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Physics > Electricity and Circuits > Flashcards

Electricity and Circuits Flashcards

1
Q

What is current?

A
  • current is the flow of electric charge
  • current will only flow through an electrical component, and if the circuit is closed
  • unit of current is ampere (A)
  • current will always be conserved, even at junctions in parrallel circuits.
  • Meaning the current leaving the positive terminal is the same as the current arriving at the negative.
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2
Q

What is potential difference?

A
  • potential difference (voltage) is the driving force that pushes the charge around
  • the current flowing through a component depends on the potential difference across it, and if the circuit is closed
  • the higher the potential difference across a component, the higher the current will be
  • the unit of potential difference is volt (V)
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3
Q

What is resistance?

A
  • resistance is anything that slows the flow down
  • the greater the resistance of a component, the smaller the current that flows
  • unit for resistance is the ohm (Ω)
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4
Q

Describe the use of a;
A) Voltmeter
B) Ammeter

A

A) Voltmeters are added in parallel with a component to measure the potential difference ( voltage ), across it.

B) Ammeters are connected in series to measure the the current passing through a component or circuit.

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5
Q

What is the atomic structure of metals?

A
  • the atoms in metals are bonded in such a way that metals are made up of a lattice of positive ions surrounded by free electrons
  • these electrons are free to move through the whole metal
  • the current in metals is the flow of these free electrons
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6
Q

Current is the rate of flow of charge

What is the equation linking charge, current and time?

A

charge = current x time
Q = I x t
current = amperes (A)
charge = coulombs (C)
time = seconds (s)

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7
Q

What is the equation linking potential difference, energy, and charge?

A

energy transferred = charge moved x potential difference
E = Q x V
energy transferred = joules (j)
charge moved = coulombs (C)
potential difference = volt (V)

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8
Q

What is the equation linking potential difference, current and resistance?

A

potential difference = current x resistance
V = I x R
potential difference = volt (V)
current = amperes (A)
resistance = ohms (Ω)

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9
Q

SP10d Resistance

Describe the design and construction of a series circuit set up for testing.

A
  • A variable resistor will be used to change the current in the circuit.
  • Measurements of the current and potential difference are recorded and the resistance is calculated.
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10
Q

Core practical - Investigating resistance

Describe how you would construct an electrical circuit to investigate the relation between P.D, current & resistance.

A

1. First you want to set up a circuit where; there is an ammeter, a resistor and a voltmeter in parrellel to the resistor.
- You then want to use a power pack that can provide different potential differences.

2. Set the power pack to its lowest voltage and switch it on. Write down the readings on the ammeter and voltmeter and then switch it on.
- Write down the readings on the ammeter and voltmeter and then switch the power pack off.

3. Repeat step 2 for 5 different voltage settings, up to a maximum of 6V.

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11
Q

Core practical - Investigating resistance

Describe how you would construct an electrical circuit to investigate Filament lamps in series and parrellel ciruits.

A

1. Set up a circuit which has; an ammeter and 2 filemant lamps in series with voltmeters parrallel.

2. Set the power pack to its lowest voltage. Write down the readings on the ammeter and the voltmeters.
- Repeat with the power pack set to provide different voltages, up to a max of 6V.

3. Now set up a circuit with; 3 ammeters in series, 2 filament lamps in parrallel along with 2 voltemeters in parrellel.
- Repeat step 2 for several different voltage settings.

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12
Q

How does resistance increase with temp?

A
  • when an electrical charge flows through a component, it has to do work against resistance, this causes an electrical transfer of energy
  • some of this energy is transferred usefully but some of it is dissipated to the thermal energy stores of the component and surroundings
  • so when a current flows through a resistor, the resistor heats up
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13
Q

Why does resistance increase with temp?

A
  • happens because the electrons collide with the ions in the lattice that make up the resistor as they flow through it
  • this gives ions energy, causing them to vibrate more
  • the more ions vibrate, the harder it is for electrons to get through the resistor (as there are more collisions)
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14
Q

SP10f Transferring energy

Describe how resistance can be reduced in a circuit.

A
  • Resistance can be reduced using wires made from metals with low resistance, such as copper.
  • Thicker wires also have lower resistance.
  • Resistance can also be decreased by cooling metals so that the lattice ions are not vibrating as much.
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15
Q

SP10f Transferring energy

Describe the benefits and drawbacks of the heating effect of an electric current.

A

Benefits;
- Its usefull in a an eletric heater or kettle due to the heat helping with their primary function

Drawbacks;
- It’s not usefull in a computer or in plugs and wires because it means that useful energy is being transferred from the circuit by heating.

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16
Q

What is the relationship between potential difference, current and resistors?

