Flashcards in Unit 3 - Physics: Motion, energy and electricity. Deck (66):
State that there are two types of charges
Unlike charges attract
LIke charges repel
Describe an electric field
A region in which an electric charge experiences a force.
Define speed and calculate speed from total time/total distance
The rate at which an object covers distance. How fast an object's distance is changing.
S = d/t
Plot and interpret a speed/time graph and a distance/time graph
Recognise from the shape of a speed/time graph when a body is
-moving with constant speed
-moving with changing speed
Distance travelled is the area under the graph.
Recognise liner motion for which the acceleration is constant and calculate the acceleration.
Straight line graph
Recognise motion for which the acceleration is not constant
Smooth curve graph.
Calculate the area under a speed/time graph to work out the distance travelled for motion with constant acceleration.
Area = distance travelled
Give and identify examples of energy in different forms, including kinetic, gravitational, chemical, nuclear, thermal (heat), electrical. light and sound.
Kinetic: person running
Gravitational potential: swings. Books on shelves
Chemical: food. Fuel. Batteries
Nuclear: fission. Fussion.
Electrical: TV, video games
Light: light bulb
Sound: voices, instruments.
Energy = joules (J)
Work done = joules (J)
Power = Watts (W)
Demonstrate understanding that an object may have energy due to its motion (kinetic) or its position (potential), and that energy may be transferred and stored.
Gravitational potential energy is work done against gravity. If an object is raised up, work is done against gravity. If the object then falls, the energy is converted into kinetic energy. ie gravitational potential at the top = kinetic energy at the bottom
Kinetic energy equation
K.E. = 1/2mv^2
= 1/2 x mass x velocity^2
Potential energy equation
P.E. = mgh
= mass x gravity x height.
Give and identify examples of the conversion of energy from one form to another, and of its transfer from one place to another.
The pool cue has kinetic energy while moving towards the ball. When it hits the ball, the kinetic energy is transferred into sound, heat and kinetic energy.
Apply the principle of energy conservation to simple examples
The Law of Conservation of Energy
"Energy cannot be created nor destroyed, merely converted from one type into another"
Demonstrate a qualitative understanding of efficiency
Whenever a machine operates energy is transferred from one form into another. Some energy will also be lost (sound, heat by friction etc). Obviously the less energy converted into the desired type then the less efficient the machine is.
Eg something that is 85% energy efficient will waste 15% of the energy supplied.
Efficiency = (useful energy)/(total energy) x 100%
Distinguish between renewable and non-renewable sources of energy
Renewable = replenish-able and limitless resources eg solar, wind
Nonrenewable = will eventually run out, cannot be replenished.
What is the source of energy for all our energy resources (except geothermal and nuclear)?
Relate (Without calculation) work done to the magnitude of a force and the distance moved.
When a force moves an object through a distance, energy is converted and work is done.
Greater the force/distance, more work is done.
Work done equation
Work done = f x d
= force x distance
or work done = power x time
P = work done / time
= (force x distance) / time
P = E/t
Describe how electricity or other useful forms of energy may be obtained from
-chemical energy stored in fuel
-water, including the energy stored in waves, in tides and in water behind hydroelectric dams
-heat and light from the sun (solar cells and panels)
-Fuels are burnt in power stations
-Heat is used to turn water into steam
-Moving steam is used to spin a turbine, generating electricity
-Chemical -> heat -> kinetic -> electric
-Waves are created by the wind
-They are channeled into a narrow chamber
-Air is pushed in and out of the chamber spinning a turbine.
-Kinetic -> electrical
-Gravitational pull of the Sun and moon causing the rising and falling tides in our seas and oceans
-a rising is used to fill a reservoir
-The reservoir is then emptied, spinning a turbine, generating electricity.
-Gravitational potential -> kinetic -> electrical
-Water is collected behind dams in large reservoirs
-This is either up a hill or on a fast flowing river
-The water is released and is used to drive a turbine
-Gravitational potential -> kinetic -> electrical
-Rocks deep underground are hot
-Water can be pumped down to these rocks and heated up
-If steam is produced it can be used to drive a turbine
-This energy can be used to drive a turbine
-Heat -> kinetic -> electrical
-Or hot water can be used for heating
-If sunlight falls on a solar panel the energy is transformed into electricity
-When sunlight hits a surface it is transformed into heat energy
-or hot water can be used for heating
-Light -> heat or light -> electricity
-solar panels can heat water
-solar cells can transfer light energy directly into electricity
Give advantages and disadvantages of each method in terms of reliability, scale and environmental impact
-combustion produces carbon dioxide and other pollutants
-pollution may cause global warming
-only works when there are tides
-barrages are expensive to build
-some impact through barrier installation can disrupt tidal flow to shore and hence the movement of nutrients and organisms
-works only if there are waves
-a large number of buoys are needed to generate enough electricity for a town. Only woks where there are big waves
-if there is good rain supply there will always be water to produce energy
-only suitable for hilly areas with rivers
-some impact on diverting rivers. This may upset the ecology of the area or the fertility of surrounding land.
