other low) is called Electric Potential difference.
It is measured in volt, so it is also called “voltage”
Potential Difference When the positive charge is moved from A to B, work is being done on the charge and that work becomes Potential Energy. There is now a difference in electrical potential between locations A and B.
Potential Difference Ex: If the potential difference between 2 locations is 1 volt, then 1 Coulomb of charge will gain 1 joule of PE when moved between those 2 locations.
Ex: if the potential difference is 2…?
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) e e e e e e e
Basic Structure of a Circuit The internal circuit is the battery with electrochemical cells The external circuit is the wire and any “load” or circuit element (fan, light bulb, heater)
Basic Structure of a Circuit 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
Basic Structure of a Circuit There is an electric field between the 2 terminals, directed from the positive towards the negative terminal Movement of a positive test charge from negative to positive terminal would require work, thus increasing the PE of any charge that moves along this path
Basic Structure of a Circuit The positive terminal is referred to as the “high potential terminal” The negative terminal is referred to as the “low potential terminal”
Basic Structure of a Circuit In battery cells, chemical energy does work on a positive test charge to move it from the negative terminal to the positive terminal The chemical energy is transformed into electrical potential energy (within the battery) Once at the positive terminal, a test charge will then move through the external circuit
Basic Structure of a Circuit As the charge travels, it will lose PE as it transforms its PE into other forms of energy (light, thermal…) at the light bulbs, fans… The test charge returns to the negative terminal at low energy and low potential – it is ready to repeat the cycle again
Basic Structure of a Circuit If a 12V battery is used in the circuit, then every C of charge gains 12 J of PE as it moves through the battery.
Then every C of charge loses 12 J of electrical PE as it passes through the external circuit (lost due to other types of energy) The loss in electrical potential while passing through a circuit element is called “voltage drop”
Producing Electric Current This flow of charged particles is an electric current.
The flow stops when the electric potential difference between the two plates is zero.
If the charge particles are pumped back to the original conductor (plate) by an outside energy source (battery), the flow of charges will continue.
Requirements to establish an electric current There must be a closed conducting path that extends from the positive terminal to the negative terminal. It is not enough that there is simply a closed conducting loop; the loop itself must extend from the positive terminal to the negative terminal of the electrochemical cell.
Requirements to establish an electric current The second requirement is that there must be an electric potential difference across the two
ends
of the circuit. Some energy source, such as a battery or charge pump, is required
Electric Circuits 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
Electric Current Current is the rate at which charge flows past a point on a circuit.
Current flows from the positive terminal to the negative terminal.
Current (I) = q/t = Coulombs/sec=amperes (A) Example: A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 s. I = _____amperes Example: A 1 mm long cross section of wire is isolated and 2 C of charge is determined to pass through it in 0.5 s. I = ________amperes
Fact or fiction?
When you turn on a light switch, electrons move rapidly from the socket to the lamp to the bulb.
What electrons really do Electrons move slowly bouncing around in many directions as they move in one direction overall.
“Electron Drift”
Power Power (measured in watts, W) measures the rate at which energy is transferred. (1 W = 1J/s) The energy carried by an electric current depends on the charge transferred and the potential difference across which it moves.
Current is the rate of charge flow (I=q/t). A flow of 1 C/s is called an ampere (A).
Power is equal to the current times the potential difference.
P = I V Total energy transferred = P x t (W .
s or J)
Practice: A 6.0 V battery delivers a 0.50 A current to an electric motor connected across its terminals.
What power is delivered to the motor?
If the motor runs for 5.0 min, how much electric energy is delivered?
The Kilowatt- Hour The electric company (power company) provides energy rather than power. When a bill is paid, the customer is paying for the electric energy used.
Because of the large amounts of energy consumed, electric companies measure sales in a units of kilowatt-hours. One kilowatt hour is equal to 1000 W being delivered for 3600 s (1 hr) or 3.6 x 10 6 J.
Power is measured in units called 1.
2.
3.
4.
amperes joules kilowatt-hours watts
Current is measured in units called 1. Amperes 2. Coulombs 3. Volts 4. Ohms
The direction that conventional current flows in the external circuit is from the ___ terminal to the ____ terminal.
1. negative, positive 2. positive, negative 3. negative, negative 4. positive, positive
Electrons move very quickly through an electric circuit.
1. True 2. False
If the current is measured as 6 A, then ____ C of charge flows past any point in the circuit in 3 s 1. 0.5
2. 2 3. 3 4. 18
1.
2.
3.
4.
If a 9.0 V battery delivers 0.50 A of current to a heater, the power consumed by the heater is 0.056 W 4.5 W 9.0 W 18 W
Resistance Resistance (R) is the opposition to the flow of charge.
Resistance is measured in units called ohms (Ω).
Factors Impacting Resistance Length: Resistance increases as length of wire increases.
Cross-sectional area (thickness): Resistance increases as cross-sectional area decreases.
Temperature: Resistance increases as temperature increases.
