Electricity

Question 1

Define ‘Electromotive force’

Answer 1

The amount of chemical energy converted to electrical energy per coulomb of charge (C) when passing through a power supply

Question 2

state one of the. formulas for emf

Answer 2

Question 3

define the terminal potential difference

Answer 3

the potential difference across the terminals of a cell
If there was no internal resistance, the terminal p.d would be equal to the e.m.f

Question 4

define internal resistance

Answer 4

The resistance of the materials within the battery

Question 5

what causes the cell to become warm after a period of time

Answer 5

It is internal resistance that causes the charge circulating to dissipate some electrical energy from the power supply itself

Question 6

Required Practical: Investigating EMF & Internal Resistance

Answer 6

1. The cell and the resistor, labelled r, should be connected in series and considered to be a single cell
2. With the switch open, record the reading V on the voltmeter
3. Set the variable resistor to its maximum value, close the switch and record V and the reading I on the ammeter - make sure to open the switch between readings
4. Vary the resistance of the variable resistor up to a minimum of 8-10 readings and record values for V and I for each resistance. Ensure to take readings for the whole range of the variable resistor

Question 7

what is the gradient of the graph of voltage (y axis) and current (x axis)?

Answer 7

negative r (negative internal resistance)

Question 8

what are some random errors in the experiment

Answer 8

1. Only use fairly new cells otherwise the e.m.f. and internal resistance of run-down batteries can vary during the experiment
2. Wait for the reading on the voltmeter and ammeter to stabilise (stop fluctuating) before recording the values
3. Take multiple repeat readings (at least 3) for each voltage and current and calculate a mean to reduce random errors

Question 9

Kirchoff’s second law

Answer 9

The total e.m.f. in a closed circuit equals the sum of the potential differences across each component

Question 10

Kirchoff’s firsrt law

Answer 10

The sum of the currents entering a junction always equal the sum of the currents out of the junction

Question 11

state two formulas for power

Answer 11

Question 12

what is a potential divider

Answer 12

Potential dividers are circuits which produce an output voltage as a fraction of its input voltage

Question 13

what are the 3 purposes for potential dividers

Answer 13

1. To provide a variable potential difference
2. To enable a specific potential difference to be chosen
3. To split the potential difference of a power source between two or more components

Question 14

potential divider formula

Answer 14

Question 15

LDR

Answer 15

The higher the light intensity, the lower the resistance and vice versa

Question 16

thermistor

Answer 16

The hotter the thermistor, the lower the resistance and vice versa

Question 17

define potential difference

Answer 17

the work done per unit charge and is measured in units of volts (V)

Question 18

define resistance

Answer 18

the opposition to current

Question 19

state ohms law

Answer 19

For a conductor at a constant temperature, the current through it is proportional to the potential difference across it

Question 20

filament lamp

Answer 20

1. As the current increases, the temperature of the filament in the lamp increases
2. Since the filament is a metal, the higher temperature causes an increase in resistance
3. Resistance opposes the current, causing the current to increase at a slower rate

Question 21

resistance equation

Answer 21