A
  • for a given p.d. the current decreases as the resistor heats up
  • If the resistor gets too hot, no current will be able to flow
  • one exception is the resistance of a thermistor decreases as the temperature increases
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17
Q

How to investigate how p.d. changes with current?

A

1) start by setting up the series circuit with an ammeter, variable resistor and a component in parallel to the voltmeter

2) as I = V/R, increasing the the resistance of the variable resistor lowers the current through the circuit at a fixed supply p.d.

3) set resistance of variable resistor
- measure the current through and the potential difference across the component (using ammeter and voltmeter)
- take measurements at a number of different resistances

4) Swap wires connected to battery to reverse direction of current and get negative readings

5) Repeat step 3 to get results for negative values of current and then plot the current against the p.d. to get I-V graphs

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18
Q

What precautions must be taken in the investiagation of how p.d. changes in current?

A
  • Have to make sure the circuit doesnt get too hot over the course of your experiment as this will allow no current through if temp. is too high
  • if circuit starts to warm up, disconnect it for a while between readings to cool down
  • do repeats and calculate means
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19
Q

A) What does the I-V graph of a fixed resistor look like and why?
B) Describe how current varies with p.d. in a fixed resistor

A

A) Graph description
- current is directly proportional to p.d. (when temp. stays the same)
- graph will have different gradients of straight lines as different resistors will have different resistances
- is a linear graph

B) Current varies as;
- An LDR, for example, has a high resistance in the dark, but the resistance gets smaller and smaller when the light instensity increases.

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20
Q

A) What does the I-V graph of a filament lamp look like and why?
B) Describe how current varies with p.d. in a filament lamp

A

A) Graph description
- The increasing current increases the temperature of the filament, which makes the resistance increase
- leads to the I-V graphs being curved
- is a non linear graph

B) Current varies as;
- The current being pushed by the p.d. causes the filament light to heat up and glow.
- The greater the p.d, more and more current will be flowing through ( making the filament light hotter )
- However, as it heats up, the filament lamp’s resistance also increases.
- Meaning that when the p.d changes, the current will not change by the same percentage.

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21
Q

A) What does the I-V graph of a diode look like and why?
B) Describe how current varies with p.d. in a diode

A
  • current will only flow through a diode in one direction
  • the diode has very high resistance in the opposite direction
  • means there is only a curved line in one half of the quadrant as the rest is around zero
  • is a non linear graph
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22
Q

What are the characteristics of a linear component’s graph?

A
  • line goes through (0,0)
  • the resistance of the component equals the inverse of of the gradient of the line, or “1/gradient”
  • the steeper the graph, the lower the resistance
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23
Q

How to find the resistance of any point for any I-V graph?

A
  • by reading the current and the p.d.
  • sticking the values into V = I x R
24
Q

What is and LDR (light dependant resistor)?

A
  • is a resistor that is dependant on the intensity of light
  • in bright light, resistance falls
  • in darkness, the resistance is the highest
  • can be used as automatic night lights, outdoor lighting and burglar detectors
25
Q

What is a thermistor?

A
  • is a temperature dependant resistor
  • for a negative temperature coefficient (NTC) thermistor, their resistance drops in hotter conditions and goes up in colder conditions
  • thermistors can be used as temperature detectors e.g. car engines and electronic thermostats
26
Q

How to investigate how resistance changes for LDRs?

A
  • can use I-V graphs for LDRs using V = I x R, but resistance of LDRs can depend on things other than current…

To test how resistance changes with brightness:
1) conduct your experiment in a dim room
2) measure the p.d. across and current through the LDR
3) change the light level near to the LDR
4) measure the p.d. and current again and repeat for a range of light levels
5) calculate resistance for each measurement using R = V/I
- should find that as the light level gets brighter, the current through LDR increases and resistance decreases

27
Q

SP10g Power

Explain the correlation between power, current, and potential difference

A
  • Power is the energy ( or current ) transferred per second.
  • Potential difference is responsible for pushing the current throughout the circuit.
  • Also, the power transfer in a circuit is directly proportional to the p.d across it and the currrent through it.

Formulas;
- Power ( W ) = energy transferred ( J ) / time taken
- Electrical power ( W ) = Current ( A ) / Potential difference ( V ) or Current^2 ( A^2 ) x resistance ( Ω )

28
Q

How to investigate how resistance changes for thermistors?

A
  • can use the method to create I-V grpahs for thermistors
  • also can test how resistance of a thermistor changes with temp. using a circuit with an ammeter, a variable resistor and a voltmeter in parallel…

1) measure the p.d. across and current through the thermistor
2) change the temp. of the thermistor by heating it
3) measure the current and p.d. for a range of different temps.
4) calculate the resistance for each measurement using R = V/I

29
Q

Why does adding resistors in series increase total resistance?

