P3 Flashcards
What is charge?
-Charge is a property of matter just like mass. You know that atoms are neutral overall
-They have no charge because there are equal numbers or protons and electrons
Electric charge:
Protons: positive charge
Electrons: negative charge
What do like charges do?
Repel
What do unlike charges do?
Attract
How do you produce static electricity?
-When you rub two insulators together you can transfer electrons from one insulator to the other.
-Positive charges do not move.
-One object ends up with extra electrons, and the other with not enough electrons to cancel out the positive charge
What is the charge on each object?
Static electricity or static
How do you discharge an object?
-To discharge an object you need to remove the charge
-you connect it to something that allows charge to flow- like a piece of metal, or you when touch a car door
-sparks also discharge charged objects such as van Der graaf generator, or clouds. A spark is a flow of charge, or a current, through the air
How can you use fields to explain static electricity?
-There is an electric field around a charged object or particle
-if you put another charged object in this field it will be attracted or repelled, even if the objects are not touching.
-You can work out what is happening by looking at the shape of electric fields
-Field lines are a bit like elastic bands
-If you change the shape of the field by putting another charged object t in it the field lines will “stretch”
-The force of an object will be in the direction that causes the field lines to shorten or straighten
-If the field lines are closer together it means that the field is stronger
-The direction if the field lines is the direction of the force on a positive charge
What is current and how do you model it?
-that little shock you experience when you touch a car door is the movement of electrons through your body. This is a current.
-Current is the rate of flow of charged particles, or charge. In the metal wires that you use to make circuits the charges that move are electrons. Each electron has a very small charge, but there are large numbers of them in the wires.
How do you make current flow?
-To make current flow you need a cell or battery, or a power supply, as well as a complete circuit.
-The current everywhere in a single closed loop is the same
Which way does current flow?
When you draw the direction of electric current in a circuit diagram, you draw it going in the direction of the positive terminal of the battery
-This is called conventional current
-This is opposite to electron flow
How do you calculate with current?
Charge flow(C) = current(A) x time(s)
What is charge measured in?
Coulombs (c)
What is current measured in?
Current is measured in amperes or amps(A)
What is time measured in?
Time is measured in seconds
What are small currents measured in?
Milliamps (mA)
1mA = 1x10^-3 Amps
How do we draw circuits?
You use circuits symbols to draw electric circuits.
What is potential difference?
-To make a current flow you need potential difference (p.d.) inside a cell or battery there are substances that react to separate the charges
-This makes one side of the battery positively charged and the other side negatively charged
-The positive terminal of the cell is at a higher electrical potential than the negative terminal
-The potential difference, and the current that a cell can deliver, depends on the construction of the cell, not it’s size.
What is potential difference measured in?
-Potential difference is measured in volts(V)
How do you know which symbol is negative or positive?
The longer line on the symbol for a cell is positive, and the shorter line is negative.
What happens when you apply a potential difference (p.d) between the ends of a piece of wire
-you set up an electrical field in the wire
-the field is set up very quickly(close to the speed of light in a vacuum )
-so the charged electrons start to move straight away.
How do you model potential difference?
-In the rope model you show the work done by the battery by pulling the rope.
-The work done by the charges in the components is the work done in the persons hand holding the rope
-The charges do very little work in the wires
What is the link between potential difference , energy and charge?
-When you connect a voltmeter across a component in the circuit you measure the work done or energy transferred by the charges in the component
-We call this electrical working
What is the equation for energy transferred(J)
Energy transferred(J) = potential difference(V) x charge(C)
How do you measure current in series circuits?
-You have learned that a series circuit has only one loop and the current is the same everywhere
-you measure the current in a circuit with an ammeter
How do you measure potential difference is series circuits?
A voltmeter measures a “difference” -the (electrical) potential difference
-you need to connect the voltmeter to both sides of a component
What does a voltmeter do?
-A voltmeter measures the “rise in potential” across a cell or battery
-It can also measure the “drop in potential” across a component
-if There is very little “drop” across the wires, a voltmeter will give a low reading such as “zero”
What do the voltmeters add up to?
The readings in voltmeters always add up to the reading across the cell
What happens in parallel circuits?
-It is helpful to think of a parallel circuit as having more than one loop
-In a parallel circuit each loop can be worked independently, this is what happens in your house.