-it is free and available day and night
-only available in certain parts of the world.
-Sometimes poisonous gases are given off
-some impact from the installation of the equipment that is needed to direct steam to turbines
-freely available whenever the sun is shining
-solar panels require continuous sunshine, unless the energy can be stored in batteries.
-Solar cells are expensive to buy
-some impact as may need large area for solar cells.
Describe energy changes in terms of work done
Whenever a force makes something move, work is done. The amount of work done is equal to the amount of energy transferred. Work, like energy, is measured in joules. When work is done by something, it loses energy; when work is done on something it gains energy.
Relate (without calculation) power to work done and time taken, using appropriate examples.
Power is a measure of how quickly work is being done and so how quickly energy is being transferred.
More powerful engines in cars can do work quicker than less powerful ones. As a result they usually travel faster and cover the same distance in less time but also require more fuel.
Describe simple experiments to show the product and detection of electrostatic charges
Scrub a plastic strip on a cloth. Positive charges on plastic and negative charges on cloth. The electrons moved. Put plastic near water to see it bend. Or cut up pieces of paper and the paper will stick to the stick
Distinguish electrical conductors and insulators and give typical examples
-Electrical conductors are materials that allow electricity to flow through them easily. Eg metal
-Electrical insulators are materials that prevent electrical flow. In the diagram to the right, the insulating material (plastic) surrounds the conducting material (copper wires). Eg wood
-Semi-conducting materials exhibit both conducting and insulating properties. The way in which the material is connected to a power supply determines whether it will conduct an electrical current or prevent it from flowing. Eg silicon.
Demonstrate understanding of current, potential difference and resistance, and use with their appropriate units
Current: the rate of flow of electric charge. I = electric current in amps.
Potential difference: A 'potential difference' across an electrical component is needed to make a current flow in it. Cells or batteries often provide the potential difference needed.
'Potential difference' is often called 'voltage'. The number of Joules transferred by each coulomb of charge. Measured in volts.
Resistance: the opposition to the flow of electric current. "The ratio of potential difference across a device to the current through that device". Measured in Ohms. Ω
Ohm's Law states that "For an object obeying Ohm's Law, the ratio of potential difference to current is proportional provided temperature stays the same." This means that if we plot a graph of potential difference (y axis) against current (x axis) then the result is a straight-line graph then the device is an ohmic resistor. The gradient of this graph will be numerically equal to the resistance of the device.
Use and describe the use of an ammeter and a voltmeter
Ammeter: measure current in amps.
Voltmeter: measures the energy going in & the energy coming out and tells you the difference. In volts.
What is current related to?
The current is related to the flow of charge.
-The current is the same at all points in a SERIES section of a circuit
-The voltage is the same across all points in a PARALLEL section of a circuit
-The voltages must add up to the supply voltage
-The current entering a junction must equal the current in the parallel branches. Ie parallel branches B and C add up to A (entering the parallel state).
Use the term "potential difference (p.d.) to describe what drives the current between two points in a circuit.
A potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.
Power equation using current and voltage
P = I V
P = power
I = current
V = voltage
Energy equation using current, voltage and time
E = I V t
E = energy
I = current
V = voltage
t = time
Identify electrical hazards
-overheating of cables
Demonstrate the understanding of use of fuses
The fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.
When operating correctly the current for an appliance flows through the live and neutral wires. The outer casing should have an earth wire connected to it. WHen a fault develops the live wire can touch the outer casing. This can be particularly dangerous if the outer casing is made of a metal. The earth wire has a very low resistance hence current can flow through it easily. The current will flow through the earth wire due to this low resistance.
A new circuit is made; the current will flow through the live wire (and the fuse) and the earth wire. The current through the earth wire will be high. The high current flows through the fuse and causes the fuse to melt. The fuse melting causes there to be a break in the circuit, preventing any current from flowing. With no current flowing, the appliance is unable to give the user an electric shock.
Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), lamps, ammeters, voltmeters and fuses
Is the current same at every point in a series circuit
Yes. The current at every point in a series circuit is the same.
What is the sum of the potential differences across the components in a series circuit equal to
The total potential difference across the supply.
The sum of the potential differences across the components in a series circuit is equal to the total potential difference across the supply.
The voltages must add up to the supply voltage.
For a parallel circuit, is the current from the source larger than the current in each branch?
The current entering a junction must equal to the current in the parallel branches.
What is the sum of the currents in the separate branches of a parallel circuit?
The current from the source
Advantages of connecting lamps in parallel in a lighting circuit
-Lamps are designed to operate at a fixed potential difference - no matter how many lamps are in parallel circuit, the potential difference across them is always the supply potential difference. In a series circuit, the potential difference is shared out between the lamps, so the more lamps there are, the smaller the potential difference across each one and the dimmer each will be.
-If the lamps are connected in series then they must be switched on or they must all be switched off, with a single switch. In the parallel circuit, the lamps can be switched separately as each lamp is in its own separate arm.