A
  • by adding a resistor in series, the two resistors have to share the total p.d.
  • the p.d. across each resistor is lower, so the current through each resistor is also lower (V = I x R)
  • in a series circuit, the current is the same everywhere so the total current in the circuit is reduced when a resistor is added, meaning the total resistance of the cirucit increases (R = V/I)
30
Q

What are the characterisitcs of series circuits?

A
  • there’s a bigger supply p.d. when more cells are in series (when all connected in the same way)
  • the current is the same everywhere, I¹ = I² . the size of the current depends on total p.d. and total resistance of circuit (I = V/R)
  • the total p.d. of supply is shared between components. the bigger a component’s resistance, the bigger its share of total p.d.
  • the total resistance of circuit increases as you add resistors
31
Q

What are the characteristics of a parallel circuit?

A
  • each component is separately connected to the +ve and -ve of the supply (except ammeters which are aloways in series)
  • all components get the full source of p.d., so the voltage is the same across all components, V¹ = V² = V³ , means that identical bulbs connected in parallel will all be the same brightness
  • total current flowing around the circuit is equal to the total of all the currents through the separate components, I (total) = I¹ + I² + I³ + …
  • if two identical components are connected in parallel, then the same current will flow through each component
32
Q

Why does adding a resistor in parallel reduce total resistance?

A
  • In parallel, both resistors have the same p.d. across them as the source
  • this means the ‘pushing force’ making the current flow has more than one direction to go in
  • this increases the total current that can flow around the circuit . Using V = I x R, an increase in current means a decrease in the total resistance of the circuit
33
Q

How do you investigate adding resistors in series?

A

1) find four identical resistors

2) build a series circuit with a battery, ammeter and one of the resistors and note the p.d. of the battery (V)

3) measure the current through the circuit using the ammeter and then use R = V / I to find the total resistance of the circuit

4) Add another resistor, in series with the first and repeat steps 2 and 3 to find the new total resistance of the circuit

5) Repeat steps until you’ve added all the resistors

6) Plot a graph of the number of resistors against the total resistance of the circuit

34
Q

Includes graph description

What should the results be after adding resistors in series?

A
  • should find that adding resistors in series increases total resistance of the circuit (decreases total current through the circuit)
  • The more resistors you add, the larger the resistance of the whole circuit
  • graph should be a straight diagonal line from (0,0)
35
Q

How do you investigate adding resistors in parallel?

A

1) Using the same equipment as the series test, build the same initial circuit and measure total current through the circuit and the p.d. of the battery to calculate total resistance (R = V / I)
2) Next add another resistor, in parallel with the first
3) measure the total current through the circuit and use this and the p.d. of the battery to calculate the total resistance of the circuit
4) Repeat steps 2 and 3 until all four resistors are added
5) Plot a graph of the num. of resistors in the circuit against the total resistance

36
Q

Includes graph description

What should the results be after adding resistors in parallel?

A
  • total current through the circuit increases - so the total resistance of the circuit has decreased
  • the more resisors you add the smaller the overall resistance becomes
  • the graph should be a negatively sloping graph decreasing with the number of resistors added
37
Q

What is the equation that links energy transferred, current, p.d. and time?

A

Energy transferred = current x potential difference x time
E = I x V x t
E= energy transferred (j)
I= current (A)
V = potential difference (V)
t = time (s)

38
Q

How does the E = I x V x t equation work?

A
  • when an electrical charge goes through a change in p.d., then energy is transferred (work done against resistance)
  • energy supplied to the charge at power source to ‘raise’ it through a potential
  • the charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit (e.g resistors)
  • to find the energy transferred we use E = I x V x t
39
Q

Examples of electrical appliances transferring energy to components in a circuit

A
  • Kettles transfer energy electrically from the mains a.c. supply to the thermal energy store of the heating element inside the kettle
  • Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor
40
Q

What are examples of heating a circuit being good?

A
  • it is very beneficial if you want to heat something
  • toasters contain a coil of wire with a really high resistance, so when a current passes through the coil, its temperature increases so much that it glows and gives off infrared radiation
  • this radiation transfers to the bread and cooks it
  • filament bulbs and electric heaters work in a similar way
41
Q

SP10h Transferring energy by electricity

Describe how energy is trasnferred from batteries in motors

A

A) In a Motor;
1. First, some of the energy in the battery is transferred by electricity to the motor,
- Where it will be trasnferred to a store of kinetic energy

2. Some energy will also be transferred by heating the wires, the motor and surroundings.

3. In the end, all the energy will be dissapated by heating, making the surroundings a little warmer.

42
Q

SP10h Transferring energy by electricity

Describe how energy is trasnferred from batteries in heating devices

A

A) In a Heating device;
1. Gloves, for example, would have wires with very high resistance.