-You do not have all the lights on at once
-If you measure the current at different points in a parallel circuit, the current in the loops add up to the current near the battery
Physics parallel circuit trigger:
Current at b + current at c = current at a
Current a is near the battery
What does current depend on?
The current depends on the potential difference of the cell or battery of the current.
What is resistance measured in?
Resistance in measured in ohms (Ω)
How do you model resistance?
-In the rope model, resistance is modelled by the person gripping the rope
-If the person grips the rope more tightly and the other person pulls with the same force then the rope will move more slowly
-To keep the same speed the person pulling the rope would have to pull harder
How do you calculate resistance?
Current(A) = potential difference (V) / resistance (Ω)
- rearrange
Resistance(Ω) =potential difference (V) / current (I)
What does the symbol V equate for?
Potential difference
What does the symbol R equate for?
Resistance
What does the symbol “I” equate for?
Current
What does V = I x R mean?
Potential difference (Volts) = current (Amps) x resistance (Ω ohms)
What does the resistance of some components change?
-A metal is made up of positively charged ions arranged in a regular pattern
-The ions are formed when electrons leave the outer shells of metal atoms
-These delocalised electrons are free to move through the structure of the metal
-Resistance is produced when electrons collide with the ions in the lattice
-This explains why the resistance of some components change with current
What is a variable resistor?
A variable resistor is a circuit component you can use to change the amount of wire or other resisting material.
-you use a variable resistor every time you use a dimmer switch.
Electrical intensity and current:
The letter I is used instead of C, for current, because the scientist who discovered electricity called it electrical “intensity”
What happens with electrons in a wire?
-Electrons in a wire are free to move in the electrical field produced by the potential difference.
What happens if you increase the resistance?
If you increase the resistance, the current decreases
How do you get measurements for a graph of current and potential difference?
-A graph of current against potential difference is called a characteristic graph.
-You collect data for the graph by taking measurements of the current flowing through a component for different potential differences.
-Then you reverse the battery or power supply connections and repeat the measurements
What can you do once you’ve recorded your measurements for a characteristic graph?
-when you have recorded measurements you can:
-calculate the resistance from each pair of measurements
-plot a characteristics graph with potential difference on the x-axis and current on the y-axis
What is the characteristic graph for a resistance wire or a resistor?
-The wire is a linear circuit element. It’s resistance does not change as you change the potential difference
-you use resistance wire or a resistor in a circuit if you need the resistance to be constant
What is ohms law?
-George ohm discovered that metal wires are linear elements
-He said that the current is proportional to the potential difference if the temperature does not change
-This is ohms law
What is the characteristic graph for a lamp?
-The electrons in a wire collide with the ions in the wire.
-The ions vibrate more as the wire gets hotter
-hit wires can be used in lamps
-As that happens there are more collisions
-The wire heats up more
-The current increases as the potential difference increases, but at a slower rate.
What is a diode?
-A diode is a component that only allows a current to flow one way.
-Some diodes emit light
-There are light emitting diodes (LEDS) in most of the electronic devices that you use
-The long leg of an LED should be connected to the positive terminal of a battery
-
what does a diodes characteristic graph look like?
-As you apply a potential difference in the “forward” direction very little current flows.
-Then the current suddenly increases.
-If you reverse the potential difference there is no current
-You use a diode if you want the current to flow only in one direction
What is a thermistor?
-At the end of many digital thermometers there is a thermistor
-The resistance of a thermistor depends on temperature
-Thermistors come in a different shapes and sizes, and detect different ranges of temperature
-a thermistor is not like a piece of wire
-It is made of a semi conducting material, such as silicon
-Electrons in the atoms of a semiconductor do not need much energy to escape from the atom to form the current
How does the resistance of a thermistor change with temperature?
-You can measure the resistance of the thermistor by measuring the current through it and the potential difference across it
-If you out a thermistor in a beaker of cold water and heat the water up the resistance of the thermistor decreases
-As you heat the thermistor many electrons gain enough energy to escape from the atoms in the semiconductor
What happens to the thermistor at different temperatures?
-At low temperatures, the current is small, so the resistance of the thermistor is high
-At higher temperatures the current is high, so the resistance of the thermistor is low
-If there are more electrons moving per second for a particular potential difference , the current will get bigger
-A bigger current for the same potential difference means the resistance is smaller.