Material: Resistance varies with the material used.
silver Ohm’s Law Resistance is equal to potential voltage divided by current. V = I R The current in a circuit is directly proportional to the voltage (potential difference) The current is indirectly proportional to the resistance. This means that the greater the voltage, the greater the current, and the greater the resistance the lower the current 1. For a wire that obeys Ohm’s Law, the resistance of a wire depends on the ___ the wire. A. current in B. length of C. power delivered by D. voltage across 2. A battery with a voltage of 9.0 V is connected to a lamp. The current flowing in the circuit is 0.30 A. The resistance of the lamp is A. 0.030 Ω B. 2.7 Ω C. 27 Ω D. 30 Ω 3. What factor impacting resistance explains why light bulbs tend to burn out more frequently when they are first switched on? A. Length B. Cross-sectional area C. Temperature D. Material 4. Which wire conducts electricity with the least resistance? A. One with a large cross-sectional area B. One with a small cross-sectional area Resistors Wires used to complete circuits have low resistance. A wire 1 m in length that is typically used in a physics lab has a resistance of 0.03 Ω. Wires used in home wiring offer as little as 0.004 Ω of resistance for each 1 m of length. Resistors are devices with specific resistances that are used to control current in circuits or parts of circuits. A variable resistor (potentiometer) is used when a smooth, continuous variation of the circuit is desired (example: volume, brightness, contrast, hue, and tone controls on a TV) The human body acts as a variable resistor. (Dry skin has high resistance-wet skin has lower resistance) 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 Bird on a Wire 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? Bird on a 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. 5. A device that can be used to change the current in a circuits in a continuous way is a A. potentiometer B. battery C. motor D. lamp 6. If a 200 Ω resistor is connected to a 5 V battery, the current in the circuit will be A. 0.025 A B. 2.5 A C. 40 A D. 1000 A 7. If you double the voltage what happens to the current (all others kept equal)? A. it doubles B. it halves 8. If you double the resistance what happens to the current (all others kept equal)? A. it doubles B. it halves Types of Current Direct Current (dc) Electrons move in one direction Alternating Current (ac) Electrons change direction Types of Circuits There are three types of circuits: • Series circuits-all current travels through each device. • Parallel circuits-a circuit in which there are several different current paths • Combination series-parallel circuit- circuits which includes both series and parallel branches. Series Circuits In a series circuit, the current is the same throughout. I total = I A =I B = I C = … The equivalent resistance (total resistance) of the circuit equals the sum of the individual resistances of the resistors. R total = R A + R B + … Series Circuits The total voltage is the sum of the individual voltages across each device. V source = V A + V B + … If the voltage source does not change, adding more devices in series will decrease the current. Because the current flows through one pathway, if one light goes out, they all go out. 1. 2. 3. 4. The current is ___ in a series circuit. higher at the beginning of the same everywhere lower at the beginning variable in In a series circuit, the equivalent resistance is ___ any single resistance 1. 2. 3. 4. larger than determined by equal to smaller than If the battery voltage does not change, adding more devices in the series ___ the current. 1. 2. 3. 4. sometimes decreases always decreases sometimes increases always increases To find the current through a series circuit, first calculate the ____. 1. 2. 3. 4. voltage equivalent resistance power equivalent voltage Parallel Circuits A parallel circuit is one in which there are several paths. The total current is the sum of the currents through each path. I total = I A + I B + I c + … Placing two or more resistors in parallel always decreases the equivalent resistance of a circuit. 1 = 1 + 1 + 1 + … R R A R B R C The potential difference across each path is the same. V total = V A = V B = V C = … Safety Devices A short circuit occurs when a circuit with very low resistance is formed. Parallel circuits are susceptible to short circuits. Safety devices are incorporated into the circuit to prevent a circuit overload. 1. Fuse-a piece of metal melts when too much current flows and breaks the circuit. 2. Circuit breaker-an automatic switch that opens when the current reaches a value greater than the rated value in the circuit. 3. Ground-fault interrupter-an outlet that prevents injuries that are caused when an electric appliance falls into water. It contains an electric circuit that detects an extra current path and opens the circuit. The current is the same everywhere throughout the circuit 1. 2. Parallel Series The total resistance is equal to the sum of the individual resistances. 1. 2. Parallel Series The voltage drop across each resistor in the circuit is the same 1. 2. Parallel Series The voltage drop in the circuit is proportional to the resistance. 1. 2. Parallel Series Adding a resistor to the circuit decreases the total resistance. 1. 2. Parallel Series Adding a resistor to the circuit increases the total resistance. 1. 2. Parallel Series If the current through one resistor goes to zero, there is no current in the entire circuit. 1. 2. Parallel Series If the current through one resistor in the circuit goes to zero, the current through all of the other resistors remains the same. 1. 2. Parallel Series This form is suitable for household wiring. 1. 2. Parallel Series An ammeter is connected in ____ in a circuit. 1. 2. Parallel Series A voltmeter is connected in ___ in a circuit. 1. 2. Parallel Series The basic unit of resistance is the 1. 2. 3. 4. volt ohm amp coulomb A short thick wire has more resistance than a long thin wire. 1. 2. True False In a DC circuit, current flows from the ___ electrode to the ___ electrode. 1. 2. 3. 4. negative, positive positive, negative positive, positive negative, negative