2. Energy stored in the battery is** transferred by electricity** to the high resistance wire where it is trasnferred by heating to a store of thermal energy in the wire

3. The energy is then transferred by heating to the gloves and hands of the wearer, and eventually dissapting to the surroundings.

43
Q

SP10h Transferring energy by electricity

Explain the differences between A.C and D.C

A
  • In A.C the direction of the current is frequently changing due to the rotating generators.
  • However in D.C the current stays facing in the same direction
  • In A.C the voltage ( p.d ) is also constantly changing, increasing to a peak voltage then back to 0.
  • This does not happen in D.C
44
Q

How do power ratings of appliances work?

A
  • the power of an appliance is the energy that it transfers per second, so the more energy it transfers in a given time, the higher its power
  • Power is measured in watts
  • the appliance is then given a power rating, labelled with the maximum safe power they can operate at (maximum operating power)
  • tells you the maximum amount of energy transferred between stores per second when appliance in use
45
Q

What does a higher power rating on an appliance mean?

A
  • means more electricity is used per second
  • does not necessarily mean it is more efficient than an appliance with a lower power rating
46
Q

What are the equations for power and electrical power?

A

Power (W) = Energy Transferred (J) / Time (s)

or

Electrical power (W) = Current (A) x Potential difference (V)

47
Q

How to calculate the power of an appliance without knowing the p.d.?

A
  • Use equation V = I x R and stick it into P (power) = I x V
  • This gives you: P = I² x R
  • P = power (W)
  • I = current (A)
  • R = resistance (Ω)
48
Q

How does the UK mains supply work?

A
  • uses an a.c. which are produced by alternating voltages (positive and negative ends of the p.d. keep alternating)
  • has an a.c. supply at around 230V
  • the frequency of the a.c. mains supply is 50 cycles per second or 50Hz
49
Q

How does battery supply work?

A
  • cells and batteries supply direct current
  • this is the movement of charges in only one direction and is created by direct voltage (a p.d. that is only positive or negative, not both)
50
Q

What do the three separate wires do inside a cable?

A

Neutral wire:
- is shown as blue always
- completes the circuit - when the appliance is operating normally, current flows through the live and neutral wires
- it is around 0V

Live Wire:
- is shown always as brown
- carries the voltage (p.d.)
- it alternates between a high ve+ and a ve- voltage of about 230V

Earth Wire:
- always shown as green and yellow
- is for safety and protecting the wiring
- it carries the current away if something goes wrong and stops the appliance casing becoming live
- also at 0V

51
Q

SP10i Electrical safety

Explain why switches and fuses should be connected in a live wire.

A

Switches;
- When the switches are turn off, no current goes through the appliance.
- This can be crucial in dangerous moments, ( e.g. water spill )

Fuses;
- A fuse is a tube with a thin wire inside. The current passes through the wire and the wire gets hotter.
- If the current exceeds a certain value, the wire melts.
- This breaks the circuit and stops the flow of current.

52
Q

SP10i Electrical Safety

Describe how Circuit Breakers work and an advantage of using them.

A

How it works;
- They are a type of automatic switch, that stops current flowing if there is a problem in the circuit, like to much current flowing or into the wrong wire.

Advantages;
- Once a fault is fixed they can be switched back on again, where a fuse has to be replaced once used.
- Another one is that some types work very quickly, so they can save lives.
- A fuse takes some time to melt and will not prevent you getting a shock if you touch the live wire

53
Q

SP10h Transferring energy by electricity

Describe, with an example, the relation between power ratings and the changes in stored energy in domestic devices.

A

1. A Kettle with a power rating of 3kW transferrs 3000 joules of energy every second
- This is done with from the main electricty supply to a supply of thermal energy in the water.

2. Applications that need a large amount of power use mains electricity. In a power station energy is transferred from a store of kinetic energy ( e.g. turbine ) by electricity.
- The electricity is carried to our homes through a network of wires and cables known as the national grid.

54
Q

SP10i Electrical Safety

Explain the dangers of providing any connection between the live wire and earth.

A
  • If a fault in a wire causes the live wire to touch a metal part, it makes a very low resistance circuit between 230V and 0V ( The earth )
  • This causes a very large current to flow to the earth, which heats up the wire and could cause a fire.
  • If this happens, current blows the fuse and cuts off the mains electricity supply.
55
Q

Why does touching a live wire give you an electric shock?

A
  • your body is at 0V
  • this means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you
  • this causes a large electric shock which could injure or kill you

Physics (10 decks)

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