Equation for resistance using potential difference and current
Resistance = potential difference / current
Understand qualitatively how changes in p.d. or resistance affect current
The greater the potential difference, the more the resistance. The greater the current, the less the resistance.
Like a fraction
Calculate the combined resistance of two or more resistors in series
R total = R1 + R2
Is the combined resistance of two resistors in parallel less than that of either resistor by itself?
Yes. The combined resistance of two resistors in parallel is less than that of either resistor by itself.
R total < R1 or R2
1/Rtotal = 1/R1 + 1/R2
Calculate resistance using voltage and current
R = V / I
Relate the resistance of a wire to its length and to its diametre
The resistance in a wire increases as:
-The length of the wire increases
-The thickness of the wire decreases
An electric current flows when electrons move through a conductor, such as a metal wire. The moving electrons can collide with the ions in the metal. This makes it more difficult for the current to flow, and causes resistance.
-The resistance of a long wire is greater than the resistance of a short wire because electrons collide with ions more often.
-The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire has fewer electrons to carry the current.
Describe an experiment to determine resistance using a voltmeter and an ammeter
a Set up the circuit shown. (One resistor, ammeter in series, voltmeter connected in parallel with resistor) Turn the power supply up until the p.d. across the lamp is 12 V (the normal operating voltage).
b Take readings of the p.d. and current.
c Calculate the resistance of the lamp at its running temperature.
d Now, for several different values of p.d., measure the current through the lamp. Plot a graph of your results; this graph is known as the voltage-current characteristic of the lamp.
Use equation R = V/I
Understand the ideas of mass and weight and the distinction between these terms
Mass - measurement of the amount of matter something contains. Mass doesn't change when an object's location changes.
Weight is the force of gravity pulling on a mass.
Weight: spring balance. Spring balances measure weight by the tension on the helical string. The greater the force, the greater the extension of the string.
Mass: Lever-arm balance
What is a source of gravitational field
Weight equation using mass and gravity
W = mg (with g = 10Nkg^-1)
What does the term "centre of mass" mean and what is the significance of it for the stability of an object
Centre of mass: the average masses factored by their distances from a reference point. Point representing the mean position of the matter in a body.
An object becomes unstable when its centre of gravity falls out of the object's base.
The lower the centre of gravity the more stable the object.
What is the moment of a force (with examples)
The turning effect of a force produced by a force acting on a body. The moment of a force about a point is the product of the force and the perpendicular distance of its line of action from the point. The turning force around the pivot is called the moment.
When no-one is on the see-saw it is level, but it tips up if someone gets onto one end. Turning forces around a pivot are called moments.
It is possible to balance the see-saw again if someone else gets onto the other end and sits in the correct place. This is because the turning forces are balanced. We say the moments are equal and opposite.
Have an understanding of the principle of moments and perform simple calculations using the principle of moments
The principle of moments states that when in equilibrium the total sum of the anti clockwise moment is equal to the total sum of the clockwise moment.
When a system is stable or balance it is said to be in equilibrium as all the forces acting on the system cancel each other out.
Total Anticlockwise Moment = Total Clockwise Moment
Moment = force x distance (perpendicular distance from the force to the pivot)
Moment in Newton metres
Force in Newtons
Distance in metres
Show familiarity with speed and acceleration and understand the meaning of each
Speed - how fast an object's distance is changing
Acceleration - the rate at which which an object's velocity changes
Distinguish between speed and velocity
Speed is how fast an object is moving whereas velocity is the change in distance (displacement) over a time.
Use a speed/time graph to determine the distance travelled for motion with constant acceleration
Distance = area under graph
Know and use the relationship between force, mass and acceleration. Equation
F = m x a
Describe qualitatively the motion of bodies falling in a uniform gravitational field with and without air resistance, including reference to terminal velocity
Without air resistance - the weight of the body is the only force acting on it, causing it to experience a constant acceleration. This body undergoes a freefall. Gradient of graph is constant throughout its rise and fall.
With air resistance - air resistance opposes its weight. The downward acceleration is less than 9.81ms^-2. As air resistance increases with speed, it eventually equals its weight (but in opposite direction). From then there will be no resultant force acting on the body and it will fall at a constant speed, TERMINAL VELOCITY.
Why is it possible for objects to orbit the Earth without falling to its surface?
It is possible for objects to orbit the Earth without falling to its surface because of whether or not the satellite's (a celestial body orbiting the earth or another planet) orbital path intersects the surface of Earth. The path that any tossed or falling object follows is just the tip of a very elliptical orbital path.
Conversion of energy
Chemical/fuel: Chemical potential -> heat -> kinetic -> electric
Solar: light -> heat or light -> electricity
Nuclear: Nuclear -> heat -> kinetic -> electric
Wind: kinetic -> electric
Waves: kinetic -> electric
Tidal: gravitation potential -> kinetic -> electrical
Hydroelectric: gravitational -> kinetic -> electrical