-You will have thermistors all Mmmm in your house
-They are used to monitor the temperature in ovens and refrigerators, and to help to prevent your laptop or tablet from overheating.
What is an LDR?
-The resistance of a light-dependent resistor (LDR) changes with light intensity.
-It is a circuit component that is also made of a semi conducting material.
-In the LDR, light causes electrons to be released into the circuit to increase the current
How does the resistance of an LDR change with light intensity?
The shape of the graph for an LDR is the same as the shape of the graph for a thermistor
-As the intensity of the light increases, more electrons are released from rbe semiconductor and the resistance decreases.
-LDRs are used to automatically control lights in a building
What is net resistance?
-You can calculate the resistance if something without knowing what the other components in a circuit are
-This is net resistance
-If you changed the components, or arrangements of the components, then the current could change.
-The net resistance would be different.
What happens if you add a lamp into a series circuit?
-The current decreases
-the net resistance increases
What happens if you add another lamp in a parallel circuit?
-The total current increases
-The net resistance decreases
What happens to the net resistance when you add a component in a parallel or series circuit?
-in a series circuit the net resistance increases.
-in a parallel circuit the net resistance decreases
What is in a series circuit?
You know that in a series circuit:
-The current is the same everywhere
-the potential difference across each component add up to the potential difference across the battery
-V= I x R
How do you analyse series circuit?
-If you know the current and the resistance you can work out the potential difference across the lamp
V = I x R
-If you know the p.d. Across the lamp and the cell you can:
Work out the potential difference across the variable resistor
V(vr) = V(cell) -V(Lamp)
-If you know the potential difference across the variable resistor and the current through it you can:
Work out the resistance of the variable resistor:
R = V / I
What do you know about parallel circuits?
-The potential difference (p.d.) is the same across each loop)
-The current in each loop adds up to the current near the battery
-V = I x R
How do you analyse parallel circuits?
If you know the potential difference across the resistor and across the lamp, and if you know the resistance you can:
-work out the current through the resistor
I = V / R
If you know the current near the battery you can:
-work out the current through the lamp I(lamp) = I(battery) - current(resistor)
If you know the potential difference across the lamp and current through it you can:
Work out the resistance of the lamp
V / I = R
How can you measure temperature with a circuit?
Thermistors can be used in sensing circuits to produce a potential difference that changes with temperature
-this means that you can use a sensing circuit to operate a heater or the air conditioning in a house, school or office.
-the output potential difference will depend on the potential difference of the battery and the magnitude of each resistor
-V(out) as measured by a voltmeter, will depend on the temperature
Sensing circuits:
An LDR
-has a resistance that changes with light level
-you can monitor: the light level in a greenhouse
A variable resistor:
-Has a resistance that changes with how much it turns
-You could monitor: the position of a door
A pressure sensor:
-has a resistance that changes with the force applied to it
-Could monitor whether there is a burglar in your house
what is power?
-You need a current to work your microwave oven or power shower.
-you select some of the appliances in your house because of their power
-you can buy a high-power hair dryer or one with low power
-you are choosing the energy transferred per second
-power is measured in watts
-Large powers are measured in kilowatts(kW) or megawatts(MW)
-You might choose the high-power appliance because you can use it for a shorter time to get a job done
What is power measured in?
power is measured in watts
How do you calculate electric power with current and resistance?
In an electrical circuit:
-the current is the rate of flow of charge- it tells you the amount of charge flowing through a component per second
-the potential difference tells you the energy transferred by each charge
What is current?
-the current is the rate of flow of charge- it tells you the amount of charge flowing through a component per second
What is potential difference?
-the potential difference tells you the energy transferred by each charge
Equation for potential difference
Potential difference = energy transferred / charge
Equation for current
Current = charge / time
Equation for charge
Charge = current x time
Equation for energy transferred
Energy transferred = potential difference x charge
How is power related to current and resistance?
This means that power = potential difference x current
You can eliminate potential difference from the lower equation. This is called “substitution”
You know that potential difference = current x resistance
(Current x resistance) x current
= current^2 x resistance
You need to be able to apply
P = I^2 x R
Equation for power:
P = I^2 x R
